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6171 lines
201 KiB
HTML
6171 lines
201 KiB
HTML
<!DOCTYPE html>
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<html>
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<head>
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<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<title>Google C++ Style Guide</title>
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<link rel="stylesheet" type="text/css" href="include/styleguide.css">
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<script language="javascript" src="include/styleguide.js"></script>
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<link rel="shortcut icon" type="image/x-icon" href="https://www.google.com/favicon.ico" />
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</head>
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<body onload="initStyleGuide();">
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<div id="content">
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<h1>Google C++ Style Guide</h1>
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<div class="horizontal_toc" id="tocDiv"></div>
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<div class="main_body">
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<h2 class="ignoreLink" id="Background">Background</h2>
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<p>C++ is one of the main development languages used by
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many of Google's open-source projects. As every C++
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programmer knows, the language has many powerful features, but
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this power brings with it complexity, which in turn can make
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code more bug-prone and harder to read and maintain.</p>
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<p>The goal of this guide is to manage this complexity by
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describing in detail the dos and don'ts of writing C++ code.
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These rules exist to
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keep the code base manageable while still allowing
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coders to use C++ language features productively.</p>
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<p><em>Style</em>, also known as readability, is what we call
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the conventions that govern our C++ code. The term Style is a
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bit of a misnomer, since these conventions cover far more than
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just source file formatting.</p>
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<p>
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Most open-source projects developed by
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Google conform to the requirements in this guide.
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</p>
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<p>Note that this guide is not a C++ tutorial: we assume that
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the reader is familiar with the language. </p>
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<h3 id="Goals">Goals of the Style Guide</h3>
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<div class="stylebody">
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<p>Why do we have this document?</p>
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<p>There are a few core goals that we believe this guide should
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serve. These are the fundamental <b>why</b>s that
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underlie all of the individual rules. By bringing these ideas to
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the fore, we hope to ground discussions and make it clearer to our
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broader community why the rules are in place and why particular
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decisions have been made. If you understand what goals each rule is
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serving, it should be clearer to everyone when a rule may be waived
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(some can be), and what sort of argument or alternative would be
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necessary to change a rule in the guide.</p>
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<p>The goals of the style guide as we currently see them are as follows:</p>
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<dl>
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<dt>Style rules should pull their weight</dt>
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<dd>The benefit of a style rule
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must be large enough to justify asking all of our engineers to
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remember it. The benefit is measured relative to the codebase we would
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get without the rule, so a rule against a very harmful practice may
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still have a small benefit if people are unlikely to do it
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anyway. This principle mostly explains the rules we don’t have, rather
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than the rules we do: for example, <code>goto</code> contravenes many
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of the following principles, but is already vanishingly rare, so the Style
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Guide doesn’t discuss it.</dd>
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<dt>Optimize for the reader, not the writer</dt>
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<dd>Our codebase (and most individual components submitted to it) is
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expected to continue for quite some time. As a result, more time will
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be spent reading most of our code than writing it. We explicitly
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choose to optimize for the experience of our average software engineer
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reading, maintaining, and debugging code in our codebase rather than
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ease when writing said code. "Leave a trace for the reader" is a
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particularly common sub-point of this principle: When something
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surprising or unusual is happening in a snippet of code (for example,
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transfer of pointer ownership), leaving textual hints for the reader
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at the point of use is valuable (<code>std::unique_ptr</code>
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demonstrates the ownership transfer unambiguously at the call
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site). </dd>
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<dt>Be consistent with existing code</dt>
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<dd>Using one style consistently through our codebase lets us focus on
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other (more important) issues. Consistency also allows for
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automation: tools that format your code or adjust
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your <code>#include</code>s only work properly when your code is
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consistent with the expectations of the tooling. In many cases, rules
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that are attributed to "Be Consistent" boil down to "Just pick one and
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stop worrying about it"; the potential value of allowing flexibility
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on these points is outweighed by the cost of having people argue over
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them. </dd>
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<dt>Be consistent with the broader C++ community when appropriate</dt>
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<dd>Consistency with the way other organizations use C++ has value for
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the same reasons as consistency within our code base. If a feature in
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the C++ standard solves a problem, or if some idiom is widely known
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and accepted, that's an argument for using it. However, sometimes
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standard features and idioms are flawed, or were just designed without
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our codebase's needs in mind. In those cases (as described below) it's
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appropriate to constrain or ban standard features. In some cases we
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prefer a homegrown or third-party library over a library defined in
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the C++ Standard, either out of perceived superiority or insufficient
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value to transition the codebase to the standard interface.</dd>
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<dt>Avoid surprising or dangerous constructs</dt>
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<dd>C++ has features that are more surprising or dangerous than one
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might think at a glance. Some style guide restrictions are in place to
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prevent falling into these pitfalls. There is a high bar for style
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guide waivers on such restrictions, because waiving such rules often
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directly risks compromising program correctness.
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</dd>
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<dt>Avoid constructs that our average C++ programmer would find tricky
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or hard to maintain</dt>
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<dd>C++ has features that may not be generally appropriate because of
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the complexity they introduce to the code. In widely used
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code, it may be more acceptable to use
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trickier language constructs, because any benefits of more complex
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implementation are multiplied widely by usage, and the cost in understanding
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the complexity does not need to be paid again when working with new
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portions of the codebase. When in doubt, waivers to rules of this type
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can be sought by asking
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your project leads. This is specifically
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important for our codebase because code ownership and team membership
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changes over time: even if everyone that works with some piece of code
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currently understands it, such understanding is not guaranteed to hold a
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few years from now.</dd>
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<dt>Be mindful of our scale</dt>
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<dd>With a codebase of 100+ million lines and thousands of engineers,
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some mistakes and simplifications for one engineer can become costly
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for many. For instance it's particularly important to
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avoid polluting the global namespace: name collisions across a
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codebase of hundreds of millions of lines are difficult to work with
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and hard to avoid if everyone puts things into the global
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namespace.</dd>
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<dt>Concede to optimization when necessary</dt>
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<dd>Performance optimizations can sometimes be necessary and
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appropriate, even when they conflict with the other principles of this
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document.</dd>
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</dl>
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<p>The intent of this document is to provide maximal guidance with
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reasonable restriction. As always, common sense and good taste should
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prevail. By this we specifically refer to the established conventions
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of the entire Google C++ community, not just your personal preferences
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or those of your team. Be skeptical about and reluctant to use
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clever or unusual constructs: the absence of a prohibition is not the
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same as a license to proceed. Use your judgment, and if you are
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unsure, please don't hesitate to ask your project leads to get additional
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input.</p>
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</div>
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<h2 id="Header_Files">Header Files</h2>
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<p>In general, every <code>.cc</code> file should have an
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associated <code>.h</code> file. There are some common
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exceptions, such as unittests and
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small <code>.cc</code> files containing just a
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<code>main()</code> function.</p>
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<p>Correct use of header files can make a huge difference to
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the readability, size and performance of your code.</p>
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<p>The following rules will guide you through the various
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pitfalls of using header files.</p>
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<a id="The_-inl.h_Files"></a>
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<h3 id="Self_contained_Headers">Self-contained Headers</h3>
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<div class="summary">
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<p>Header files should be self-contained (compile on their own) and
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end in <code>.h</code>. Non-header files that are meant for inclusion
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should end in <code>.inc</code> and be used sparingly.</p>
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</div>
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<div class="stylebody">
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<p>All header files should be self-contained. Users and refactoring
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tools should not have to adhere to special conditions to include the
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header. Specifically, a header should
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have <a href="#The__define_Guard">header guards</a> and include all
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other headers it needs.</p>
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<p>Prefer placing the definitions for template and inline functions in
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the same file as their declarations. The definitions of these
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constructs must be included into every <code>.cc</code> file that uses
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them, or the program may fail to link in some build configurations. If
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declarations and definitions are in different files, including the
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former should transitively include the latter. Do not move these
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definitions to separately included header files (<code>-inl.h</code>);
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this practice was common in the past, but is no longer allowed.</p>
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<p>As an exception, a template that is explicitly instantiated for
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all relevant sets of template arguments, or that is a private
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implementation detail of a class, is allowed to be defined in the one
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and only <code>.cc</code> file that instantiates the template.</p>
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<p>There are rare cases where a file designed to be included is not
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self-contained. These are typically intended to be included at unusual
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locations, such as the middle of another file. They might not
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use <a href="#The__define_Guard">header guards</a>, and might not include
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their prerequisites. Name such files with the <code>.inc</code>
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extension. Use sparingly, and prefer self-contained headers when
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possible.</p>
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</div>
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<h3 id="The__define_Guard">The #define Guard</h3>
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<div class="summary">
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<p>All header files should have <code>#define</code> guards to
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prevent multiple inclusion. The format of the symbol name
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should be
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<code><i><PROJECT></i>_<i><PATH></i>_<i><FILE></i>_H_</code>.</p>
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</div>
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<div class="stylebody">
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<p>To guarantee uniqueness, they should
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be based on the full path in a project's source tree. For
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example, the file <code>foo/src/bar/baz.h</code> in
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project <code>foo</code> should have the following
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guard:</p>
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<pre>#ifndef FOO_BAR_BAZ_H_
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#define FOO_BAR_BAZ_H_
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...
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#endif // FOO_BAR_BAZ_H_
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</pre>
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</div>
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<h3 id="Forward_Declarations">Forward Declarations</h3>
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<div class="summary">
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<p>Avoid using forward declarations where possible.
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Just <code>#include</code> the headers you need.</p>
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</div>
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<div class="stylebody">
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<div class="definition">
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<p>A "forward declaration" is a declaration of a class,
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function, or template without an associated definition.</p>
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</div>
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<div class="pros">
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<ul>
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<li>Forward declarations can save compile time, as
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<code>#include</code>s force the compiler to open
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more files and process more input.</li>
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<li>Forward declarations can save on unnecessary
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recompilation. <code>#include</code>s can force
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your code to be recompiled more often, due to unrelated
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changes in the header.</li>
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</ul>
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</div>
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<div class="cons">
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<ul>
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<li>Forward declarations can hide a dependency, allowing
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user code to skip necessary recompilation when headers
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change.</li>
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<li>A forward declaration may be broken by subsequent
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changes to the library. Forward declarations of functions
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and templates can prevent the header owners from making
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otherwise-compatible changes to their APIs, such as
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widening a parameter type, adding a template parameter
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with a default value, or migrating to a new namespace.</li>
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<li>Forward declaring symbols from namespace
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<code>std::</code> yields undefined behavior.</li>
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<li>It can be difficult to determine whether a forward
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declaration or a full <code>#include</code> is needed.
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Replacing an <code>#include</code> with a forward
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declaration can silently change the meaning of
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code:
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<pre> // b.h:
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struct B {};
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struct D : B {};
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// good_user.cc:
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#include "b.h"
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void f(B*);
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void f(void*);
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void test(D* x) { f(x); } // calls f(B*)
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</pre>
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If the <code>#include</code> was replaced with forward
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decls for <code>B</code> and <code>D</code>,
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<code>test()</code> would call <code>f(void*)</code>.
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</li>
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<li>Forward declaring multiple symbols from a header
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can be more verbose than simply
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<code>#include</code>ing the header.</li>
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<li>Structuring code to enable forward declarations
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(e.g. using pointer members instead of object members)
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can make the code slower and more complex.</li>
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</ul>
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</div>
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<div class="decision">
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<ul>
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<li>Try to avoid forward declarations of entities
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defined in another project.</li>
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<li>When using a function declared in a header file,
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always <code>#include</code> that header.</li>
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<li>When using a class template, prefer to
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<code>#include</code> its header file.</li>
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</ul>
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<p>Please see <a href="#Names_and_Order_of_Includes">Names and Order
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of Includes</a> for rules about when to #include a header.</p>
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</div>
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</div>
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<h3 id="Inline_Functions">Inline Functions</h3>
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<div class="summary">
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<p>Define functions inline only when they are small, say, 10
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lines or fewer.</p>
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</div>
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<div class="stylebody">
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<div class="definition">
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<p>You can declare functions in a way that allows the compiler to expand
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them inline rather than calling them through the usual
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function call mechanism.</p>
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</div>
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<div class="pros">
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<p>Inlining a function can generate more efficient object
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code, as long as the inlined function is small. Feel free
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to inline accessors and mutators, and other short,
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performance-critical functions.</p>
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</div>
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<div class="cons">
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<p>Overuse of inlining can actually make programs slower.
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Depending on a function's size, inlining it can cause the
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code size to increase or decrease. Inlining a very small
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accessor function will usually decrease code size while
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inlining a very large function can dramatically increase
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code size. On modern processors smaller code usually runs
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faster due to better use of the instruction cache.</p>
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</div>
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<div class="decision">
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<p>A decent rule of thumb is to not inline a function if
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it is more than 10 lines long. Beware of destructors,
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which are often longer than they appear because of
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implicit member- and base-destructor calls!</p>
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<p>Another useful rule of thumb: it's typically not cost
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effective to inline functions with loops or switch
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statements (unless, in the common case, the loop or
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switch statement is never executed).</p>
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<p>It is important to know that functions are not always
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inlined even if they are declared as such; for example,
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virtual and recursive functions are not normally inlined.
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Usually recursive functions should not be inline. The
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main reason for making a virtual function inline is to
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place its definition in the class, either for convenience
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or to document its behavior, e.g., for accessors and
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mutators.</p>
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</div>
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</div>
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<h3 id="Names_and_Order_of_Includes">Names and Order of Includes</h3>
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<div class="summary">
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<p>Use standard order for readability and to avoid hidden
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dependencies: Related header, C library, C++ library, other libraries'
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<code>.h</code>, your project's <code>.h</code>.</p>
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</div>
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<div class="stylebody">
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<p>
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All of a project's header files should be
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listed as descendants of the project's source
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directory without use of UNIX directory shortcuts
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<code>.</code> (the current directory) or <code>..</code>
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(the parent directory). For example,
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<code>google-awesome-project/src/base/logging.h</code>
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should be included as:</p>
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<pre>#include "base/logging.h"
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</pre>
|
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|
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<p>In <code><var>dir/foo</var>.cc</code> or
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<code><var>dir/foo_test</var>.cc</code>, whose main
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purpose is to implement or test the stuff in
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<code><var>dir2/foo2</var>.h</code>, order your includes
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as follows:</p>
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<ol>
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<li><code><var>dir2/foo2</var>.h</code>.</li>
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<li>C system files.</li>
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<li>C++ system files.</li>
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<li>Other libraries' <code>.h</code>
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files.</li>
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<li>
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Your project's <code>.h</code>
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files.</li>
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</ol>
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<p>With the preferred ordering, if
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<code><var>dir2/foo2</var>.h</code> omits any necessary
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includes, the build of <code><var>dir/foo</var>.cc</code>
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or <code><var>dir/foo</var>_test.cc</code> will break.
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Thus, this rule ensures that build breaks show up first
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for the people working on these files, not for innocent
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people in other packages.</p>
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<p><code><var>dir/foo</var>.cc</code> and
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<code><var>dir2/foo2</var>.h</code> are usually in the same
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directory (e.g. <code>base/basictypes_test.cc</code> and
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<code>base/basictypes.h</code>), but may sometimes be in different
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directories too.</p>
|
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|
|
|
|
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<p>Within each section the includes should be ordered
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alphabetically. Note that older code might not conform to
|
|
this rule and should be fixed when convenient.</p>
|
|
|
|
<p>You should include all the headers that define the symbols you rely
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upon, except in the unusual case of <a href="#Forward_Declarations">forward
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|
declaration</a>. If you rely on symbols from <code>bar.h</code>,
|
|
don't count on the fact that you included <code>foo.h</code> which
|
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(currently) includes <code>bar.h</code>: include <code>bar.h</code>
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|
yourself, unless <code>foo.h</code> explicitly demonstrates its intent
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to provide you the symbols of <code>bar.h</code>. However, any
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includes present in the related header do not need to be included
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again in the related <code>cc</code> (i.e., <code>foo.cc</code> can
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rely on <code>foo.h</code>'s includes).</p>
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|
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<p>For example, the includes in
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<code>google-awesome-project/src/foo/internal/fooserver.cc</code>
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might look like this:</p>
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|
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<pre>#include "foo/server/fooserver.h"
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|
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#include <sys/types.h>
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#include <unistd.h>
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#include <hash_map>
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#include <vector>
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#include "base/basictypes.h"
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#include "base/commandlineflags.h"
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|
#include "foo/server/bar.h"
|
|
</pre>
|
|
|
|
<p class="exception">Sometimes, system-specific code needs
|
|
conditional includes. Such code can put conditional
|
|
includes after other includes. Of course, keep your
|
|
system-specific code small and localized. Example:</p>
|
|
|
|
<pre>#include "foo/public/fooserver.h"
|
|
|
|
#include "base/port.h" // For LANG_CXX11.
|
|
|
|
#ifdef LANG_CXX11
|
|
#include <initializer_list>
|
|
#endif // LANG_CXX11
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h2 id="Scoping">Scoping</h2>
|
|
|
|
<h3 id="Namespaces">Namespaces</h3>
|
|
|
|
<div class="summary">
|
|
<p>With few exceptions, place code in a namespace. Namespaces
|
|
should have unique names based on the project name, and possibly
|
|
its path. Do not use <i>using-directives</i> (e.g.
|
|
<code>using namespace foo</code>). Do not use
|
|
inline namespaces. For unnamed namespaces, see
|
|
<a href="#Unnamed_Namespaces_and_Static_Variables">Unnamed Namespaces and
|
|
Static Variables</a>.
|
|
</p></div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>Namespaces subdivide the global scope
|
|
into distinct, named scopes, and so are useful for preventing
|
|
name collisions in the global scope.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
|
|
<p>Namespaces provide a method for preventing name conflicts
|
|
in large programs while allowing most code to use reasonably
|
|
short names.</p>
|
|
|
|
<p>For example, if two different projects have a class
|
|
<code>Foo</code> in the global scope, these symbols may
|
|
collide at compile time or at runtime. If each project
|
|
places their code in a namespace, <code>project1::Foo</code>
|
|
and <code>project2::Foo</code> are now distinct symbols that
|
|
do not collide, and code within each project's namespace
|
|
can continue to refer to <code>Foo</code> without the prefix.</p>
|
|
|
|
<p>Inline namespaces automatically place their names in
|
|
the enclosing scope. Consider the following snippet, for
|
|
example:</p>
|
|
|
|
<pre>namespace X {
|
|
inline namespace Y {
|
|
void foo();
|
|
} // namespace Y
|
|
} // namespace X
|
|
</pre>
|
|
|
|
<p>The expressions <code>X::Y::foo()</code> and
|
|
<code>X::foo()</code> are interchangeable. Inline
|
|
namespaces are primarily intended for ABI compatibility
|
|
across versions.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
|
|
<p>Namespaces can be confusing, because they complicate
|
|
the mechanics of figuring out what definition a name refers
|
|
to.</p>
|
|
|
|
<p>Inline namespaces, in particular, can be confusing
|
|
because names aren't actually restricted to the namespace
|
|
where they are declared. They are only useful as part of
|
|
some larger versioning policy.</p>
|
|
|
|
<p>In some contexts, it's necessary to repeatedly refer to
|
|
symbols by their fully-qualified names. For deeply-nested
|
|
namespaces, this can add a lot of clutter.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
|
|
<p>Namespaces should be used as follows:</p>
|
|
|
|
<ul>
|
|
<li>Follow the rules on <a href="#Namespace_Names">Namespace Names</a>.
|
|
</li><li>Terminate namespaces with comments as shown in the given examples.
|
|
</li><li>
|
|
|
|
<p>Namespaces wrap the entire source file after
|
|
includes,
|
|
<a href="https://gflags.github.io/gflags/">
|
|
gflags</a> definitions/declarations
|
|
and forward declarations of classes from other namespaces.</p>
|
|
|
|
<pre>// In the .h file
|
|
namespace mynamespace {
|
|
|
|
// All declarations are within the namespace scope.
|
|
// Notice the lack of indentation.
|
|
class MyClass {
|
|
public:
|
|
...
|
|
void Foo();
|
|
};
|
|
|
|
} // namespace mynamespace
|
|
</pre>
|
|
|
|
<pre>// In the .cc file
|
|
namespace mynamespace {
|
|
|
|
// Definition of functions is within scope of the namespace.
|
|
void MyClass::Foo() {
|
|
...
|
|
}
|
|
|
|
} // namespace mynamespace
|
|
</pre>
|
|
|
|
<p>More complex <code>.cc</code> files might have additional details,
|
|
like flags or using-declarations.</p>
|
|
|
|
<pre>#include "a.h"
|
|
|
|
DEFINE_FLAG(bool, someflag, false, "dummy flag");
|
|
|
|
namespace a {
|
|
|
|
using ::foo::bar;
|
|
|
|
...code for a... // Code goes against the left margin.
|
|
|
|
} // namespace a
|
|
</pre>
|
|
</li>
|
|
|
|
|
|
|
|
<li>Do not declare anything in namespace
|
|
<code>std</code>, including forward declarations of
|
|
standard library classes. Declaring entities in
|
|
namespace <code>std</code> is undefined behavior, i.e.,
|
|
not portable. To declare entities from the standard
|
|
library, include the appropriate header file.</li>
|
|
|
|
<li><p>You may not use a <i>using-directive</i>
|
|
to make all names from a namespace available.</p>
|
|
|
|
<pre class="badcode">// Forbidden -- This pollutes the namespace.
|
|
using namespace foo;
|
|
</pre>
|
|
</li>
|
|
|
|
<li><p>Do not use <i>Namespace aliases</i> at namespace scope
|
|
in header files except in explicitly marked
|
|
internal-only namespaces, because anything imported into a namespace
|
|
in a header file becomes part of the public
|
|
API exported by that file.</p>
|
|
|
|
<pre>// Shorten access to some commonly used names in .cc files.
|
|
namespace baz = ::foo::bar::baz;
|
|
</pre>
|
|
|
|
<pre>// Shorten access to some commonly used names (in a .h file).
|
|
namespace librarian {
|
|
namespace impl { // Internal, not part of the API.
|
|
namespace sidetable = ::pipeline_diagnostics::sidetable;
|
|
} // namespace impl
|
|
|
|
inline void my_inline_function() {
|
|
// namespace alias local to a function (or method).
|
|
namespace baz = ::foo::bar::baz;
|
|
...
|
|
}
|
|
} // namespace librarian
|
|
</pre>
|
|
|
|
</li><li>Do not use inline namespaces.</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
|
|
<h3 id="Unnamed_Namespaces_and_Static_Variables">Unnamed Namespaces and Static
|
|
Variables</h3>
|
|
|
|
<div class="summary">
|
|
<p>When definitions in a <code>.cc</code> file do not need to be
|
|
referenced outside that file, place them in an unnamed
|
|
namespace or declare them <code>static</code>. Do not use either
|
|
of these constructs in <code>.h</code> files.
|
|
</p></div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>All declarations can be given internal linkage by placing them in
|
|
unnamed namespaces, and functions and variables can be given internal linkage by
|
|
declaring them <code>static</code>. This means that anything you're declaring
|
|
can't be accessed from another file. If a different file declares something
|
|
with the same name, then the two entities are completely independent.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
|
|
<p>Use of internal linkage in <code>.cc</code> files is encouraged
|
|
for all code that does not need to be referenced elsewhere.
|
|
Do not use internal linkage in <code>.h</code> files.</p>
|
|
|
|
<p>Format unnamed namespaces like named namespaces. In the
|
|
terminating comment, leave the namespace name empty:</p>
|
|
|
|
<pre>namespace {
|
|
...
|
|
} // namespace
|
|
</pre>
|
|
</div>
|
|
</div>
|
|
|
|
<h3 id="Nonmember,_Static_Member,_and_Global_Functions">Nonmember, Static Member, and Global Functions</h3>
|
|
|
|
<div class="summary">
|
|
<p>Prefer placing nonmember functions in a namespace; use completely global
|
|
functions rarely. Prefer grouping functions with a namespace instead of
|
|
using a class as if it were a namespace. Static methods of a class should
|
|
generally be closely related to instances of the class or the class's static
|
|
data.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="pros">
|
|
<p>Nonmember and static member functions can be useful in
|
|
some situations. Putting nonmember functions in a
|
|
namespace avoids polluting the global namespace.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>Nonmember and static member functions may make more sense
|
|
as members of a new class, especially if they access
|
|
external resources or have significant dependencies.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>Sometimes it is useful to define a
|
|
function not bound to a class instance. Such a function
|
|
can be either a static member or a nonmember function.
|
|
Nonmember functions should not depend on external
|
|
variables, and should nearly always exist in a namespace.
|
|
Rather than creating classes only to group static member
|
|
functions which do not share static data, use
|
|
<a href="#Namespaces">namespaces</a> instead. For a header
|
|
<code>myproject/foo_bar.h</code>, for example, write</p>
|
|
<pre>namespace myproject {
|
|
namespace foo_bar {
|
|
void Function1();
|
|
void Function2();
|
|
} // namespace foo_bar
|
|
} // namespace myproject
|
|
</pre>
|
|
<p>instead of</p>
|
|
<pre class="badcode">namespace myproject {
|
|
class FooBar {
|
|
public:
|
|
static void Function1();
|
|
static void Function2();
|
|
};
|
|
} // namespace myproject
|
|
</pre>
|
|
|
|
<p>If you define a nonmember function and it is only
|
|
needed in its <code>.cc</code> file, use
|
|
<a href="#Unnamed_Namespaces_and_Static_Variables">internal linkage</a> to limit
|
|
its scope.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Local_Variables">Local Variables</h3>
|
|
|
|
<div class="summary">
|
|
<p>Place a function's variables in the narrowest scope
|
|
possible, and initialize variables in the declaration.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>C++ allows you to declare variables anywhere in a
|
|
function. We encourage you to declare them in as local a
|
|
scope as possible, and as close to the first use as
|
|
possible. This makes it easier for the reader to find the
|
|
declaration and see what type the variable is and what it
|
|
was initialized to. In particular, initialization should
|
|
be used instead of declaration and assignment, e.g.:</p>
|
|
|
|
<pre class="badcode">int i;
|
|
i = f(); // Bad -- initialization separate from declaration.
|
|
</pre>
|
|
|
|
<pre>int j = g(); // Good -- declaration has initialization.
|
|
</pre>
|
|
|
|
<pre class="badcode">std::vector<int> v;
|
|
v.push_back(1); // Prefer initializing using brace initialization.
|
|
v.push_back(2);
|
|
</pre>
|
|
|
|
<pre>std::vector<int> v = {1, 2}; // Good -- v starts initialized.
|
|
</pre>
|
|
|
|
<p>Variables needed for <code>if</code>, <code>while</code>
|
|
and <code>for</code> statements should normally be declared
|
|
within those statements, so that such variables are confined
|
|
to those scopes. E.g.:</p>
|
|
|
|
<pre>while (const char* p = strchr(str, '/')) str = p + 1;
|
|
</pre>
|
|
|
|
<p>There is one caveat: if the variable is an object, its
|
|
constructor is invoked every time it enters scope and is
|
|
created, and its destructor is invoked every time it goes
|
|
out of scope.</p>
|
|
|
|
<pre class="badcode">// Inefficient implementation:
|
|
for (int i = 0; i < 1000000; ++i) {
|
|
Foo f; // My ctor and dtor get called 1000000 times each.
|
|
f.DoSomething(i);
|
|
}
|
|
</pre>
|
|
|
|
<p>It may be more efficient to declare such a variable
|
|
used in a loop outside that loop:</p>
|
|
|
|
<pre>Foo f; // My ctor and dtor get called once each.
|
|
for (int i = 0; i < 1000000; ++i) {
|
|
f.DoSomething(i);
|
|
}
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Static_and_Global_Variables">Static and Global Variables</h3>
|
|
|
|
<div class="summary">
|
|
<p>Variables of class type with <a href="http://en.cppreference.com/w/cpp/language/storage_duration#Storage_duration">
|
|
static storage duration</a> are forbidden: they cause hard-to-find bugs due
|
|
to indeterminate order of construction and destruction. However, such
|
|
variables are allowed if they are <code>constexpr</code>: they have no
|
|
dynamic initialization or destruction.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Objects with static storage duration, including global
|
|
variables, static variables, static class member
|
|
variables, and function static variables, must be Plain
|
|
Old Data (POD): only ints, chars, floats, or pointers, or
|
|
arrays/structs of POD.</p>
|
|
|
|
<p>The order in which class constructors and initializers
|
|
for static variables are called is only partially
|
|
specified in C++ and can even change from build to build,
|
|
which can cause bugs that are difficult to find.
|
|
Therefore in addition to banning globals of class type,
|
|
we do not allow non-local static variables to be initialized
|
|
with the result of a function, unless that function (such
|
|
as getenv(), or getpid()) does not itself depend on any
|
|
other globals. However, a static POD variable within
|
|
function scope may be initialized with the result of a
|
|
function, since its initialization order is well-defined
|
|
and does not occur until control passes through its
|
|
declaration.</p>
|
|
|
|
<p>Likewise, global and static variables are destroyed
|
|
when the program terminates, regardless of whether the
|
|
termination is by returning from <code>main()</code> or
|
|
by calling <code>exit()</code>. The order in which
|
|
destructors are called is defined to be the reverse of
|
|
the order in which the constructors were called. Since
|
|
constructor order is indeterminate, so is destructor
|
|
order. For example, at program-end time a static variable
|
|
might have been destroyed, but code still running
|
|
— perhaps in another thread
|
|
— tries to access it and fails. Or the
|
|
destructor for a static <code>string</code> variable
|
|
might be run prior to the destructor for another variable
|
|
that contains a reference to that string.</p>
|
|
|
|
<p>One way to alleviate the destructor problem is to
|
|
terminate the program by calling
|
|
<code>quick_exit()</code> instead of <code>exit()</code>.
|
|
The difference is that <code>quick_exit()</code> does not
|
|
invoke destructors and does not invoke any handlers that
|
|
were registered by calling <code>atexit()</code>. If you
|
|
have a handler that needs to run when a program
|
|
terminates via <code>quick_exit()</code> (flushing logs,
|
|
for example), you can register it using
|
|
<code>at_quick_exit()</code>. (If you have a handler that
|
|
needs to run at both <code>exit()</code> and
|
|
<code>quick_exit()</code>, you need to register it in
|
|
both places.)</p>
|
|
|
|
<p>As a result we only allow static variables to contain
|
|
POD data. This rule completely disallows
|
|
<code>std::vector</code> (use C arrays instead), or
|
|
<code>string</code> (use <code>const char []</code>).</p>
|
|
|
|
|
|
|
|
<p>If you need a static or global
|
|
variable of a class type, consider initializing a pointer
|
|
(which will never be freed), from either your main()
|
|
function or from pthread_once(). Note that this must be a
|
|
raw pointer, not a "smart" pointer, since the smart
|
|
pointer's destructor will have the order-of-destructor
|
|
issue that we are trying to avoid.</p>
|
|
|
|
|
|
|
|
|
|
|
|
</div>
|
|
|
|
<h2 id="Classes">Classes</h2>
|
|
|
|
<p>Classes are the fundamental unit of code in C++. Naturally,
|
|
we use them extensively. This section lists the main dos and
|
|
don'ts you should follow when writing a class.</p>
|
|
|
|
<h3 id="Doing_Work_in_Constructors">Doing Work in Constructors</h3>
|
|
|
|
<div class="summary">
|
|
<p>Avoid virtual method calls in constructors, and avoid
|
|
initialization that can fail if you can't signal an error.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>It is possible to perform arbitrary initialization in the body
|
|
of the constructor.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<ul>
|
|
<li>No need to worry about whether the class has been initialized or
|
|
not.</li>
|
|
|
|
<li>Objects that are fully initialized by constructor call can
|
|
be <code>const</code> and may also be easier to use with standard containers
|
|
or algorithms.</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<ul>
|
|
<li>If the work calls virtual functions, these calls
|
|
will not get dispatched to the subclass
|
|
implementations. Future modification to your class can
|
|
quietly introduce this problem even if your class is
|
|
not currently subclassed, causing much confusion.</li>
|
|
|
|
<li>There is no easy way for constructors to signal errors, short of
|
|
crashing the program (not always appropriate) or using exceptions
|
|
(which are <a href="#Exceptions">forbidden</a>).</li>
|
|
|
|
<li>If the work fails, we now have an object whose initialization
|
|
code failed, so it may be an unusual state requiring a <code>bool
|
|
IsValid()</code> state checking mechanism (or similar) which is easy
|
|
to forget to call.</li>
|
|
|
|
<li>You cannot take the address of a constructor, so whatever work
|
|
is done in the constructor cannot easily be handed off to, for
|
|
example, another thread.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
|
|
<div class="decision">
|
|
<p>Constructors should never call virtual functions. If appropriate
|
|
for your code
|
|
,
|
|
terminating the program may be an appropriate error handling
|
|
response. Otherwise, consider a factory function
|
|
or <code>Init()</code> method. Avoid <code>Init()</code> methods on objects with
|
|
no other states that affect which public methods may be called
|
|
(semi-constructed objects of this form are particularly hard to work
|
|
with correctly).</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<a id="Explicit_Constructors"></a>
|
|
<h3 id="Implicit_Conversions">Implicit Conversions</h3>
|
|
|
|
<div class="summary">
|
|
<p>Do not define implicit conversions. Use the <code>explicit</code>
|
|
keyword for conversion operators and single-argument
|
|
constructors.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>Implicit conversions allow an
|
|
object of one type (called the <dfn>source type</dfn>) to
|
|
be used where a different type (called the <dfn>destination
|
|
type</dfn>) is expected, such as when passing an
|
|
<code>int</code> argument to a function that takes a
|
|
<code>double</code> parameter.</p>
|
|
|
|
<p>In addition to the implicit conversions defined by the language,
|
|
users can define their own, by adding appropriate members to the
|
|
class definition of the source or destination type. An implicit
|
|
conversion in the source type is defined by a type conversion operator
|
|
named after the destination type (e.g. <code>operator
|
|
bool()</code>). An implicit conversion in the destination
|
|
type is defined by a constructor that can take the source type as
|
|
its only argument (or only argument with no default value).</p>
|
|
|
|
<p>The <code>explicit</code> keyword can be applied to a constructor
|
|
or (since C++11) a conversion operator, to ensure that it can only be
|
|
used when the destination type is explicit at the point of use,
|
|
e.g. with a cast. This applies not only to implicit conversions, but to
|
|
C++11's list initialization syntax:</p>
|
|
<pre>class Foo {
|
|
explicit Foo(int x, double y);
|
|
...
|
|
};
|
|
|
|
void Func(Foo f);
|
|
</pre>
|
|
<pre class="badcode">Func({42, 3.14}); // Error
|
|
</pre>
|
|
This kind of code isn't technically an implicit conversion, but the
|
|
language treats it as one as far as <code>explicit</code> is concerned.
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<ul>
|
|
<li>Implicit conversions can make a type more usable and
|
|
expressive by eliminating the need to explicitly name a type
|
|
when it's obvious.</li>
|
|
<li>Implicit conversions can be a simpler alternative to
|
|
overloading.</li>
|
|
<li>List initialization syntax is a concise and expressive
|
|
way of initializing objects.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<ul>
|
|
<li>Implicit conversions can hide type-mismatch bugs, where the
|
|
destination type does not match the user's expectation, or
|
|
the user is unaware that any conversion will take place.</li>
|
|
|
|
<li>Implicit conversions can make code harder to read, particularly
|
|
in the presence of overloading, by making it less obvious what
|
|
code is actually getting called.</li>
|
|
|
|
<li>Constructors that take a single argument may accidentally
|
|
be usable as implicit type conversions, even if they are not
|
|
intended to do so.</li>
|
|
|
|
<li>When a single-argument constructor is not marked
|
|
<code>explicit</code>, there's no reliable way to tell whether
|
|
it's intended to define an implicit conversion, or the author
|
|
simply forgot to mark it.</li>
|
|
|
|
<li>It's not always clear which type should provide the conversion,
|
|
and if they both do, the code becomes ambiguous.</li>
|
|
|
|
<li>List initialization can suffer from the same problems if
|
|
the destination type is implicit, particularly if the
|
|
list has only a single element.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>Type conversion operators, and constructors that are
|
|
callable with a single argument, must be marked
|
|
<code>explicit</code> in the class definition. As an
|
|
exception, copy and move constructors should not be
|
|
<code>explicit</code>, since they do not perform type
|
|
conversion. Implicit conversions can sometimes be necessary and
|
|
appropriate for types that are designed to transparently wrap other
|
|
types. In that case, contact
|
|
your project leads to request
|
|
a waiver of this rule.</p>
|
|
|
|
<p>Constructors that cannot be called with a single argument
|
|
should usually omit <code>explicit</code>. Constructors that
|
|
take a single <code>std::initializer_list</code> parameter should
|
|
also omit <code>explicit</code>, in order to support copy-initialization
|
|
(e.g. <code>MyType m = {1, 2};</code>).</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Copyable_Movable_Types">Copyable and Movable Types</h3>
|
|
<a id="Copy_Constructors"></a>
|
|
<div class="summary">
|
|
<p>Support copying and/or moving if these operations are clear and meaningful
|
|
for your type. Otherwise, disable the implicitly generated special functions
|
|
that perform copies and moves.
|
|
</p></div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>A copyable type allows its objects to be initialized or assigned
|
|
from any other object of the same type, without changing the value of the source.
|
|
For user-defined types, the copy behavior is defined by the copy
|
|
constructor and the copy-assignment operator.
|
|
<code>string</code> is an example of a copyable type.</p>
|
|
|
|
<p>A movable type is one that can be initialized and assigned
|
|
from temporaries (all copyable types are therefore movable).
|
|
<code>std::unique_ptr<int></code> is an example of a movable but not
|
|
copyable type. For user-defined types, the move behavior is defined by the move
|
|
constructor and the move-assignment operator.</p>
|
|
|
|
<p>The copy/move constructors can be implicitly invoked by the compiler
|
|
in some situations, e.g. when passing objects by value.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>Objects of copyable and movable types can be passed and returned by value,
|
|
which makes APIs simpler, safer, and more general. Unlike when passing objects
|
|
by pointer or reference, there's no risk of confusion over ownership,
|
|
lifetime, mutability, and similar issues, and no need to specify them in the
|
|
contract. It also prevents non-local interactions between the client and the
|
|
implementation, which makes them easier to understand, maintain, and optimize by
|
|
the compiler. Further, such objects can be used with generic APIs that
|
|
require pass-by-value, such as most containers, and they allow for additional
|
|
flexibility in e.g., type composition.</p>
|
|
|
|
<p>Copy/move constructors and assignment operators are usually
|
|
easier to define correctly than alternatives
|
|
like <code>Clone()</code>, <code>CopyFrom()</code> or <code>Swap()</code>,
|
|
because they can be generated by the compiler, either implicitly or
|
|
with <code>= default</code>. They are concise, and ensure
|
|
that all data members are copied. Copy and move
|
|
constructors are also generally more efficient, because they don't
|
|
require heap allocation or separate initialization and assignment
|
|
steps, and they're eligible for optimizations such as
|
|
|
|
<a href="http://en.cppreference.com/w/cpp/language/copy_elision">
|
|
copy elision</a>.</p>
|
|
|
|
<p>Move operations allow the implicit and efficient transfer of
|
|
resources out of rvalue objects. This allows a plainer coding style
|
|
in some cases.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>Some types do not need to be copyable, and providing copy
|
|
operations for such types can be confusing, nonsensical, or outright
|
|
incorrect. Types representing singleton objects (<code>Registerer</code>),
|
|
objects tied to a specific scope (<code>Cleanup</code>), or closely coupled to
|
|
object identity (<code>Mutex</code>) cannot be copied meaningfully.
|
|
Copy operations for base class types that are to be used
|
|
polymorphically are hazardous, because use of them can lead to
|
|
<a href="https://en.wikipedia.org/wiki/Object_slicing">object slicing</a>.
|
|
Defaulted or carelessly-implemented copy operations can be incorrect, and the
|
|
resulting bugs can be confusing and difficult to diagnose.</p>
|
|
|
|
<p>Copy constructors are invoked implicitly, which makes the
|
|
invocation easy to miss. This may cause confusion for programmers used to
|
|
languages where pass-by-reference is conventional or mandatory. It may also
|
|
encourage excessive copying, which can cause performance problems.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
|
|
<p>Provide the copy and move operations if their meaning is clear to a casual
|
|
user and the copying/moving does not incur unexpected costs. If you define a
|
|
copy or move constructor, define the corresponding assignment operator, and
|
|
vice-versa. If your type is copyable, do not define move operations unless they
|
|
are significantly more efficient than the corresponding copy operations. If your
|
|
type is not copyable, but the correctness of a move is obvious to users of the
|
|
type, you may make the type move-only by defining both of the move operations.
|
|
</p>
|
|
|
|
<p>If your type provides copy operations, it is recommended that you design
|
|
your class so that the default implementation of those operations is correct.
|
|
Remember to review the correctness of any defaulted operations as you would any
|
|
other code, and to document that your class is copyable and/or cheaply movable
|
|
if that's an API guarantee.</p>
|
|
|
|
<pre class="badcode">class Foo {
|
|
public:
|
|
Foo(Foo&& other) : field_(other.field) {}
|
|
// Bad, defines only move constructor, but not operator=.
|
|
|
|
private:
|
|
Field field_;
|
|
};
|
|
</pre>
|
|
|
|
<p>Due to the risk of slicing, avoid providing an assignment
|
|
operator or public copy/move constructor for a class that's
|
|
intended to be derived from (and avoid deriving from a class
|
|
with such members). If your base class needs to be
|
|
copyable, provide a public virtual <code>Clone()</code>
|
|
method, and a protected copy constructor that derived classes
|
|
can use to implement it.</p>
|
|
|
|
<p>If you do not want to support copy/move operations on your type,
|
|
explicitly disable them using <code>= delete</code> in
|
|
the <code>public:</code> section:</p>
|
|
|
|
<pre class="code">// MyClass is neither copyable nor movable.
|
|
MyClass(const MyClass&) = delete;
|
|
MyClass& operator=(const MyClass&) = delete;
|
|
</pre>
|
|
|
|
<p></p>
|
|
|
|
</div>
|
|
</div>
|
|
|
|
<h3 id="Structs_vs._Classes">Structs vs. Classes</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use a <code>struct</code> only for passive objects that
|
|
carry data; everything else is a <code>class</code>.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>The <code>struct</code> and <code>class</code>
|
|
keywords behave almost identically in C++. We add our own
|
|
semantic meanings to each keyword, so you should use the
|
|
appropriate keyword for the data-type you're
|
|
defining.</p>
|
|
|
|
<p><code>structs</code> should be used for passive
|
|
objects that carry data, and may have associated
|
|
constants, but lack any functionality other than
|
|
access/setting the data members. The accessing/setting of
|
|
fields is done by directly accessing the fields rather
|
|
than through method invocations. Methods should not
|
|
provide behavior but should only be used to set up the
|
|
data members, e.g., constructor, destructor,
|
|
<code>Initialize()</code>, <code>Reset()</code>,
|
|
<code>Validate()</code>.</p>
|
|
|
|
<p>If more functionality is required, a
|
|
<code>class</code> is more appropriate. If in doubt, make
|
|
it a <code>class</code>.</p>
|
|
|
|
<p>For consistency with STL, you can use
|
|
<code>struct</code> instead of <code>class</code> for
|
|
functors and traits.</p>
|
|
|
|
<p>Note that member variables in structs and classes have
|
|
<a href="#Variable_Names">different naming rules</a>.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Inheritance">Inheritance</h3>
|
|
|
|
<div class="summary">
|
|
<p>Composition is often more appropriate than inheritance.
|
|
When using inheritance, make it <code>public</code>.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p> When a sub-class
|
|
inherits from a base class, it includes the definitions
|
|
of all the data and operations that the parent base class
|
|
defines. In practice, inheritance is used in two major
|
|
ways in C++: implementation inheritance, in which actual
|
|
code is inherited by the child, and
|
|
<a href="#Interfaces">interface inheritance</a>, in which
|
|
only method names are inherited.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>Implementation inheritance reduces code size by re-using
|
|
the base class code as it specializes an existing type.
|
|
Because inheritance is a compile-time declaration, you
|
|
and the compiler can understand the operation and detect
|
|
errors. Interface inheritance can be used to
|
|
programmatically enforce that a class expose a particular
|
|
API. Again, the compiler can detect errors, in this case,
|
|
when a class does not define a necessary method of the
|
|
API.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>For implementation inheritance, because the code
|
|
implementing a sub-class is spread between the base and
|
|
the sub-class, it can be more difficult to understand an
|
|
implementation. The sub-class cannot override functions
|
|
that are not virtual, so the sub-class cannot change
|
|
implementation. The base class may also define some data
|
|
members, so that specifies physical layout of the base
|
|
class.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
|
|
<p>All inheritance should be <code>public</code>. If you
|
|
want to do private inheritance, you should be including
|
|
an instance of the base class as a member instead.</p>
|
|
|
|
<p>Do not overuse implementation inheritance. Composition
|
|
is often more appropriate. Try to restrict use of
|
|
inheritance to the "is-a" case: <code>Bar</code>
|
|
subclasses <code>Foo</code> if it can reasonably be said
|
|
that <code>Bar</code> "is a kind of"
|
|
<code>Foo</code>.</p>
|
|
|
|
<p>Make your destructor <code>virtual</code> if
|
|
necessary. If your class has virtual methods, its
|
|
destructor should be virtual.</p>
|
|
|
|
<p>Limit the use of <code>protected</code> to those
|
|
member functions that might need to be accessed from
|
|
subclasses. Note that <a href="#Access_Control">data
|
|
members should be private</a>.</p>
|
|
|
|
<p>Explicitly annotate overrides of virtual functions
|
|
or virtual destructors with an <code>override</code>
|
|
or (less frequently) <code>final</code> specifier.
|
|
Older (pre-C++11) code will use the
|
|
<code>virtual</code> keyword as an inferior
|
|
alternative annotation. For clarity, use exactly one of
|
|
<code>override</code>, <code>final</code>, or
|
|
<code>virtual</code> when declaring an override.
|
|
Rationale: A function or destructor marked
|
|
<code>override</code> or <code>final</code> that is
|
|
not an override of a base class virtual function will
|
|
not compile, and this helps catch common errors. The
|
|
specifiers serve as documentation; if no specifier is
|
|
present, the reader has to check all ancestors of the
|
|
class in question to determine if the function or
|
|
destructor is virtual or not.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Multiple_Inheritance">Multiple Inheritance</h3>
|
|
|
|
<div class="summary">
|
|
<p>Only very rarely is multiple implementation inheritance
|
|
actually useful. We allow multiple inheritance only when at
|
|
most one of the base classes has an implementation; all
|
|
other base classes must be <a href="#Interfaces">pure
|
|
interface</a> classes tagged with the
|
|
<code>Interface</code> suffix.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>Multiple inheritance allows a sub-class to have more than
|
|
one base class. We distinguish between base classes that are
|
|
<em>pure interfaces</em> and those that have an
|
|
<em>implementation</em>.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>Multiple implementation inheritance may let you re-use
|
|
even more code than single inheritance (see <a href="#Inheritance">Inheritance</a>).</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>Only very rarely is multiple <em>implementation</em>
|
|
inheritance actually useful. When multiple implementation
|
|
inheritance seems like the solution, you can usually find
|
|
a different, more explicit, and cleaner solution.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p> Multiple inheritance is allowed only when all
|
|
superclasses, with the possible exception of the first one,
|
|
are <a href="#Interfaces">pure interfaces</a>. In order to
|
|
ensure that they remain pure interfaces, they must end with
|
|
the <code>Interface</code> suffix.</p>
|
|
</div>
|
|
|
|
<div class="note">
|
|
<p>There is an <a href="#Windows_Code">exception</a> to
|
|
this rule on Windows.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Interfaces">Interfaces</h3>
|
|
|
|
<div class="summary">
|
|
<p>Classes that satisfy certain conditions are allowed, but
|
|
not required, to end with an <code>Interface</code> suffix.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>A class is a pure interface if it meets the following
|
|
requirements:</p>
|
|
|
|
<ul>
|
|
<li>It has only public pure virtual ("<code>=
|
|
0</code>") methods and static methods (but see below
|
|
for destructor).</li>
|
|
|
|
<li>It may not have non-static data members.</li>
|
|
|
|
<li>It need not have any constructors defined. If a
|
|
constructor is provided, it must take no arguments and
|
|
it must be protected.</li>
|
|
|
|
<li>If it is a subclass, it may only be derived from
|
|
classes that satisfy these conditions and are tagged
|
|
with the <code>Interface</code> suffix.</li>
|
|
</ul>
|
|
|
|
<p>An interface class can never be directly instantiated
|
|
because of the pure virtual method(s) it declares. To
|
|
make sure all implementations of the interface can be
|
|
destroyed correctly, the interface must also declare a
|
|
virtual destructor (in an exception to the first rule,
|
|
this should not be pure). See Stroustrup, <cite>The C++
|
|
Programming Language</cite>, 3rd edition, section 12.4
|
|
for details.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>Tagging a class with the <code>Interface</code> suffix
|
|
lets others know that they must not add implemented
|
|
methods or non static data members. This is particularly
|
|
important in the case of <a href="#Multiple_Inheritance">multiple inheritance</a>.
|
|
Additionally, the interface concept is already
|
|
well-understood by Java programmers.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>The <code>Interface</code> suffix lengthens the class
|
|
name, which can make it harder to read and understand.
|
|
Also, the interface property may be considered an
|
|
implementation detail that shouldn't be exposed to
|
|
clients.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>A class may end
|
|
with <code>Interface</code> only if it meets the above
|
|
requirements. We do not require the converse, however:
|
|
classes that meet the above requirements are not required
|
|
to end with <code>Interface</code>.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Operator_Overloading">Operator Overloading</h3>
|
|
|
|
<div class="summary">
|
|
<p>Overload operators judiciously. Do not create user-defined literals.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>C++ permits user code to
|
|
<a href="http://en.cppreference.com/w/cpp/language/operators">declare
|
|
overloaded versions of the built-in operators</a> using the
|
|
<code>operator</code> keyword, so long as one of the parameters
|
|
is a user-defined type. The <code>operator</code> keyword also
|
|
permits user code to define new kinds of literals using
|
|
<code>operator""</code>, and to define type-conversion functions
|
|
such as <code>operator bool()</code>.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>Operator overloading can make code more concise and
|
|
intuitive by enabling user-defined types to behave the same
|
|
as built-in types. Overloaded operators are the idiomatic names
|
|
for certain operations (e.g. <code>==</code>, <code><</code>,
|
|
<code>=</code>, and <code><<</code>), and adhering to
|
|
those conventions can make user-defined types more readable
|
|
and enable them to interoperate with libraries that expect
|
|
those names.</p>
|
|
|
|
<p>User-defined literals are a very concise notation for
|
|
creating objects of user-defined types.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<ul>
|
|
<li>Providing a correct, consistent, and unsurprising
|
|
set of operator overloads requires some care, and failure
|
|
to do so can lead to confusion and bugs.</li>
|
|
|
|
<li>Overuse of operators can lead to obfuscated code,
|
|
particularly if the overloaded operator's semantics
|
|
don't follow convention.</li>
|
|
|
|
<li>The hazards of function overloading apply just as
|
|
much to operator overloading, if not more so.</li>
|
|
|
|
<li>Operator overloads can fool our intuition into
|
|
thinking that expensive operations are cheap, built-in
|
|
operations.</li>
|
|
|
|
<li>Finding the call sites for overloaded operators may
|
|
requre a search tool that's aware of C++ syntax, rather
|
|
than e.g. grep.</li>
|
|
|
|
<li>If you get the argument type of an overloaded operator
|
|
wrong, you may get a different overload rather than a
|
|
compiler error. For example, <code>foo < bar</code>
|
|
may do one thing, while <code>&foo < &bar</code>
|
|
does something totally different.</li>
|
|
|
|
<li>Certain operator overloads are inherently hazardous.
|
|
Overloading unary <code>&</code> can cause the same
|
|
code to have different meanings depending on whether
|
|
the overload declaration is visible. Overloads of
|
|
<code>&&</code>, <code>||</code>, and <code>,</code>
|
|
(comma) cannot match the evaluation-order semantics of the
|
|
built-in operators.</li>
|
|
|
|
<li>Operators are often defined outside the class,
|
|
so there's a risk of different files introducing
|
|
different definitions of the same operator. If both
|
|
definitions are linked into the same binary, this results
|
|
in undefined behavior, which can manifest as subtle
|
|
run-time bugs.</li>
|
|
|
|
<li>User-defined literals allow the creation of new
|
|
syntactic forms that are unfamiliar even to experienced C++
|
|
programmers.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>Define overloaded operators only if their meaning is
|
|
obvious, unsurprising, and consistent with the corresponding
|
|
built-in operators. For example, use <code>|</code> as a
|
|
bitwise- or logical-or, not as a shell-style pipe.</p>
|
|
|
|
<p>Define operators only on your own types. More precisely,
|
|
define them in the same headers, .cc files, and namespaces
|
|
as the types they operate on. That way, the operators are available
|
|
wherever the type is, minimizing the risk of multiple
|
|
definitions. If possible, avoid defining operators as templates,
|
|
because they must satisfy this rule for any possible template
|
|
arguments. If you define an operator, also define
|
|
any related operators that make sense, and make sure they
|
|
are defined consistently. For example, if you overload
|
|
<code><</code>, overload all the comparison operators,
|
|
and make sure <code><</code> and <code>></code> never
|
|
return true for the same arguments.</p>
|
|
|
|
<p>Prefer to define non-modifying binary operators as
|
|
non-member functions. If a binary operator is defined as a
|
|
class member, implicit conversions will apply to the
|
|
right-hand argument, but not the left-hand one. It will
|
|
confuse your users if <code>a < b</code> compiles but
|
|
<code>b < a</code> doesn't.</p>
|
|
|
|
<p>Don't go out of your way to avoid defining operator
|
|
overloads. For example, prefer to define <code>==</code>,
|
|
<code>=</code>, and <code><<</code>, rather than
|
|
<code>Equals()</code>, <code>CopyFrom()</code>, and
|
|
<code>PrintTo()</code>. Conversely, don't define
|
|
operator overloads just because other libraries expect
|
|
them. For example, if your type doesn't have a natural
|
|
ordering, but you want to store it in a <code>std::set</code>,
|
|
use a custom comparator rather than overloading
|
|
<code><</code>.</p>
|
|
|
|
<p>Do not overload <code>&&</code>, <code>||</code>,
|
|
<code>,</code> (comma), or unary <code>&</code>. Do not overload
|
|
<code>operator""</code>, i.e. do not introduce user-defined
|
|
literals.</p>
|
|
|
|
<p>Type conversion operators are covered in the section on
|
|
<a href="#Implicit_Conversions">implicit conversions</a>.
|
|
The <code>=</code> operator is covered in the section on
|
|
<a href="#Copy_Constructors">copy constructors</a>. Overloading
|
|
<code><<</code> for use with streams is covered in the
|
|
section on <a href="#Streams">streams</a>. See also the rules on
|
|
<a href="#Function_Overloading">function overloading</a>, which
|
|
apply to operator overloading as well.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Access_Control">Access Control</h3>
|
|
|
|
<div class="summary">
|
|
<p> Make data members <code>private</code>, unless they are
|
|
<code>static const</code> (and follow the <a href="#Constant_Names">
|
|
naming convention for constants</a>). For technical
|
|
reasons, we allow data members of a test fixture class to
|
|
be <code>protected</code> when using
|
|
|
|
|
|
<a href="https://github.com/google/googletest">Google
|
|
Test</a>).</p>
|
|
</div>
|
|
|
|
<h3 id="Declaration_Order">Declaration Order</h3>
|
|
|
|
<div class="summary">
|
|
<p>Group similar declarations together, placing public parts
|
|
earlier.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>A class definition should usually start with a
|
|
<code>public:</code> section, followed by
|
|
<code>protected:</code>, then <code>private:</code>. Omit
|
|
sections that would be empty.</p>
|
|
|
|
<p>Within each section, generally prefer grouping similar
|
|
kinds of declarations together, and generally prefer the
|
|
following order: types (including <code>typedef</code>,
|
|
<code>using</code>, and nested structs and classes),
|
|
constants, factory functions, constructors, assignment
|
|
operators, destructor, all other methods, data members.</p>
|
|
|
|
<p>Do not put large method definitions inline in the
|
|
class definition. Usually, only trivial or
|
|
performance-critical, and very short, methods may be
|
|
defined inline. See <a href="#Inline_Functions">Inline
|
|
Functions</a> for more details.</p>
|
|
|
|
</div>
|
|
|
|
<h2 id="Functions">Functions</h2>
|
|
|
|
<h3 id="Function_Parameter_Ordering">Parameter Ordering</h3>
|
|
|
|
<div class="summary">
|
|
<p>When defining a function, parameter order is: inputs, then
|
|
outputs.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
<p>Parameters to C/C++ functions are either input to the
|
|
function, output from the function, or both. Input
|
|
parameters are usually values or <code>const</code>
|
|
references, while output and input/output parameters will
|
|
be pointers to non-<code>const</code>. When ordering
|
|
function parameters, put all input-only parameters before
|
|
any output parameters. In particular, do not add new
|
|
parameters to the end of the function just because they
|
|
are new; place new input-only parameters before the
|
|
output parameters.</p>
|
|
|
|
<p>This is not a hard-and-fast rule. Parameters that are
|
|
both input and output (often classes/structs) muddy the
|
|
waters, and, as always, consistency with related
|
|
functions may require you to bend the rule.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Write_Short_Functions">Write Short Functions</h3>
|
|
|
|
<div class="summary">
|
|
<p>Prefer small and focused functions.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
<p>We recognize that long functions are sometimes
|
|
appropriate, so no hard limit is placed on functions
|
|
length. If a function exceeds about 40 lines, think about
|
|
whether it can be broken up without harming the structure
|
|
of the program.</p>
|
|
|
|
<p>Even if your long function works perfectly now,
|
|
someone modifying it in a few months may add new
|
|
behavior. This could result in bugs that are hard to
|
|
find. Keeping your functions short and simple makes it
|
|
easier for other people to read and modify your code.</p>
|
|
|
|
<p>You could find long and complicated functions when
|
|
working with
|
|
some code. Do not be
|
|
intimidated by modifying existing code: if working with
|
|
such a function proves to be difficult, you find that
|
|
errors are hard to debug, or you want to use a piece of
|
|
it in several different contexts, consider breaking up
|
|
the function into smaller and more manageable pieces.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Reference_Arguments">Reference Arguments</h3>
|
|
|
|
<div class="summary">
|
|
<p>All parameters passed by reference must be labeled
|
|
<code>const</code>.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>In C, if a
|
|
function needs to modify a variable, the parameter must
|
|
use a pointer, eg <code>int foo(int *pval)</code>. In
|
|
C++, the function can alternatively declare a reference
|
|
parameter: <code>int foo(int &val)</code>.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>Defining a parameter as reference avoids ugly code like
|
|
<code>(*pval)++</code>. Necessary for some applications
|
|
like copy constructors. Makes it clear, unlike with
|
|
pointers, that a null pointer is not a possible
|
|
value.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>References can be confusing, as they have value syntax
|
|
but pointer semantics.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>Within function parameter lists all references must be
|
|
<code>const</code>:</p>
|
|
|
|
<pre>void Foo(const string &in, string *out);
|
|
</pre>
|
|
|
|
<p>In fact it is a very strong convention in Google code
|
|
that input arguments are values or <code>const</code>
|
|
references while output arguments are pointers. Input
|
|
parameters may be <code>const</code> pointers, but we
|
|
never allow non-<code>const</code> reference parameters
|
|
except when required by convention, e.g.,
|
|
<code>swap()</code>.</p>
|
|
|
|
<p>However, there are some instances where using
|
|
<code>const T*</code> is preferable to <code>const
|
|
T&</code> for input parameters. For example:</p>
|
|
|
|
<ul>
|
|
<li>You want to pass in a null pointer.</li>
|
|
|
|
<li>The function saves a pointer or reference to the
|
|
input.</li>
|
|
</ul>
|
|
|
|
<p> Remember that most of the time input
|
|
parameters are going to be specified as <code>const
|
|
T&</code>. Using <code>const T*</code> instead
|
|
communicates to the reader that the input is somehow
|
|
treated differently. So if you choose <code>const
|
|
T*</code> rather than <code>const T&</code>, do so
|
|
for a concrete reason; otherwise it will likely confuse
|
|
readers by making them look for an explanation that
|
|
doesn't exist.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Function_Overloading">Function Overloading</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use overloaded functions (including constructors) only if a
|
|
reader looking at a call site can get a good idea of what
|
|
is happening without having to first figure out exactly
|
|
which overload is being called.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>You may write a function that takes a <code>const
|
|
string&</code> and overload it with another that
|
|
takes <code>const char*</code>.</p>
|
|
|
|
<pre>class MyClass {
|
|
public:
|
|
void Analyze(const string &text);
|
|
void Analyze(const char *text, size_t textlen);
|
|
};
|
|
</pre>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>Overloading can make code more intuitive by allowing an
|
|
identically-named function to take different arguments.
|
|
It may be necessary for templatized code, and it can be
|
|
convenient for Visitors.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>If a function is overloaded by the argument types alone,
|
|
a reader may have to understand C++'s complex matching
|
|
rules in order to tell what's going on. Also many people
|
|
are confused by the semantics of inheritance if a derived
|
|
class overrides only some of the variants of a
|
|
function.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>If you want to overload a function, consider qualifying
|
|
the name with some information about the arguments, e.g.,
|
|
<code>AppendString()</code>, <code>AppendInt()</code>
|
|
rather than just <code>Append()</code>. If you are
|
|
overloading a function to support variable number of
|
|
arguments of the same type, consider making it take a
|
|
<code>std::vector</code> so that the user can use an
|
|
<a href="#Braced_Initializer_List">initializer list
|
|
</a> to specify the arguments.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Default_Arguments">Default Arguments</h3>
|
|
|
|
<div class="summary">
|
|
<p>Default arguments are allowed on non-virtual functions
|
|
when the default is guaranteed to always have the same
|
|
value. Follow the same restrictions as for <a href="#Function_Overloading">function overloading</a>, and
|
|
prefer overloaded functions if the readability gained with
|
|
default arguments doesn't outweigh the downsides below.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="pros">
|
|
<p>Often you have a function that uses default values, but
|
|
occasionally you want to override the defaults. Default
|
|
parameters allow an easy way to do this without having to
|
|
define many functions for the rare exceptions. Compared
|
|
to overloading the function, default arguments have a
|
|
cleaner syntax, with less boilerplate and a clearer
|
|
distinction between 'required' and 'optional'
|
|
arguments.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>Defaulted arguments are another way to achieve the
|
|
semantics of overloaded functions, so all the <a href="#Function_Overloading">reasons not to overload
|
|
functions</a> apply.</p>
|
|
|
|
<p>The defaults for arguments in a virtual function call are
|
|
determined by the static type of the target object, and
|
|
there's no guarantee that all overrides of a given function
|
|
declare the same defaults.</p>
|
|
|
|
<p>Default parameters are re-evaluated at each call site,
|
|
which can bloat the generated code. Readers may also expect
|
|
the default's value to be fixed at the declaration instead
|
|
of varying at each call.</p>
|
|
|
|
<p>Function pointers are confusing in the presence of
|
|
default arguments, since the function signature often
|
|
doesn't match the call signature. Adding
|
|
function overloads avoids these problems.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>Default arguments are banned on virtual functions, where
|
|
they don't work properly, and in cases where the specified
|
|
default might not evaluate to the same value depending on
|
|
when it was evaluated. (For example, don't write <code>void
|
|
f(int n = counter++);</code>.)</p>
|
|
|
|
<p>In some other cases, default arguments can improve the
|
|
readability of their function declarations enough to
|
|
overcome the downsides above, so they are allowed. When in
|
|
doubt, use overloads.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="trailing_return">Trailing Return Type Syntax</h3>
|
|
<div class="summary">
|
|
<p>Use trailing return types only where using the ordinary syntax (leading
|
|
return types) is impractical or much less readable.</p>
|
|
</div>
|
|
|
|
<div class="definition">
|
|
<p>C++ allows two different forms of function declarations. In the older
|
|
form, the return type appears before the function name. For example:</p>
|
|
<pre>int foo(int x);
|
|
</pre>
|
|
<p>The new form, introduced in C++11, uses the <code>auto</code>
|
|
keyword before the function name and a trailing return type after
|
|
the argument list. For example, the declaration above could
|
|
equivalently be written:</p>
|
|
<pre>auto foo(int x) -> int;
|
|
</pre>
|
|
<p>The trailing return type is in the function's scope. This doesn't
|
|
make a difference for a simple case like <code>int</code> but it matters
|
|
for more complicated cases, like types declared in class scope or
|
|
types written in terms of the function parameters.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
<div class="pros">
|
|
<p>Trailing return types are the only way to explicitly specify the
|
|
return type of a <a href="#Lambda_expressions">lambda expression</a>.
|
|
In some cases the compiler is able to deduce a lambda's return type,
|
|
but not in all cases. Even when the compiler can deduce it automatically,
|
|
sometimes specifying it explicitly would be clearer for readers.
|
|
</p>
|
|
<p>Sometimes it's easier and more readable to specify a return type
|
|
after the function's parameter list has already appeared. This is
|
|
particularly true when the return type depends on template parameters.
|
|
For example:</p>
|
|
<pre>template <class T, class U> auto add(T t, U u) -> decltype(t + u);</pre>
|
|
versus
|
|
<pre>template <class T, class U> decltype(declval<T&>() + declval<U&>()) add(T t, U u);</pre>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>Trailing return type syntax is relatively new and it has no
|
|
analogue in C++-like languages like C and Java, so some readers may
|
|
find it unfamiliar.</p>
|
|
<p>Existing code bases have an enormous number of function
|
|
declarations that aren't going to get changed to use the new syntax,
|
|
so the realistic choices are using the old syntax only or using a mixture
|
|
of the two. Using a single version is better for uniformity of style.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>In most cases, continue to use the older style of function
|
|
declaration where the return type goes before the function name.
|
|
Use the new trailing-return-type form only in cases where it's
|
|
required (such as lambdas) or where, by putting the type after the
|
|
function's parameter list, it allows you to write the type in a much
|
|
more readable way. The latter case should be rare; it's mostly an
|
|
issue in fairly complicated template code, which is
|
|
<a href="#Template_metaprogramming">discouraged in most cases</a>.</p>
|
|
|
|
</div>
|
|
</div>
|
|
|
|
<h2 id="Google-Specific_Magic">Google-Specific Magic</h2>
|
|
|
|
|
|
|
|
<p>There are various tricks and utilities that
|
|
we use to make C++ code more robust, and various ways we use
|
|
C++ that may differ from what you see elsewhere.</p>
|
|
|
|
|
|
|
|
<h3 id="Ownership_and_Smart_Pointers">Ownership and Smart Pointers</h3>
|
|
|
|
<div class="summary">
|
|
<p>Prefer to have single, fixed owners for dynamically
|
|
allocated objects. Prefer to transfer ownership with smart
|
|
pointers.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>"Ownership" is a bookkeeping technique for managing
|
|
dynamically allocated memory (and other resources). The
|
|
owner of a dynamically allocated object is an object or
|
|
function that is responsible for ensuring that it is
|
|
deleted when no longer needed. Ownership can sometimes be
|
|
shared, in which case the last owner is typically
|
|
responsible for deleting it. Even when ownership is not
|
|
shared, it can be transferred from one piece of code to
|
|
another.</p>
|
|
|
|
<p>"Smart" pointers are classes that act like pointers,
|
|
e.g. by overloading the <code>*</code> and
|
|
<code>-></code> operators. Some smart pointer types
|
|
can be used to automate ownership bookkeeping, to ensure
|
|
these responsibilities are met.
|
|
<a href="http://en.cppreference.com/w/cpp/memory/unique_ptr">
|
|
<code>std::unique_ptr</code></a> is a smart pointer type
|
|
introduced in C++11, which expresses exclusive ownership
|
|
of a dynamically allocated object; the object is deleted
|
|
when the <code>std::unique_ptr</code> goes out of scope.
|
|
It cannot be copied, but can be <em>moved</em> to
|
|
represent ownership transfer.
|
|
<a href="http://en.cppreference.com/w/cpp/memory/shared_ptr">
|
|
<code>std::shared_ptr</code></a> is a smart pointer type
|
|
that expresses shared ownership of
|
|
a dynamically allocated object. <code>std::shared_ptr</code>s
|
|
can be copied; ownership of the object is shared among
|
|
all copies, and the object is deleted when the last
|
|
<code>std::shared_ptr</code> is destroyed. </p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<ul>
|
|
<li>It's virtually impossible to manage dynamically
|
|
allocated memory without some sort of ownership
|
|
logic.</li>
|
|
|
|
<li>Transferring ownership of an object can be cheaper
|
|
than copying it (if copying it is even possible).</li>
|
|
|
|
<li>Transferring ownership can be simpler than
|
|
'borrowing' a pointer or reference, because it reduces
|
|
the need to coordinate the lifetime of the object
|
|
between the two users.</li>
|
|
|
|
<li>Smart pointers can improve readability by making
|
|
ownership logic explicit, self-documenting, and
|
|
unambiguous.</li>
|
|
|
|
<li>Smart pointers can eliminate manual ownership
|
|
bookkeeping, simplifying the code and ruling out large
|
|
classes of errors.</li>
|
|
|
|
<li>For const objects, shared ownership can be a simple
|
|
and efficient alternative to deep copying.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<ul>
|
|
<li>Ownership must be represented and transferred via
|
|
pointers (whether smart or plain). Pointer semantics
|
|
are more complicated than value semantics, especially
|
|
in APIs: you have to worry not just about ownership,
|
|
but also aliasing, lifetime, and mutability, among
|
|
other issues.</li>
|
|
|
|
<li>The performance costs of value semantics are often
|
|
overestimated, so the performance benefits of ownership
|
|
transfer might not justify the readability and
|
|
complexity costs.</li>
|
|
|
|
<li>APIs that transfer ownership force their clients
|
|
into a single memory management model.</li>
|
|
|
|
<li>Code using smart pointers is less explicit about
|
|
where the resource releases take place.</li>
|
|
|
|
<li><code>std::unique_ptr</code> expresses ownership
|
|
transfer using C++11's move semantics, which are
|
|
relatively new and may confuse some programmers.</li>
|
|
|
|
<li>Shared ownership can be a tempting alternative to
|
|
careful ownership design, obfuscating the design of a
|
|
system.</li>
|
|
|
|
<li>Shared ownership requires explicit bookkeeping at
|
|
run-time, which can be costly.</li>
|
|
|
|
<li>In some cases (e.g. cyclic references), objects
|
|
with shared ownership may never be deleted.</li>
|
|
|
|
<li>Smart pointers are not perfect substitutes for
|
|
plain pointers.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>If dynamic allocation is necessary, prefer to keep
|
|
ownership with the code that allocated it. If other code
|
|
needs access to the object, consider passing it a copy,
|
|
or passing a pointer or reference without transferring
|
|
ownership. Prefer to use <code>std::unique_ptr</code> to
|
|
make ownership transfer explicit. For example:</p>
|
|
|
|
<pre>std::unique_ptr<Foo> FooFactory();
|
|
void FooConsumer(std::unique_ptr<Foo> ptr);
|
|
</pre>
|
|
|
|
|
|
|
|
<p>Do not design your code to use shared ownership
|
|
without a very good reason. One such reason is to avoid
|
|
expensive copy operations, but you should only do this if
|
|
the performance benefits are significant, and the
|
|
underlying object is immutable (i.e.
|
|
<code>std::shared_ptr<const Foo></code>). If you
|
|
do use shared ownership, prefer to use
|
|
<code>std::shared_ptr</code>.</p>
|
|
|
|
<p>Never use <code>std::auto_ptr</code>. Instead, use
|
|
<code>std::unique_ptr</code>.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="cpplint">cpplint</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use <code>cpplint.py</code>
|
|
to detect style errors.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p><code>cpplint.py</code>
|
|
is a tool that reads a source file and identifies many
|
|
style errors. It is not perfect, and has both false
|
|
positives and false negatives, but it is still a valuable
|
|
tool. False positives can be ignored by putting <code>//
|
|
NOLINT</code> at the end of the line or
|
|
<code>// NOLINTNEXTLINE</code> in the previous line.</p>
|
|
|
|
|
|
|
|
<p>Some projects have instructions on
|
|
how to run <code>cpplint.py</code> from their project
|
|
tools. If the project you are contributing to does not,
|
|
you can download
|
|
<a href="https://raw.githubusercontent.com/google/styleguide/gh-pages/cpplint/cpplint.py">
|
|
<code>cpplint.py</code></a> separately.</p>
|
|
|
|
</div>
|
|
|
|
|
|
|
|
<h2 id="Other_C++_Features">Other C++ Features</h2>
|
|
|
|
<h3 id="Rvalue_references">Rvalue References</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use rvalue references only to define move constructors and move assignment
|
|
operators, or for perfect forwarding.
|
|
</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p> Rvalue references
|
|
are a type of reference that can only bind to temporary
|
|
objects. The syntax is similar to traditional reference
|
|
syntax. For example, <code>void f(string&&
|
|
s);</code> declares a function whose argument is an
|
|
rvalue reference to a string.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<ul>
|
|
<li>Defining a move constructor (a constructor taking
|
|
an rvalue reference to the class type) makes it
|
|
possible to move a value instead of copying it. If
|
|
<code>v1</code> is a <code>std::vector<string></code>,
|
|
for example, then <code>auto v2(std::move(v1))</code>
|
|
will probably just result in some simple pointer
|
|
manipulation instead of copying a large amount of data.
|
|
In some cases this can result in a major performance
|
|
improvement.</li>
|
|
|
|
<li>Rvalue references make it possible to write a
|
|
generic function wrapper that forwards its arguments to
|
|
another function, and works whether or not its
|
|
arguments are temporary objects. (This is sometimes called
|
|
"perfect forwarding".)</li>
|
|
|
|
<li>Rvalue references make it possible to implement
|
|
types that are movable but not copyable, which can be
|
|
useful for types that have no sensible definition of
|
|
copying but where you might still want to pass them as
|
|
function arguments, put them in containers, etc.</li>
|
|
|
|
<li><code>std::move</code> is necessary to make
|
|
effective use of some standard-library types, such as
|
|
<code>std::unique_ptr</code>.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<ul>
|
|
<li>Rvalue references are a relatively new feature
|
|
(introduced as part of C++11), and not yet widely
|
|
understood. Rules like reference collapsing, and
|
|
automatic synthesis of move constructors, are
|
|
complicated.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>Use rvalue references only to define move constructors and move assignment
|
|
operators (as described in <a href="#Copyable_Movable_Types">Copyable and
|
|
Movable Types</a>) and, in conjunction with <code><a href="http://en.cppreference.com/w/cpp/utility/forward">std::forward</a></code>,
|
|
to support perfect forwarding. You may use <code>std::move</code> to express
|
|
moving a value from one object to another rather than copying it. </p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Friends">Friends</h3>
|
|
|
|
<div class="summary">
|
|
<p>We allow use of <code>friend</code> classes and functions,
|
|
within reason.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Friends should usually be defined in the same file so
|
|
that the reader does not have to look in another file to
|
|
find uses of the private members of a class. A common use
|
|
of <code>friend</code> is to have a
|
|
<code>FooBuilder</code> class be a friend of
|
|
<code>Foo</code> so that it can construct the inner state
|
|
of <code>Foo</code> correctly, without exposing this
|
|
state to the world. In some cases it may be useful to
|
|
make a unittest class a friend of the class it tests.</p>
|
|
|
|
<p>Friends extend, but do not break, the encapsulation
|
|
boundary of a class. In some cases this is better than
|
|
making a member public when you want to give only one
|
|
other class access to it. However, most classes should
|
|
interact with other classes solely through their public
|
|
members.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Exceptions">Exceptions</h3>
|
|
|
|
<div class="summary">
|
|
<p>We do not use C++ exceptions.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="pros">
|
|
<ul>
|
|
<li>Exceptions allow higher levels of an application to
|
|
decide how to handle "can't happen" failures in deeply
|
|
nested functions, without the obscuring and error-prone
|
|
bookkeeping of error codes.</li>
|
|
|
|
|
|
|
|
<li>Exceptions are used by most other
|
|
modern languages. Using them in C++ would make it more
|
|
consistent with Python, Java, and the C++ that others
|
|
are familiar with.</li>
|
|
|
|
<li>Some third-party C++ libraries use exceptions, and
|
|
turning them off internally makes it harder to
|
|
integrate with those libraries.</li>
|
|
|
|
<li>Exceptions are the only way for a constructor to
|
|
fail. We can simulate this with a factory function or
|
|
an <code>Init()</code> method, but these require heap
|
|
allocation or a new "invalid" state, respectively.</li>
|
|
|
|
<li>Exceptions are really handy in testing
|
|
frameworks.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<ul>
|
|
<li>When you add a <code>throw</code> statement to an
|
|
existing function, you must examine all of its
|
|
transitive callers. Either they must make at least the
|
|
basic exception safety guarantee, or they must never
|
|
catch the exception and be happy with the program
|
|
terminating as a result. For instance, if
|
|
<code>f()</code> calls <code>g()</code> calls
|
|
<code>h()</code>, and <code>h</code> throws an
|
|
exception that <code>f</code> catches, <code>g</code>
|
|
has to be careful or it may not clean up properly.</li>
|
|
|
|
<li>More generally, exceptions make the control flow of
|
|
programs difficult to evaluate by looking at code:
|
|
functions may return in places you don't expect. This
|
|
causes maintainability and debugging difficulties. You
|
|
can minimize this cost via some rules on how and where
|
|
exceptions can be used, but at the cost of more that a
|
|
developer needs to know and understand.</li>
|
|
|
|
<li>Exception safety requires both RAII and different
|
|
coding practices. Lots of supporting machinery is
|
|
needed to make writing correct exception-safe code
|
|
easy. Further, to avoid requiring readers to understand
|
|
the entire call graph, exception-safe code must isolate
|
|
logic that writes to persistent state into a "commit"
|
|
phase. This will have both benefits and costs (perhaps
|
|
where you're forced to obfuscate code to isolate the
|
|
commit). Allowing exceptions would force us to always
|
|
pay those costs even when they're not worth it.</li>
|
|
|
|
<li>Turning on exceptions adds data to each binary
|
|
produced, increasing compile time (probably slightly)
|
|
and possibly increasing address space pressure.
|
|
</li>
|
|
|
|
<li>The availability of exceptions may encourage
|
|
developers to throw them when they are not appropriate
|
|
or recover from them when it's not safe to do so. For
|
|
example, invalid user input should not cause exceptions
|
|
to be thrown. We would need to make the style guide
|
|
even longer to document these restrictions!</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>On their face, the benefits of using exceptions
|
|
outweigh the costs, especially in new projects. However,
|
|
for existing code, the introduction of exceptions has
|
|
implications on all dependent code. If exceptions can be
|
|
propagated beyond a new project, it also becomes
|
|
problematic to integrate the new project into existing
|
|
exception-free code. Because most existing C++ code at
|
|
Google is not prepared to deal with exceptions, it is
|
|
comparatively difficult to adopt new code that generates
|
|
exceptions.</p>
|
|
|
|
<p>Given that Google's existing code is not
|
|
exception-tolerant, the costs of using exceptions are
|
|
somewhat greater than the costs in a new project. The
|
|
conversion process would be slow and error-prone. We
|
|
don't believe that the available alternatives to
|
|
exceptions, such as error codes and assertions, introduce
|
|
a significant burden. </p>
|
|
|
|
<p>Our advice against using exceptions is not predicated
|
|
on philosophical or moral grounds, but practical ones.
|
|
Because we'd like to use our open-source
|
|
projects at Google and it's difficult to do so if those
|
|
projects use exceptions, we need to advise against
|
|
exceptions in Google open-source projects as well.
|
|
Things would probably be different if we had to do it all
|
|
over again from scratch.</p>
|
|
|
|
<p>This prohibition also applies to the exception-related
|
|
features added in C++11, such as <code>noexcept</code>,
|
|
<code>std::exception_ptr</code>, and
|
|
<code>std::nested_exception</code>.</p>
|
|
|
|
<p>There is an <a href="#Windows_Code">exception</a> to
|
|
this rule (no pun intended) for Windows code.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Run-Time_Type_Information__RTTI_">Run-Time Type
|
|
Information (RTTI)</h3>
|
|
|
|
<div class="summary">
|
|
<p>Avoid using Run Time Type Information (RTTI).</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p> RTTI allows a
|
|
programmer to query the C++ class of an object at run
|
|
time. This is done by use of <code>typeid</code> or
|
|
<code>dynamic_cast</code>.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>Querying the type of an object at run-time frequently
|
|
means a design problem. Needing to know the type of an
|
|
object at runtime is often an indication that the design
|
|
of your class hierarchy is flawed.</p>
|
|
|
|
<p>Undisciplined use of RTTI makes code hard to maintain.
|
|
It can lead to type-based decision trees or switch
|
|
statements scattered throughout the code, all of which
|
|
must be examined when making further changes.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>The standard alternatives to RTTI (described below)
|
|
require modification or redesign of the class hierarchy
|
|
in question. Sometimes such modifications are infeasible
|
|
or undesirable, particularly in widely-used or mature
|
|
code.</p>
|
|
|
|
<p>RTTI can be useful in some unit tests. For example, it
|
|
is useful in tests of factory classes where the test has
|
|
to verify that a newly created object has the expected
|
|
dynamic type. It is also useful in managing the
|
|
relationship between objects and their mocks.</p>
|
|
|
|
<p>RTTI is useful when considering multiple abstract
|
|
objects. Consider</p>
|
|
|
|
<pre>bool Base::Equal(Base* other) = 0;
|
|
bool Derived::Equal(Base* other) {
|
|
Derived* that = dynamic_cast<Derived*>(other);
|
|
if (that == NULL)
|
|
return false;
|
|
...
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>RTTI has legitimate uses but is prone to abuse, so you
|
|
must be careful when using it. You may use it freely in
|
|
unittests, but avoid it when possible in other code. In
|
|
particular, think twice before using RTTI in new code. If
|
|
you find yourself needing to write code that behaves
|
|
differently based on the class of an object, consider one
|
|
of the following alternatives to querying the type:</p>
|
|
|
|
<ul>
|
|
<li>Virtual methods are the preferred way of executing
|
|
different code paths depending on a specific subclass
|
|
type. This puts the work within the object itself.</li>
|
|
|
|
<li>If the work belongs outside the object and instead
|
|
in some processing code, consider a double-dispatch
|
|
solution, such as the Visitor design pattern. This
|
|
allows a facility outside the object itself to
|
|
determine the type of class using the built-in type
|
|
system.</li>
|
|
</ul>
|
|
|
|
<p>When the logic of a program guarantees that a given
|
|
instance of a base class is in fact an instance of a
|
|
particular derived class, then a
|
|
<code>dynamic_cast</code> may be used freely on the
|
|
object. Usually one
|
|
can use a <code>static_cast</code> as an alternative in
|
|
such situations.</p>
|
|
|
|
<p>Decision trees based on type are a strong indication
|
|
that your code is on the wrong track.</p>
|
|
|
|
<pre class="badcode">if (typeid(*data) == typeid(D1)) {
|
|
...
|
|
} else if (typeid(*data) == typeid(D2)) {
|
|
...
|
|
} else if (typeid(*data) == typeid(D3)) {
|
|
...
|
|
</pre>
|
|
|
|
<p>Code such as this usually breaks when additional
|
|
subclasses are added to the class hierarchy. Moreover,
|
|
when properties of a subclass change, it is difficult to
|
|
find and modify all the affected code segments.</p>
|
|
|
|
<p>Do not hand-implement an RTTI-like workaround. The
|
|
arguments against RTTI apply just as much to workarounds
|
|
like class hierarchies with type tags. Moreover,
|
|
workarounds disguise your true intent.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Casting">Casting</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use C++-style casts
|
|
like <code>static_cast<float>(double_value)</code>, or brace
|
|
initialization for conversion of arithmetic types like
|
|
<code>int64 y = int64{1} << 42</code>. Do not use
|
|
cast formats like
|
|
<code>int y = (int)x</code> or <code>int y = int(x)</code> (but the latter
|
|
is okay when invoking a constructor of a class type).</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p> C++ introduced a
|
|
different cast system from C that distinguishes the types
|
|
of cast operations.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>The problem with C casts is the ambiguity of the operation;
|
|
sometimes you are doing a <em>conversion</em>
|
|
(e.g., <code>(int)3.5</code>) and sometimes you are doing
|
|
a <em>cast</em> (e.g., <code>(int)"hello"</code>). Brace
|
|
initialization and C++ casts can often help avoid this
|
|
ambiguity. Additionally, C++ casts are more visible when searching for
|
|
them.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>The C++-style cast syntax is verbose and cumbersome.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>Do not use C-style casts. Instead, use these C++-style casts when
|
|
explicit type conversion is necessary. </p>
|
|
|
|
<ul>
|
|
<li>Use brace initialization to convert arithmetic types
|
|
(e.g. <code>int64{x}</code>). This is the safest approach because code
|
|
will not compile if conversion can result in information loss. The
|
|
syntax is also concise.</li>
|
|
|
|
|
|
|
|
<li>Use <code>static_cast</code> as the equivalent of a C-style cast
|
|
that does value conversion, when you need to
|
|
explicitly up-cast a pointer from a class to its superclass, or when
|
|
you need to explicitly cast a pointer from a superclass to a
|
|
subclass. In this last case, you must be sure your object is
|
|
actually an instance of the subclass.</li>
|
|
|
|
|
|
|
|
<li>Use <code>const_cast</code> to remove the
|
|
<code>const</code> qualifier (see <a href="#Use_of_const">const</a>).</li>
|
|
|
|
<li>Use <code>reinterpret_cast</code> to do unsafe
|
|
conversions of pointer types to and from integer and
|
|
other pointer types. Use this only if you know what you
|
|
are doing and you understand the aliasing issues.
|
|
</li>
|
|
|
|
|
|
</ul>
|
|
|
|
<p>See the <a href="#Run-Time_Type_Information__RTTI_">
|
|
RTTI section</a> for guidance on the use of
|
|
<code>dynamic_cast</code>.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Streams">Streams</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use streams where appropriate, and stick to "simple"
|
|
usages.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>Streams are the standard I/O abstraction in C++, as
|
|
exemplified by the standard header <code><iostream></code>.
|
|
They are widely used in Google code, but only for debug logging
|
|
and test diagnostics.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>The <code><<</code> and <code>>></code>
|
|
stream operators provide an API for formatted I/O that
|
|
is easily learned, portable, reusable, and extensible.
|
|
<code>printf</code>, by contrast, doesn't even support
|
|
<code>string</code>, to say nothing of user-defined types,
|
|
and is very difficult to use portably.
|
|
<code>printf</code> also obliges you to choose among the
|
|
numerous slightly different versions of that function,
|
|
and navigate the dozens of conversion specifiers.</p>
|
|
|
|
<p>Streams provide first-class support for console I/O
|
|
via <code>std::cin</code>, <code>std::cout</code>,
|
|
<code>std::cerr</code>, and <code>std::clog</code>.
|
|
The C APIs do as well, but are hampered by the need to
|
|
manually buffer the input. </p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<ul>
|
|
<li>Stream formatting can be configured by mutating the
|
|
state of the stream. Such mutations are persistent, so
|
|
the behavior of your code can be affected by the entire
|
|
previous history of the stream, unless you go out of your
|
|
way to restore it to a known state every time other code
|
|
might have touched it. User code can not only modify the
|
|
built-in state, it can add new state variables and behaviors
|
|
through a registration system.</li>
|
|
|
|
<li>It is difficult to precisely control stream output, due
|
|
to the above issues, the way code and data are mixed in
|
|
streaming code, and the use of operator overloading (which
|
|
may select a different overload than you expect).</li>
|
|
|
|
<li>The practice of building up output through chains
|
|
of <code><<</code> operators interferes with
|
|
internationalization, because it bakes word order into the
|
|
code, and streams' support for localization is <a href="http://www.boost.org/doc/libs/1_48_0/libs/locale/doc/html/rationale.html#rationale_why">
|
|
flawed</a>.</li>
|
|
|
|
|
|
|
|
|
|
|
|
<li>The streams API is subtle and complex, so programmers must
|
|
develop experience with it in order to use it effectively.
|
|
However, streams were historically banned in Google code (except
|
|
for logging and diagnostics), so Google engineers tend not to
|
|
have that experience. Consequently, streams-based code is likely
|
|
to be less readable and maintainable by Googlers than code based
|
|
on more familiar abstractions.</li>
|
|
|
|
<li>Resolving the many overloads of <code><<</code> is
|
|
extremely costly for the compiler. When used pervasively in a
|
|
large code base, it can consume as much as 20% of the parsing
|
|
and semantic analysis time.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>Use streams only when they are the best tool for the job.
|
|
This is typically the case when the I/O is ad-hoc, local,
|
|
human-readable, and targeted at other developers rather than
|
|
end-users. Be consistent with the code around you, and with the
|
|
codebase as a whole; if there's an established tool for
|
|
your problem, use that tool instead. </p>
|
|
|
|
<p>Avoid using streams for I/O that faces external users or
|
|
handles untrusted data. Instead, find and use the appropriate
|
|
templating libraries to handle issues like internationalization,
|
|
localization, and security hardening.</p>
|
|
|
|
<p>If you do use streams, avoid the stateful parts of the
|
|
streams API (other than error state), such as <code>imbue()</code>,
|
|
<code>xalloc()</code>, and <code>register_callback()</code>.
|
|
Use explicit formatting functions rather than
|
|
stream manipulators or formatting flags to control formatting
|
|
details such as number base, precision, or padding.</p>
|
|
|
|
<p>Overload <code><<</code> as a streaming operator
|
|
for your type only if your type represents a value, and
|
|
<code><<</code> writes out a human-readable string
|
|
representation of that value. Avoid exposing implementation
|
|
details in the output of <code><<</code>; if you need to print
|
|
object internals for debugging, use named functions instead
|
|
(a method named <code>DebugString()</code> is the most common
|
|
convention).</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Preincrement_and_Predecrement">Preincrement and Predecrement</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use prefix form (<code>++i</code>) of the increment and
|
|
decrement operators with iterators and other template
|
|
objects.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p> When a variable
|
|
is incremented (<code>++i</code> or <code>i++</code>) or
|
|
decremented (<code>--i</code> or <code>i--</code>) and
|
|
the value of the expression is not used, one must decide
|
|
whether to preincrement (decrement) or postincrement
|
|
(decrement).</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>When the return value is ignored, the "pre" form
|
|
(<code>++i</code>) is never less efficient than the
|
|
"post" form (<code>i++</code>), and is often more
|
|
efficient. This is because post-increment (or decrement)
|
|
requires a copy of <code>i</code> to be made, which is
|
|
the value of the expression. If <code>i</code> is an
|
|
iterator or other non-scalar type, copying <code>i</code>
|
|
could be expensive. Since the two types of increment
|
|
behave the same when the value is ignored, why not just
|
|
always pre-increment?</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>The tradition developed, in C, of using post-increment
|
|
when the expression value is not used, especially in
|
|
<code>for</code> loops. Some find post-increment easier
|
|
to read, since the "subject" (<code>i</code>) precedes
|
|
the "verb" (<code>++</code>), just like in English.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p> For simple scalar
|
|
(non-object) values there is no reason to prefer one form
|
|
and we allow either. For iterators and other template
|
|
types, use pre-increment.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Use_of_const">Use of const</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use <code>const</code> whenever it makes sense. With C++11,
|
|
<code>constexpr</code> is a better choice for some uses of
|
|
const.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p> Declared variables and parameters can be preceded
|
|
by the keyword <code>const</code> to indicate the variables
|
|
are not changed (e.g., <code>const int foo</code>). Class
|
|
functions can have the <code>const</code> qualifier to
|
|
indicate the function does not change the state of the
|
|
class member variables (e.g., <code>class Foo { int
|
|
Bar(char c) const; };</code>).</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>Easier for people to understand how variables are being
|
|
used. Allows the compiler to do better type checking,
|
|
and, conceivably, generate better code. Helps people
|
|
convince themselves of program correctness because they
|
|
know the functions they call are limited in how they can
|
|
modify your variables. Helps people know what functions
|
|
are safe to use without locks in multi-threaded
|
|
programs.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p><code>const</code> is viral: if you pass a
|
|
<code>const</code> variable to a function, that function
|
|
must have <code>const</code> in its prototype (or the
|
|
variable will need a <code>const_cast</code>). This can
|
|
be a particular problem when calling library
|
|
functions.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p><code>const</code> variables, data members, methods
|
|
and arguments add a level of compile-time type checking;
|
|
it is better to detect errors as soon as possible.
|
|
Therefore we strongly recommend that you use
|
|
<code>const</code> whenever it makes sense to do so:</p>
|
|
|
|
<ul>
|
|
<li>If a function guarantees that it will not modify an argument
|
|
passed by reference or by pointer, the corresponding function parameter
|
|
should be a reference-to-const (<code>const T&</code>) or
|
|
pointer-to-const (<code>const T*</code>), respectively.</li>
|
|
|
|
<li>Declare methods to be <code>const</code> whenever
|
|
possible. Accessors should almost always be
|
|
<code>const</code>. Other methods should be const if
|
|
they do not modify any data members, do not call any
|
|
non-<code>const</code> methods, and do not return a
|
|
non-<code>const</code> pointer or
|
|
non-<code>const</code> reference to a data member.</li>
|
|
|
|
<li>Consider making data members <code>const</code>
|
|
whenever they do not need to be modified after
|
|
construction.</li>
|
|
</ul>
|
|
|
|
<p>The <code>mutable</code> keyword is allowed but is
|
|
unsafe when used with threads, so thread safety should be
|
|
carefully considered first.</p>
|
|
</div>
|
|
|
|
<div class="stylepoint_subsection">
|
|
<h4>Where to put the const</h4>
|
|
|
|
<p>Some people favor the form <code>int const *foo</code>
|
|
to <code>const int* foo</code>. They argue that this is
|
|
more readable because it's more consistent: it keeps the
|
|
rule that <code>const</code> always follows the object
|
|
it's describing. However, this consistency argument
|
|
doesn't apply in codebases with few deeply-nested pointer
|
|
expressions since most <code>const</code> expressions
|
|
have only one <code>const</code>, and it applies to the
|
|
underlying value. In such cases, there's no consistency
|
|
to maintain. Putting the <code>const</code> first is
|
|
arguably more readable, since it follows English in
|
|
putting the "adjective" (<code>const</code>) before the
|
|
"noun" (<code>int</code>).</p>
|
|
|
|
<p>That said, while we encourage putting
|
|
<code>const</code> first, we do not require it. But be
|
|
consistent with the code around you!</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Use_of_constexpr">Use of constexpr</h3>
|
|
|
|
<div class="summary">
|
|
<p>In C++11, use <code>constexpr</code> to define true
|
|
constants or to ensure constant initialization.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p> Some variables can be declared <code>constexpr</code>
|
|
to indicate the variables are true constants, i.e. fixed at
|
|
compilation/link time. Some functions and constructors
|
|
can be declared <code>constexpr</code> which enables them
|
|
to be used in defining a <code>constexpr</code>
|
|
variable.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>Use of <code>constexpr</code> enables definition of
|
|
constants with floating-point expressions rather than
|
|
just literals; definition of constants of user-defined
|
|
types; and definition of constants with function
|
|
calls.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>Prematurely marking something as constexpr may cause
|
|
migration problems if later on it has to be downgraded.
|
|
Current restrictions on what is allowed in constexpr
|
|
functions and constructors may invite obscure workarounds
|
|
in these definitions.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p><code>constexpr</code> definitions enable a more
|
|
robust specification of the constant parts of an
|
|
interface. Use <code>constexpr</code> to specify true
|
|
constants and the functions that support their
|
|
definitions. Avoid complexifying function definitions to
|
|
enable their use with <code>constexpr</code>. Do not use
|
|
<code>constexpr</code> to force inlining.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Integer_Types">Integer Types</h3>
|
|
|
|
<div class="summary">
|
|
<p>Of the built-in C++ integer types, the only one used
|
|
is
|
|
<code>int</code>. If a program needs a variable of a
|
|
different size, use
|
|
a precise-width integer type from
|
|
<code><stdint.h></code>, such as
|
|
<code>int16_t</code>. If your variable represents a
|
|
value that could ever be greater than or equal to 2^31
|
|
(2GiB), use a 64-bit type such as
|
|
<code>int64_t</code>.
|
|
Keep in mind that even if your value won't ever be too large
|
|
for an <code>int</code>, it may be used in intermediate
|
|
calculations which may require a larger type. When in doubt,
|
|
choose a larger type.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p> C++ does not specify the sizes of its integer types.
|
|
Typically people assume that <code>short</code> is 16 bits,
|
|
<code>int</code> is 32 bits, <code>long</code> is 32 bits
|
|
and <code>long long</code> is 64 bits.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>Uniformity of declaration.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>The sizes of integral types in C++ can vary based on
|
|
compiler and architecture.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
|
|
<p>
|
|
<code><stdint.h></code> defines types
|
|
like <code>int16_t</code>, <code>uint32_t</code>,
|
|
<code>int64_t</code>, etc. You should always use
|
|
those in preference to <code>short</code>, <code>unsigned
|
|
long long</code> and the like, when you need a guarantee
|
|
on the size of an integer. Of the C integer types, only
|
|
<code>int</code> should be used. When appropriate, you
|
|
are welcome to use standard types like
|
|
<code>size_t</code> and <code>ptrdiff_t</code>.</p>
|
|
|
|
<p>We use <code>int</code> very often, for integers we
|
|
know are not going to be too big, e.g., loop counters.
|
|
Use plain old <code>int</code> for such things. You
|
|
should assume that an <code>int</code> is
|
|
|
|
at least 32 bits, but don't
|
|
assume that it has more than 32 bits. If you need a 64-bit
|
|
integer type, use
|
|
<code>int64_t</code>
|
|
or
|
|
<code>uint64_t</code>.</p>
|
|
|
|
<p>For integers we know can be "big",
|
|
use
|
|
<code>int64_t</code>.
|
|
</p>
|
|
|
|
<p>You should not use the unsigned integer types such as
|
|
<code>uint32_t</code>, unless there is a valid
|
|
reason such as representing a bit pattern rather than a
|
|
number, or you need defined overflow modulo 2^N. In
|
|
particular, do not use unsigned types to say a number
|
|
will never be negative. Instead, use
|
|
assertions for this.</p>
|
|
|
|
|
|
|
|
<p>If your code is a container that returns a size, be
|
|
sure to use a type that will accommodate any possible
|
|
usage of your container. When in doubt, use a larger type
|
|
rather than a smaller type.</p>
|
|
|
|
<p>Use care when converting integer types. Integer
|
|
conversions and promotions can cause non-intuitive
|
|
behavior. </p>
|
|
</div>
|
|
|
|
<div class="stylepoint_subsection">
|
|
|
|
<h4>On Unsigned Integers</h4>
|
|
|
|
<p>Some people, including some textbook authors,
|
|
recommend using unsigned types to represent numbers that
|
|
are never negative. This is intended as a form of
|
|
self-documentation. However, in C, the advantages of such
|
|
documentation are outweighed by the real bugs it can
|
|
introduce. Consider:</p>
|
|
|
|
<pre>for (unsigned int i = foo.Length()-1; i >= 0; --i) ...
|
|
</pre>
|
|
|
|
<p>This code will never terminate! Sometimes gcc will
|
|
notice this bug and warn you, but often it will not.
|
|
Equally bad bugs can occur when comparing signed and
|
|
unsigned variables. Basically, C's type-promotion scheme
|
|
causes unsigned types to behave differently than one
|
|
might expect.</p>
|
|
|
|
<p>So, document that a variable is non-negative using
|
|
assertions. Don't use an unsigned
|
|
type.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="64-bit_Portability">64-bit Portability</h3>
|
|
|
|
<div class="summary">
|
|
<p>Code should be 64-bit and 32-bit friendly. Bear in mind
|
|
problems of printing, comparisons, and structure alignment.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<ul>
|
|
<li>
|
|
<p><code>printf()</code> specifiers for some types
|
|
are not cleanly portable between 32-bit and 64-bit
|
|
systems. C99 defines some portable format specifiers.
|
|
Unfortunately, MSVC 7.1 does not understand some of
|
|
these specifiers and the standard is missing a few,
|
|
so we
|
|
have to define our own ugly versions in some cases
|
|
(in the style of the standard include file
|
|
<code>inttypes.h</code>):</p>
|
|
|
|
<div>
|
|
<pre>// printf macros for size_t, in the style of inttypes.h
|
|
#ifdef _LP64
|
|
#define __PRIS_PREFIX "z"
|
|
#else
|
|
#define __PRIS_PREFIX
|
|
#endif
|
|
|
|
// Use these macros after a % in a printf format string
|
|
// to get correct 32/64 bit behavior, like this:
|
|
// size_t size = records.size();
|
|
// printf("%" PRIuS "\n", size);
|
|
|
|
#define PRIdS __PRIS_PREFIX "d"
|
|
#define PRIxS __PRIS_PREFIX "x"
|
|
#define PRIuS __PRIS_PREFIX "u"
|
|
#define PRIXS __PRIS_PREFIX "X"
|
|
#define PRIoS __PRIS_PREFIX "o"
|
|
</pre>
|
|
</div>
|
|
|
|
<table border="1" summary="portable printf specifiers">
|
|
<tbody><tr align="center">
|
|
<th>Type</th>
|
|
<th>DO NOT use</th>
|
|
<th>DO use</th>
|
|
<th>Notes</th>
|
|
</tr>
|
|
|
|
<tr align="center">
|
|
<td><code>void *</code> (or any pointer)</td>
|
|
<td><code>%lx</code></td>
|
|
<td><code>%p</code></td>
|
|
<td></td>
|
|
</tr>
|
|
|
|
|
|
|
|
<tr align="center">
|
|
<td><code>int64_t</code></td>
|
|
<td><code>%qd</code>, <code>%lld</code></td>
|
|
<td><code>%" PRId64 "</code></td>
|
|
<td></td>
|
|
</tr>
|
|
|
|
|
|
|
|
<tr align="center">
|
|
<td><code>uint64_t</code></td>
|
|
<td><code>%qu</code>, <code>%llu</code>,
|
|
<code>%llx</code></td>
|
|
<td><code>%" PRIu64 "</code>,
|
|
<code>%" PRIx64 "</code></td>
|
|
<td></td>
|
|
</tr>
|
|
|
|
|
|
|
|
<tr align="center">
|
|
<td><code>size_t</code></td>
|
|
<td><code>%u</code></td>
|
|
<td><code>%" PRIuS "</code>, <code>%" PRIxS "</code></td>
|
|
<td>
|
|
C99 specifies <code>%zu</code></td>
|
|
</tr>
|
|
|
|
<tr align="center">
|
|
<td><code>ptrdiff_t</code></td>
|
|
<td><code>%d</code></td>
|
|
<td><code>%" PRIdS "</code></td>
|
|
<td>
|
|
C99 specifies <code>%td</code></td>
|
|
</tr>
|
|
|
|
|
|
</tbody></table>
|
|
|
|
<p>Note that the <code>PRI*</code> macros expand to
|
|
independent strings which are concatenated by the
|
|
compiler. Hence if you are using a non-constant
|
|
format string, you need to insert the value of the
|
|
macro into the format, rather than the name. Note also
|
|
that spaces are required around the macro identifier to
|
|
separate it from the string literal. It is
|
|
still possible, as usual, to include length
|
|
specifiers, etc., after the <code>%</code> when using
|
|
the <code>PRI*</code> macros. So, e.g.
|
|
<code>printf("x = %30" PRIuS "\n", x)</code> would
|
|
expand on 32-bit Linux to <code>printf("x = %30" "u"
|
|
"\n", x)</code>, which the compiler will treat as
|
|
<code>printf("x = %30u\n", x)</code>.</p>
|
|
|
|
|
|
</li>
|
|
|
|
<li>Remember that <code>sizeof(void *)</code> !=
|
|
<code>sizeof(int)</code>. Use <code>intptr_t</code> if
|
|
you want a pointer-sized integer.</li>
|
|
|
|
<li>You may need to be careful with structure
|
|
alignments, particularly for structures being stored on
|
|
disk. Any class/structure with a
|
|
<code>int64_t</code>/<code>uint64_t</code>
|
|
member will by default end up being 8-byte aligned on a
|
|
64-bit system. If you have such structures being shared
|
|
on disk between 32-bit and 64-bit code, you will need
|
|
to ensure that they are packed the same on both
|
|
architectures.
|
|
Most compilers offer a way to
|
|
alter structure alignment. For gcc, you can use
|
|
<code>__attribute__((packed))</code>. MSVC offers
|
|
<code>#pragma pack()</code> and
|
|
<code>__declspec(align())</code>.</li>
|
|
|
|
<li>
|
|
<p>Use the <code>LL</code> or <code>ULL</code>
|
|
suffixes as needed to create 64-bit constants. For
|
|
example:</p>
|
|
|
|
|
|
<pre>int64_t my_value = 0x123456789LL;
|
|
uint64_t my_mask = 3ULL << 48;
|
|
</pre>
|
|
</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<h3 id="Preprocessor_Macros">Preprocessor Macros</h3>
|
|
|
|
<div class="summary">
|
|
<p>Avoid defining macros, especially in headers; prefer
|
|
inline functions, enums, and <code>const</code> variables.
|
|
Name macros with a project-specific prefix. Do not use
|
|
macros to define pieces of a C++ API.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Macros mean that the code you see is not the same as
|
|
the code the compiler sees. This can introduce unexpected
|
|
behavior, especially since macros have global scope.</p>
|
|
|
|
<p>The problems introduced by macros are especially severe
|
|
when they are used to define pieces of a C++ API,
|
|
and still more so for public APIs. Every error message from
|
|
the compiler when developers incorrectly use that interface
|
|
now must explain how the macros formed the interface.
|
|
Refactoring and analysis tools have a dramatically harder
|
|
time updating the interface. As a consequence, we
|
|
specifically disallow using macros in this way.
|
|
For example, avoid patterns like:</p>
|
|
|
|
<pre class="badcode">class WOMBAT_TYPE(Foo) {
|
|
// ...
|
|
|
|
public:
|
|
EXPAND_PUBLIC_WOMBAT_API(Foo)
|
|
|
|
EXPAND_WOMBAT_COMPARISONS(Foo, ==, <)
|
|
};
|
|
</pre>
|
|
|
|
<p>Luckily, macros are not nearly as necessary in C++ as
|
|
they are in C. Instead of using a macro to inline
|
|
performance-critical code, use an inline function.
|
|
Instead of using a macro to store a constant, use a
|
|
<code>const</code> variable. Instead of using a macro to
|
|
"abbreviate" a long variable name, use a reference.
|
|
Instead of using a macro to conditionally compile code
|
|
... well, don't do that at all (except, of course, for
|
|
the <code>#define</code> guards to prevent double
|
|
inclusion of header files). It makes testing much more
|
|
difficult.</p>
|
|
|
|
<p>Macros can do things these other techniques cannot,
|
|
and you do see them in the codebase, especially in the
|
|
lower-level libraries. And some of their special features
|
|
(like stringifying, concatenation, and so forth) are not
|
|
available through the language proper. But before using a
|
|
macro, consider carefully whether there's a non-macro way
|
|
to achieve the same result. If you need to use a macro to
|
|
define an interface, contact
|
|
your project leads to request
|
|
a waiver of this rule.</p>
|
|
|
|
<p>The following usage pattern will avoid many problems
|
|
with macros; if you use macros, follow it whenever
|
|
possible:</p>
|
|
|
|
<ul>
|
|
<li>Don't define macros in a <code>.h</code> file.</li>
|
|
|
|
<li><code>#define</code> macros right before you use
|
|
them, and <code>#undef</code> them right after.</li>
|
|
|
|
<li>Do not just <code>#undef</code> an existing macro
|
|
before replacing it with your own; instead, pick a name
|
|
that's likely to be unique.</li>
|
|
|
|
<li>Try not to use macros that expand to unbalanced C++
|
|
constructs, or at least document that behavior
|
|
well.</li>
|
|
|
|
<li>Prefer not using <code>##</code> to generate
|
|
function/class/variable names.</li>
|
|
</ul>
|
|
|
|
<p>Exporting macros from headers (i.e. defining them in a header
|
|
without <code>#undef</code>ing them before the end of the header)
|
|
is extremely strongly discouraged. If you do export a macro from a
|
|
header, it must have a globally unique name. To achieve this, it
|
|
must be named with a prefix consisting of your project's namespace
|
|
name (but upper case). </p>
|
|
|
|
</div>
|
|
|
|
<h3 id="0_and_nullptr/NULL">0 and nullptr/NULL</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use <code>0</code> for integers, <code>0.0</code> for
|
|
reals, <code>nullptr</code> (or <code>NULL</code>) for
|
|
pointers, and <code>'\0'</code> for chars.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Use <code>0</code> for integers and <code>0.0</code>
|
|
for reals. This is not controversial.</p>
|
|
|
|
<p> For
|
|
pointers (address values), there is a choice between
|
|
<code>0</code>, <code>NULL</code>, and
|
|
<code>nullptr</code>. For projects that allow C++11
|
|
features, use <code>nullptr</code>. For C++03 projects,
|
|
we prefer <code>NULL</code> because it looks like a
|
|
pointer. In fact, some C++ compilers provide special
|
|
definitions of <code>NULL</code> which enable them to
|
|
give useful warnings, particularly in situations where
|
|
<code>sizeof(NULL)</code> is not equal to
|
|
<code>sizeof(0)</code>.</p>
|
|
|
|
<p>Use <code>'\0'</code> for chars. This is the correct
|
|
type and also makes code more readable.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="sizeof">sizeof</h3>
|
|
|
|
<div class="summary">
|
|
<p>Prefer <code>sizeof(<var>varname</var>)</code> to
|
|
<code>sizeof(<var>type</var>)</code>.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Use <code>sizeof(<var>varname</var>)</code> when you
|
|
take the size of a particular variable.
|
|
<code>sizeof(<var>varname</var>)</code> will update
|
|
appropriately if someone changes the variable type either
|
|
now or later. You may use
|
|
<code>sizeof(<var>type</var>)</code> for code unrelated
|
|
to any particular variable, such as code that manages an
|
|
external or internal data format where a variable of an
|
|
appropriate C++ type is not convenient.</p>
|
|
|
|
<pre>Struct data;
|
|
memset(&data, 0, sizeof(data));
|
|
</pre>
|
|
|
|
<pre class="badcode">memset(&data, 0, sizeof(Struct));
|
|
</pre>
|
|
|
|
<pre>if (raw_size < sizeof(int)) {
|
|
LOG(ERROR) << "compressed record not big enough for count: " << raw_size;
|
|
return false;
|
|
}
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="auto">auto</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use <code>auto</code> to avoid type names that are noisy, obvious,
|
|
or unimportant - cases where the type doesn't aid in clarity for the
|
|
reader. Continue to use manifest type declarations when it helps
|
|
readability.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="pros">
|
|
<p>
|
|
</p><ul>
|
|
<li>C++ type names can be long and cumbersome, especially when they
|
|
involve templates or namespaces.</li>
|
|
<li>When a C++ type name is repeated within a single declaration or a
|
|
small code region, the repetition may not be aiding readability.</li>
|
|
<li>It is sometimes safer to let the type be specified by the type of
|
|
the initialization expression, since that avoids the possibility of
|
|
unintended copies or type conversions.</li>
|
|
</ul>
|
|
</div>
|
|
<div class="cons">
|
|
|
|
<p>Sometimes code is clearer when types are manifest,
|
|
especially when a variable's initialization depends on
|
|
things that were declared far away. In expressions
|
|
like:</p>
|
|
|
|
<pre class="badcode">auto foo = x.add_foo();
|
|
auto i = y.Find(key);
|
|
</pre>
|
|
|
|
<p>it may not be obvious what the resulting types are if the type
|
|
of <code>y</code> isn't very well known, or if <code>y</code> was
|
|
declared many lines earlier.</p>
|
|
|
|
<p>Programmers have to understand the difference between
|
|
<code>auto</code> and <code>const auto&</code> or
|
|
they'll get copies when they didn't mean to.</p>
|
|
|
|
<p>If an <code>auto</code> variable is used as part of an
|
|
interface, e.g. as a constant in a header, then a
|
|
programmer might change its type while only intending to
|
|
change its value, leading to a more radical API change
|
|
than intended.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
|
|
<p><code>auto</code> is permitted when it increases readability,
|
|
particularly as described below. Never initialize an <code>auto</code>-typed
|
|
variable with a braced initializer list.</p>
|
|
|
|
<p>Specific cases where <code>auto</code> is allowed or encouraged:
|
|
</p><ul>
|
|
<li>(Encouraged) For iterators and other long/cluttery type names, particularly
|
|
when the type is clear from context (calls
|
|
to <code>find</code>, <code>begin</code>, or <code>end</code> for
|
|
instance).</li>
|
|
<li>(Allowed) When the type is clear from local context (in the same expression
|
|
or within a few lines). Initialization of a pointer or smart pointer
|
|
with calls
|
|
to <code>new</code>
|
|
commonly falls into this category, as does use of <code>auto</code> in
|
|
a range-based loop over a container whose type is spelled out
|
|
nearby.</li>
|
|
<li>(Allowed) When the type doesn't matter because it isn't being used for
|
|
anything other than equality comparison.</li>
|
|
<li>(Encouraged) When iterating over a map with a range-based loop
|
|
(because it is often assumed that the correct type
|
|
is <code>std::pair<KeyType, ValueType></code> whereas it is actually
|
|
<code>std::pair<const KeyType, ValueType></code>). This is
|
|
particularly well paired with local <code>key</code>
|
|
and <code>value</code> aliases for <code>.first</code>
|
|
and <code>.second</code> (often const-ref).
|
|
<pre class="code">for (const auto& item : some_map) {
|
|
const KeyType& key = item.first;
|
|
const ValType& value = item.second;
|
|
// The rest of the loop can now just refer to key and value,
|
|
// a reader can see the types in question, and we've avoided
|
|
// the too-common case of extra copies in this iteration.
|
|
}
|
|
</pre>
|
|
</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Braced_Initializer_List">Braced Initializer List</h3>
|
|
|
|
<div class="summary">
|
|
<p>You may use braced initializer lists.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>In C++03, aggregate types (arrays and structs with no
|
|
constructor) could be initialized with braced initializer lists.</p>
|
|
|
|
<pre>struct Point { int x; int y; };
|
|
Point p = {1, 2};
|
|
</pre>
|
|
|
|
<p>In C++11, this syntax was generalized, and any object type can now
|
|
be created with a braced initializer list, known as a
|
|
<i>braced-init-list</i> in the C++ grammar. Here are a few examples
|
|
of its use.</p>
|
|
|
|
<pre>// Vector takes a braced-init-list of elements.
|
|
std::vector<string> v{"foo", "bar"};
|
|
|
|
// Basically the same, ignoring some small technicalities.
|
|
// You may choose to use either form.
|
|
std::vector<string> v = {"foo", "bar"};
|
|
|
|
// Usable with 'new' expressions.
|
|
auto p = new vector<string>{"foo", "bar"};
|
|
|
|
// A map can take a list of pairs. Nested braced-init-lists work.
|
|
std::map<int, string> m = {{1, "one"}, {2, "2"}};
|
|
|
|
// A braced-init-list can be implicitly converted to a return type.
|
|
std::vector<int> test_function() { return {1, 2, 3}; }
|
|
|
|
// Iterate over a braced-init-list.
|
|
for (int i : {-1, -2, -3}) {}
|
|
|
|
// Call a function using a braced-init-list.
|
|
void TestFunction2(std::vector<int> v) {}
|
|
TestFunction2({1, 2, 3});
|
|
</pre>
|
|
|
|
<p>A user-defined type can also define a constructor and/or assignment operator
|
|
that take <code>std::initializer_list<T></code>, which is automatically
|
|
created from <i>braced-init-list</i>:</p>
|
|
|
|
<pre>class MyType {
|
|
public:
|
|
// std::initializer_list references the underlying init list.
|
|
// It should be passed by value.
|
|
MyType(std::initializer_list<int> init_list) {
|
|
for (int i : init_list) append(i);
|
|
}
|
|
MyType& operator=(std::initializer_list<int> init_list) {
|
|
clear();
|
|
for (int i : init_list) append(i);
|
|
}
|
|
};
|
|
MyType m{2, 3, 5, 7};
|
|
</pre>
|
|
|
|
<p>Finally, brace initialization can also call ordinary
|
|
constructors of data types, even if they do not have
|
|
<code>std::initializer_list<T></code> constructors.</p>
|
|
|
|
<pre>double d{1.23};
|
|
// Calls ordinary constructor as long as MyOtherType has no
|
|
// std::initializer_list constructor.
|
|
class MyOtherType {
|
|
public:
|
|
explicit MyOtherType(string);
|
|
MyOtherType(int, string);
|
|
};
|
|
MyOtherType m = {1, "b"};
|
|
// If the constructor is explicit, you can't use the "= {}" form.
|
|
MyOtherType m{"b"};
|
|
</pre>
|
|
|
|
<p>Never assign a <i>braced-init-list</i> to an auto
|
|
local variable. In the single element case, what this
|
|
means can be confusing.</p>
|
|
|
|
<pre class="badcode">auto d = {1.23}; // d is a std::initializer_list<double>
|
|
</pre>
|
|
|
|
<pre>auto d = double{1.23}; // Good -- d is a double, not a std::initializer_list.
|
|
</pre>
|
|
|
|
<p>See <a href="#Braced_Initializer_List_Format">Braced_Initializer_List_Format</a> for formatting.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Lambda_expressions">Lambda expressions</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use lambda expressions where appropriate. Prefer explicit captures
|
|
when the lambda will escape the current scope.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
|
|
<p> Lambda expressions are a concise way of creating anonymous
|
|
function objects. They're often useful when passing
|
|
functions as arguments. For example:</p>
|
|
|
|
<pre>std::sort(v.begin(), v.end(), [](int x, int y) {
|
|
return Weight(x) < Weight(y);
|
|
});
|
|
</pre>
|
|
|
|
<p> They further allow capturing variables from the enclosing scope either
|
|
explicitly by name, or implicitly using a default capture. Explicit captures
|
|
require each variable to be listed, as
|
|
either a value or reference capture:</p>
|
|
|
|
<pre>int weight = 3;
|
|
int sum = 0;
|
|
// Captures `weight` by value and `sum` by reference.
|
|
std::for_each(v.begin(), v.end(), [weight, &sum](int x) {
|
|
sum += weight * x;
|
|
});
|
|
</pre>
|
|
|
|
|
|
Default captures implicitly capture any variable referenced in the
|
|
lambda body, including <code>this</code> if any members are used:
|
|
|
|
<pre>const std::vector<int> lookup_table = ...;
|
|
std::vector<int> indices = ...;
|
|
// Captures `lookup_table` by reference, sorts `indices` by the value
|
|
// of the associated element in `lookup_table`.
|
|
std::sort(indices.begin(), indices.end(), [&](int a, int b) {
|
|
return lookup_table[a] < lookup_table[b];
|
|
});
|
|
</pre>
|
|
|
|
<p>Lambdas were introduced in C++11 along with a set of utilities
|
|
for working with function objects, such as the polymorphic
|
|
wrapper <code>std::function</code>.
|
|
</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<ul>
|
|
<li>Lambdas are much more concise than other ways of
|
|
defining function objects to be passed to STL
|
|
algorithms, which can be a readability
|
|
improvement.</li>
|
|
|
|
<li>Appropriate use of default captures can remove
|
|
redundancy and highlight important exceptions from
|
|
the default.</li>
|
|
|
|
<li>Lambdas, <code>std::function</code>, and
|
|
<code>std::bind</code> can be used in combination as a
|
|
general purpose callback mechanism; they make it easy
|
|
to write functions that take bound functions as
|
|
arguments.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<ul>
|
|
<li>Variable capture in lambdas can be a source of dangling-pointer
|
|
bugs, particularly if a lambda escapes the current scope.</li>
|
|
|
|
<li>Default captures by value can be misleading because they do not prevent
|
|
dangling-pointer bugs. Capturing a pointer by value doesn't cause a deep
|
|
copy, so it often has the same lifetime issues as capture by reference.
|
|
This is especially confusing when capturing 'this' by value, since the use
|
|
of 'this' is often implicit.</li>
|
|
|
|
<li>It's possible for use of lambdas to get out of
|
|
hand; very long nested anonymous functions can make
|
|
code harder to understand.</li>
|
|
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<ul>
|
|
<li>Use lambda expressions where appropriate, with formatting as
|
|
described <a href="#Formatting_Lambda_Expressions">below</a>.</li>
|
|
<li>Prefer explicit captures if the lambda may escape the current scope.
|
|
For example, instead of:
|
|
<pre class="badcode">{
|
|
Foo foo;
|
|
...
|
|
executor->Schedule([&] { Frobnicate(foo); })
|
|
...
|
|
}
|
|
// BAD! The fact that the lambda makes use of a reference to `foo` and
|
|
// possibly `this` (if `Frobnicate` is a member function) may not be
|
|
// apparent on a cursory inspection. If the lambda is invoked after
|
|
// the function returns, that would be bad, because both `foo`
|
|
// and the enclosing object could have been destroyed.
|
|
</pre>
|
|
prefer to write:
|
|
<pre>{
|
|
Foo foo;
|
|
...
|
|
executor->Schedule([&foo] { Frobnicate(foo); })
|
|
...
|
|
}
|
|
// BETTER - The compile will fail if `Frobnicate` is a member
|
|
// function, and it's clearer that `foo` is dangerously captured by
|
|
// reference.
|
|
</pre>
|
|
</li>
|
|
<li>Use default capture by reference ([&]) only when the
|
|
lifetime of the lambda is obviously shorter than any potential
|
|
captures.
|
|
</li>
|
|
<li>Use default capture by value ([=]) only as a means of binding a
|
|
few variables for a short lambda, where the set of captured
|
|
variables is obvious at a glance. Prefer not to write long or
|
|
complex lambdas with default capture by value.
|
|
</li>
|
|
<li>Keep unnamed lambdas short. If a lambda body is more than
|
|
maybe five lines long, prefer to give the lambda a name, or to
|
|
use a named function instead of a lambda.</li>
|
|
<li>Specify the return type of the lambda explicitly if that will
|
|
make it more obvious to readers, as with
|
|
<a href="#auto"><code>auto</code></a>.</li>
|
|
|
|
</ul>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Template_metaprogramming">Template metaprogramming</h3>
|
|
<div class="summary">
|
|
<p>Avoid complicated template programming.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p>Template metaprogramming refers to a family of techniques that
|
|
exploit the fact that the C++ template instantiation mechanism is
|
|
Turing complete and can be used to perform arbitrary compile-time
|
|
computation in the type domain.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>Template metaprogramming allows extremely flexible interfaces that
|
|
are type safe and high performance. Facilities like
|
|
|
|
<a href="https://code.google.com/p/googletest/">Google Test</a>,
|
|
<code>std::tuple</code>, <code>std::function</code>, and
|
|
Boost.Spirit would be impossible without it.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>The techniques used in template metaprogramming are often obscure
|
|
to anyone but language experts. Code that uses templates in
|
|
complicated ways is often unreadable, and is hard to debug or
|
|
maintain.</p>
|
|
|
|
<p>Template metaprogramming often leads to extremely poor compiler
|
|
time error messages: even if an interface is simple, the complicated
|
|
implementation details become visible when the user does something
|
|
wrong.</p>
|
|
|
|
<p>Template metaprogramming interferes with large scale refactoring by
|
|
making the job of refactoring tools harder. First, the template code
|
|
is expanded in multiple contexts, and it's hard to verify that the
|
|
transformation makes sense in all of them. Second, some refactoring
|
|
tools work with an AST that only represents the structure of the code
|
|
after template expansion. It can be difficult to automatically work
|
|
back to the original source construct that needs to be
|
|
rewritten.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>Template metaprogramming sometimes allows cleaner and easier-to-use
|
|
interfaces than would be possible without it, but it's also often a
|
|
temptation to be overly clever. It's best used in a small number of
|
|
low level components where the extra maintenance burden is spread out
|
|
over a large number of uses.</p>
|
|
|
|
<p>Think twice before using template metaprogramming or other
|
|
complicated template techniques; think about whether the average
|
|
member of your team will be able to understand your code well enough
|
|
to maintain it after you switch to another project, or whether a
|
|
non-C++ programmer or someone casually browsing the code base will be
|
|
able to understand the error messages or trace the flow of a function
|
|
they want to call. If you're using recursive template instantiations
|
|
or type lists or metafunctions or expression templates, or relying on
|
|
SFINAE or on the <code>sizeof</code> trick for detecting function
|
|
overload resolution, then there's a good chance you've gone too
|
|
far.</p>
|
|
|
|
<p>If you use template metaprogramming, you should expect to put
|
|
considerable effort into minimizing and isolating the complexity. You
|
|
should hide metaprogramming as an implementation detail whenever
|
|
possible, so that user-facing headers are readable, and you should
|
|
make sure that tricky code is especially well commented. You should
|
|
carefully document how the code is used, and you should say something
|
|
about what the "generated" code looks like. Pay extra attention to the
|
|
error messages that the compiler emits when users make mistakes. The
|
|
error messages are part of your user interface, and your code should
|
|
be tweaked as necessary so that the error messages are understandable
|
|
and actionable from a user point of view.</p>
|
|
|
|
</div>
|
|
</div>
|
|
|
|
|
|
<h3 id="Boost">Boost</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use only approved libraries from the Boost library
|
|
collection.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p> The
|
|
<a href="https://www.boost.org/">
|
|
Boost library collection</a> is a popular collection of
|
|
peer-reviewed, free, open-source C++ libraries.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>Boost code is generally very high-quality, is widely
|
|
portable, and fills many important gaps in the C++
|
|
standard library, such as type traits and better binders.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>Some Boost libraries encourage coding practices which can
|
|
hamper readability, such as metaprogramming and other
|
|
advanced template techniques, and an excessively
|
|
"functional" style of programming. </p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
|
|
|
|
|
|
<div>
|
|
<p>In order to maintain a high level of readability for
|
|
all contributors who might read and maintain code, we
|
|
only allow an approved subset of Boost features.
|
|
Currently, the following libraries are permitted:</p>
|
|
|
|
<ul>
|
|
<li>
|
|
<a href="https://www.boost.org/libs/utility/call_traits.htm">
|
|
Call Traits</a> from <code>boost/call_traits.hpp</code></li>
|
|
|
|
<li><a href="https://www.boost.org/libs/utility/compressed_pair.htm">
|
|
Compressed Pair</a> from <code>boost/compressed_pair.hpp</code></li>
|
|
|
|
<li><a href="https://www.boost.org/libs/graph/">
|
|
The Boost Graph Library (BGL)</a> from <code>boost/graph</code>,
|
|
except serialization (<code>adj_list_serialize.hpp</code>) and
|
|
parallel/distributed algorithms and data structures
|
|
(<code>boost/graph/parallel/*</code> and
|
|
<code>boost/graph/distributed/*</code>).</li>
|
|
|
|
<li><a href="https://www.boost.org/libs/property_map/">
|
|
Property Map</a> from <code>boost/property_map</code>, except
|
|
parallel/distributed property maps (<code>boost/property_map/parallel/*</code>).</li>
|
|
|
|
<li><a href="https://www.boost.org/libs/iterator/">
|
|
Iterator</a> from <code>boost/iterator</code></li>
|
|
|
|
<li>The part of <a href="https://www.boost.org/libs/polygon/">
|
|
Polygon</a> that deals with Voronoi diagram
|
|
construction and doesn't depend on the rest of
|
|
Polygon:
|
|
<code>boost/polygon/voronoi_builder.hpp</code>,
|
|
<code>boost/polygon/voronoi_diagram.hpp</code>, and
|
|
<code>boost/polygon/voronoi_geometry_type.hpp</code></li>
|
|
|
|
<li><a href="https://www.boost.org/libs/bimap/">
|
|
Bimap</a> from <code>boost/bimap</code></li>
|
|
|
|
<li><a href="https://www.boost.org/libs/math/doc/html/dist.html">
|
|
Statistical Distributions and Functions</a> from
|
|
<code>boost/math/distributions</code></li>
|
|
|
|
<li><a href="https://www.boost.org/libs/math/doc/html/special.html">
|
|
Special Functions</a> from <code>boost/math/special_functions</code></li>
|
|
|
|
<li><a href="https://www.boost.org/libs/multi_index/">
|
|
Multi-index</a> from <code>boost/multi_index</code></li>
|
|
|
|
<li><a href="https://www.boost.org/libs/heap/">
|
|
Heap</a> from <code>boost/heap</code></li>
|
|
|
|
<li>The flat containers from
|
|
<a href="https://www.boost.org/libs/container/">Container</a>:
|
|
<code>boost/container/flat_map</code>, and
|
|
<code>boost/container/flat_set</code></li>
|
|
|
|
<li><a href="https://www.boost.org/libs/intrusive/">Intrusive</a>
|
|
from <code>boost/intrusive</code>.</li>
|
|
|
|
<li><a href="https://www.boost.org/libs/sort/">The
|
|
<code>boost/sort</code> library</a>.</li>
|
|
|
|
<li><a href="https://www.boost.org/libs/preprocessor/">Preprocessor</a>
|
|
from <code>boost/preprocessor</code>.</li>
|
|
</ul>
|
|
|
|
<p>We are actively considering adding other Boost
|
|
features to the list, so this list may be expanded in
|
|
the future.</p>
|
|
</div>
|
|
|
|
<p>The following libraries are permitted, but their use
|
|
is discouraged because they've been superseded by
|
|
standard libraries in C++11:</p>
|
|
|
|
<ul>
|
|
<li><a href="https://www.boost.org/libs/array/">
|
|
Array</a> from <code>boost/array.hpp</code>: use
|
|
<a href="http://en.cppreference.com/w/cpp/container/array">
|
|
<code>std::array</code></a> instead.</li>
|
|
|
|
<li><a href="https://www.boost.org/libs/ptr_container/">
|
|
Pointer Container</a> from <code>boost/ptr_container</code>: use containers of
|
|
<a href="http://en.cppreference.com/w/cpp/memory/unique_ptr">
|
|
<code>std::unique_ptr</code></a> instead.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
|
|
|
|
<h3 id="std_hash">std::hash</h3>
|
|
|
|
<div class="summary">
|
|
<p>Do not define specializations of <code>std::hash</code>.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p><code>std::hash<T></code> is the function object that the
|
|
C++11 hash containers use to hash keys of type <code>T</code>,
|
|
unless the user explicitly specifies a different hash function. For
|
|
example, <code>std::unordered_map<int, string></code> is a hash
|
|
map that uses <code>std::hash<int></code> to hash its keys,
|
|
whereas <code>std::unordered_map<int, string, MyIntHash></code>
|
|
uses <code>MyIntHash</code>.</p>
|
|
|
|
<p><code>std::hash</code> is defined for all integral, floating-point,
|
|
pointer, and <code>enum</code> types, as well as some standard library
|
|
types such as <code>string</code> and <code>unique_ptr</code>. Users
|
|
can enable it to work for their own types by defining specializations
|
|
of it for those types.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p><code>std::hash</code> is easy to use, and simplifies the code
|
|
since you don't have to name it explicitly. Specializing
|
|
<code>std::hash</code> is the standard way of specifying how to
|
|
hash a type, so it's what outside resources will teach, and what
|
|
new engineers will expect.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p><code>std::hash</code> is hard to specialize. It requires a lot
|
|
of boilerplate code, and more importantly, it combines responsibility
|
|
for identifying the hash inputs with responsibility for executing the
|
|
hashing algorithm itself. The type author has to be responsible for
|
|
the former, but the latter requires expertise that a type author
|
|
usually doesn't have, and shouldn't need. The stakes here are high
|
|
because low-quality hash functions can be security vulnerabilities,
|
|
due to the emergence of
|
|
<a href="https://emboss.github.io/blog/2012/12/14/breaking-murmur-hash-flooding-dos-reloaded/">
|
|
hash flooding attacks</a>.</p>
|
|
|
|
<p>Even for experts, <code>std::hash</code> specializations are
|
|
inordinately difficult to implement correctly for compound types,
|
|
because the implementation cannot recursively call <code>std::hash</code>
|
|
on data members. High-quality hash algorithms maintain large
|
|
amounts of internal state, and reducing that state to the
|
|
<code>size_t</code> bytes that <code>std::hash</code>
|
|
returns is usually the slowest part of the computation, so it
|
|
should not be done more than once.</p>
|
|
|
|
<p>Due to exactly that issue, <code>std::hash</code> does not work
|
|
with <code>std::pair</code> or <code>std::tuple</code>, and the
|
|
language does not allow us to extend it to support them.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>You can use <code>std::hash</code> with the types that it supports
|
|
"out of the box", but do not specialize it to support additional types.
|
|
If you need a hash table with a key type that <code>std::hash</code>
|
|
does not support, consider using legacy hash containers (e.g.
|
|
<code>hash_map</code>) for now; they use a different default hasher,
|
|
which is unaffected by this prohibition.</p>
|
|
|
|
<p>If you want to use the standard hash containers anyway, you will
|
|
need to specify a custom hasher for the key type, e.g.</p>
|
|
<pre>std::unordered_map<MyKeyType, Value, MyKeyTypeHasher> my_map;
|
|
</pre><p>
|
|
Consult with the type's owners to see if there is an existing hasher
|
|
that you can use; otherwise work with them to provide one,
|
|
or roll your own.</p>
|
|
|
|
<p>We are planning to provide a hash function that can work with any type,
|
|
using a new customization mechanism that doesn't have the drawbacks of
|
|
<code>std::hash</code>.</p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="C++11">C++11</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use libraries and language extensions from C++11 when appropriate.
|
|
Consider portability to other environments
|
|
before using C++11 features in your
|
|
project. </p>
|
|
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<div class="definition">
|
|
<p> C++11 contains <a href="https://en.wikipedia.org/wiki/C%2B%2B11">
|
|
significant changes</a> both to the language and
|
|
libraries. </p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<p>C++11 was the official standard until august 2014, and
|
|
is supported by most C++ compilers. It standardizes
|
|
some common C++ extensions that we use already, allows
|
|
shorthands for some operations, and has some performance
|
|
and safety improvements.</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>The C++11 standard is substantially more complex than
|
|
its predecessor (1,300 pages versus 800 pages), and is
|
|
unfamiliar to many developers. The long-term effects of
|
|
some features on code readability and maintenance are
|
|
unknown. We cannot predict when its various features will
|
|
be implemented uniformly by tools that may be of
|
|
interest, particularly in the case of projects that are
|
|
forced to use older versions of tools.</p>
|
|
|
|
<p>As with <a href="#Boost">Boost</a>, some C++11
|
|
extensions encourage coding practices that hamper
|
|
readability—for example by removing
|
|
checked redundancy (such as type names) that may be
|
|
helpful to readers, or by encouraging template
|
|
metaprogramming. Other extensions duplicate functionality
|
|
available through existing mechanisms, which may lead to confusion
|
|
and conversion costs.</p>
|
|
|
|
|
|
</div>
|
|
|
|
<div class="decision">
|
|
|
|
<p>C++11 features may be used unless specified otherwise.
|
|
In addition to what's described in the rest of the style
|
|
guide, the following C++11 features may not be used:</p>
|
|
|
|
<ul>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<li>Compile-time rational numbers
|
|
(<code><ratio></code>), because of concerns that
|
|
it's tied to a more template-heavy interface
|
|
style.</li>
|
|
|
|
<li>The <code><cfenv></code> and
|
|
<code><fenv.h></code> headers, because many
|
|
compilers do not support those features reliably.</li>
|
|
|
|
<li>Ref-qualifiers on member functions, such as <code>void X::Foo()
|
|
&</code> or <code>void X::Foo() &&</code>, because of concerns
|
|
that they're an overly obscure feature.</li>
|
|
|
|
|
|
|
|
|
|
</ul>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Nonstandard_Extensions">Nonstandard Extensions</h3>
|
|
|
|
<div class="summary">
|
|
<p>Nonstandard extensions to C++ may not be used unless otherwise specified.</p>
|
|
</div>
|
|
<div class="stylebody">
|
|
<div class="definition">
|
|
<p>Compilers support various extensions that are not part of standard C++. Such
|
|
extensions include GCC's <code>__attribute__</code>, intrinsic functions such
|
|
as <code>__builtin_prefetch</code>, designated initializers (e.g.
|
|
<code>Foo f = {.field = 3}</code>), inline assembly, <code>__COUNTER__</code>,
|
|
<code>__PRETTY_FUNCTION__</code>, compound statement expressions (e.g.
|
|
<code>foo = ({ int x; Bar(&x); x })</code>, variable-length arrays and
|
|
<code>alloca()</code>, and the <code>a?:b</code> syntax.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<ul>
|
|
<li>Nonstandard extensions may provide useful features that do not exist
|
|
in standard C++. For example, some people think that designated
|
|
initializers are more readable than standard C++ features like
|
|
constructors.</li>
|
|
<li>Important performance guidance to the compiler can only be specified
|
|
using extensions.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<ul>
|
|
<li>Nonstandard extensions do not work in all compilers. Use of nonstandard
|
|
extensions reduces portability of code.</li>
|
|
<li>Even if they are supported in all targeted compilers, the extensions
|
|
are often not well-specified, and there may be subtle behavior differences
|
|
between compilers.</li>
|
|
<li>Nonstandard extensions add to the language features that a reader must
|
|
know to understand the code.</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>Do not use nonstandard extensions. You may use portability wrappers that
|
|
are implemented using nonstandard extensions, so long as those wrappers
|
|
|
|
are provided by a designated project-wide
|
|
portability header.</p>
|
|
</div>
|
|
</div>
|
|
|
|
<h3 id="Aliases">Aliases</h3>
|
|
|
|
<div class="summary">
|
|
<p>Public aliases are for the benefit of an API's user, and should be clearly documented.</p>
|
|
</div>
|
|
<div class="stylebody">
|
|
<div class="definition">
|
|
<p>There are several ways to create names that are aliases of other entities:</p>
|
|
<pre>typedef Foo Bar;
|
|
using Bar = Foo;
|
|
using other_namespace::Foo;
|
|
</pre>
|
|
|
|
<p>Like other declarations, aliases declared in a header file are part of that
|
|
header's public API unless they're in a function definition, in the private portion of a class,
|
|
or in an explicitly-marked internal namespace. Aliases in such areas or in .cc files are
|
|
implementation details (because client code can't refer to them), and are not restricted by this
|
|
rule.</p>
|
|
</div>
|
|
|
|
<div class="pros">
|
|
<ul>
|
|
<li>Aliases can improve readability by simplifying a long or complicated name.</li>
|
|
<li>Aliases can reduce duplication by naming in one place a type used repeatedly in an API,
|
|
which <em>might</em> make it easier to change the type later.
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<ul>
|
|
<li>When placed in a header where client code can refer to them, aliases increase the
|
|
number of entities in that header's API, increasing its complexity.</li>
|
|
<li>Clients can easily rely on unintended details of public aliases, making
|
|
changes difficult.</li>
|
|
<li>It can be tempting to create a public alias that is only intended for use
|
|
in the implementation, without considering its impact on the API, or on maintainability.</li>
|
|
<li>Aliases can create risk of name collisions</li>
|
|
<li>Aliases can reduce readability by giving a familiar construct an unfamiliar name</li>
|
|
<li>Type aliases can create an unclear API contract:
|
|
it is unclear whether the alias is guaranteed to be identical to the type it aliases,
|
|
to have the same API, or only to be usable in specified narrow ways</li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p>Don't put an alias in your public API just to save typing in the implementation;
|
|
do so only if you intend it to be used by your clients.</p>
|
|
<p>When defining a public alias, document the intent of
|
|
the new name, including whether it is guaranteed to always be the same as the type
|
|
it's currently aliased to, or whether a more limited compatibility is
|
|
intended. This lets the user know whether they can treat the types as
|
|
substitutable or whether more specific rules must be followed, and can help the
|
|
implementation retain some degree of freedom to change the alias.</p>
|
|
<p>Don't put namespace aliases in your public API. (See also <a href="#Namespaces">Namespaces</a>).
|
|
</p>
|
|
|
|
<p>For example, these aliases document how they are intended to be used in client code:</p>
|
|
<pre>namespace a {
|
|
// Used to store field measurements. DataPoint may change from Bar* to some internal type.
|
|
// Client code should treat it as an opaque pointer.
|
|
using DataPoint = foo::bar::Bar*;
|
|
|
|
// A set of measurements. Just an alias for user convenience.
|
|
using TimeSeries = std::unordered_set<DataPoint, std::hash<DataPoint>, DataPointComparator>;
|
|
} // namespace a
|
|
</pre>
|
|
|
|
<p>These aliases don't document intended use, and half of them aren't meant for client use:</p>
|
|
|
|
<pre class="badcode">namespace a {
|
|
// Bad: none of these say how they should be used.
|
|
using DataPoint = foo::bar::Bar*;
|
|
using std::unordered_set; // Bad: just for local convenience
|
|
using std::hash; // Bad: just for local convenience
|
|
typedef unordered_set<DataPoint, hash<DataPoint>, DataPointComparator> TimeSeries;
|
|
} // namespace a
|
|
</pre>
|
|
|
|
<p>However, local convenience aliases are fine in function definitions, private sections of
|
|
classes, explicitly marked internal namespaces, and in .cc files:</p>
|
|
|
|
<pre>// In a .cc file
|
|
using std::unordered_set;
|
|
</pre>
|
|
|
|
</div>
|
|
</div>
|
|
|
|
<h2 id="Naming">Naming</h2>
|
|
|
|
<p>The most important consistency rules are those that govern
|
|
naming. The style of a name immediately informs us what sort of
|
|
thing the named entity is: a type, a variable, a function, a
|
|
constant, a macro, etc., without requiring us to search for the
|
|
declaration of that entity. The pattern-matching engine in our
|
|
brains relies a great deal on these naming rules.
|
|
</p>
|
|
|
|
<p>Naming rules are pretty arbitrary, but
|
|
we feel that
|
|
consistency is more important than individual preferences in this
|
|
area, so regardless of whether you find them sensible or not,
|
|
the rules are the rules.</p>
|
|
|
|
<h3 id="General_Naming_Rules">General Naming Rules</h3>
|
|
|
|
<div class="summary">
|
|
<p>Names should be descriptive; avoid abbreviation.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
<p>Give as descriptive a name as possible, within reason.
|
|
Do not worry about saving horizontal space as it is far
|
|
more important to make your code immediately
|
|
understandable by a new reader. Do not use abbreviations
|
|
that are ambiguous or unfamiliar to readers outside your
|
|
project, and do not abbreviate by deleting letters within
|
|
a word.</p>
|
|
|
|
<pre>int price_count_reader; // No abbreviation.
|
|
int num_errors; // "num" is a widespread convention.
|
|
int num_dns_connections; // Most people know what "DNS" stands for.
|
|
</pre>
|
|
|
|
<pre class="badcode">int n; // Meaningless.
|
|
int nerr; // Ambiguous abbreviation.
|
|
int n_comp_conns; // Ambiguous abbreviation.
|
|
int wgc_connections; // Only your group knows what this stands for.
|
|
int pc_reader; // Lots of things can be abbreviated "pc".
|
|
int cstmr_id; // Deletes internal letters.
|
|
</pre>
|
|
|
|
<p>Note that certain universally-known abbreviations are OK, such as
|
|
<code>i</code> for an iteration variable and <code>T</code> for a
|
|
template parameter.</p>
|
|
|
|
<p>Template parameters should follow the naming style for their
|
|
category: type template parameters should follow the rules for
|
|
<a href="#Type_Names">type names</a>, and non-type template
|
|
parameters should follow the rules for <a href="#Variable_Names">
|
|
variable names</a>.
|
|
|
|
</p></div>
|
|
|
|
<h3 id="File_Names">File Names</h3>
|
|
|
|
<div class="summary">
|
|
<p>Filenames should be all lowercase and can include
|
|
underscores (<code>_</code>) or dashes (<code>-</code>).
|
|
Follow the convention that your
|
|
|
|
project uses. If there is no consistent
|
|
local pattern to follow, prefer "_".</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Examples of acceptable file names:</p>
|
|
|
|
<ul>
|
|
<li><code>my_useful_class.cc</code></li>
|
|
<li><code>my-useful-class.cc</code></li>
|
|
<li><code>myusefulclass.cc</code></li>
|
|
<li><code>myusefulclass_test.cc // _unittest and _regtest are deprecated.</code></li>
|
|
</ul>
|
|
|
|
<p>C++ files should end in <code>.cc</code> and header files should end in
|
|
<code>.h</code>. Files that rely on being textually included at specific points
|
|
should end in <code>.inc</code> (see also the section on
|
|
<a href="#Self_contained_Headers">self-contained headers</a>).</p>
|
|
|
|
<p>Do not use filenames that already exist in
|
|
<code>/usr/include</code>, such as <code>db.h</code>.</p>
|
|
|
|
<p>In general, make your filenames very specific. For
|
|
example, use <code>http_server_logs.h</code> rather than
|
|
<code>logs.h</code>. A very common case is to have a pair
|
|
of files called, e.g., <code>foo_bar.h</code> and
|
|
<code>foo_bar.cc</code>, defining a class called
|
|
<code>FooBar</code>.</p>
|
|
|
|
<p>Inline functions must be in a <code>.h</code> file. If
|
|
your inline functions are very short, they should go
|
|
directly into your <code>.h</code> file. </p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Type_Names">Type Names</h3>
|
|
|
|
<div class="summary">
|
|
<p>Type names start with a capital letter and have a capital
|
|
letter for each new word, with no underscores:
|
|
<code>MyExcitingClass</code>, <code>MyExcitingEnum</code>.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>The names of all types — classes, structs, type aliases,
|
|
enums, and type template parameters — have the same naming convention.
|
|
Type names should start with a capital letter and have a capital letter
|
|
for each new word. No underscores. For example:</p>
|
|
|
|
<pre>// classes and structs
|
|
class UrlTable { ...
|
|
class UrlTableTester { ...
|
|
struct UrlTableProperties { ...
|
|
|
|
// typedefs
|
|
typedef hash_map<UrlTableProperties *, string> PropertiesMap;
|
|
|
|
// using aliases
|
|
using PropertiesMap = hash_map<UrlTableProperties *, string>;
|
|
|
|
// enums
|
|
enum UrlTableErrors { ...
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Variable_Names">Variable Names</h3>
|
|
|
|
<div class="summary">
|
|
<p>The names of variables (including function parameters) and data members are
|
|
all lowercase, with underscores between words. Data members of classes (but not
|
|
structs) additionally have trailing underscores. For instance:
|
|
<code>a_local_variable</code>, <code>a_struct_data_member</code>,
|
|
<code>a_class_data_member_</code>.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<h4 class="stylepoint_subsection">Common Variable names</h4>
|
|
|
|
<p>For example:</p>
|
|
|
|
<pre>string table_name; // OK - uses underscore.
|
|
string tablename; // OK - all lowercase.
|
|
</pre>
|
|
|
|
<pre class="badcode">string tableName; // Bad - mixed case.
|
|
</pre>
|
|
|
|
<h4 class="stylepoint_subsection">Class Data Members</h4>
|
|
|
|
<p>Data members of classes, both static and non-static, are
|
|
named like ordinary nonmember variables, but with a
|
|
trailing underscore.</p>
|
|
|
|
<pre>class TableInfo {
|
|
...
|
|
private:
|
|
string table_name_; // OK - underscore at end.
|
|
string tablename_; // OK.
|
|
static Pool<TableInfo>* pool_; // OK.
|
|
};
|
|
</pre>
|
|
|
|
<h4 class="stylepoint_subsection">Struct Data Members</h4>
|
|
|
|
<p>Data members of structs, both static and non-static,
|
|
are named like ordinary nonmember variables. They do not have
|
|
the trailing underscores that data members in classes have.</p>
|
|
|
|
<pre>struct UrlTableProperties {
|
|
string name;
|
|
int num_entries;
|
|
static Pool<UrlTableProperties>* pool;
|
|
};
|
|
</pre>
|
|
|
|
|
|
<p>See <a href="#Structs_vs._Classes">Structs vs.
|
|
Classes</a> for a discussion of when to use a struct
|
|
versus a class.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Constant_Names">Constant Names</h3>
|
|
|
|
<div class="summary">
|
|
<p>Variables declared constexpr or const, and whose value is fixed for
|
|
the duration of the program, are named with a leading "k" followed
|
|
by mixed case. For example:</p>
|
|
</div>
|
|
|
|
<pre>const int kDaysInAWeek = 7;
|
|
</pre>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>All such variables with static storage duration (i.e. statics and globals,
|
|
see <a href="http://en.cppreference.com/w/cpp/language/storage_duration#Storage_duration">
|
|
Storage Duration</a> for details) should be named this way. This
|
|
convention is optional for variables of other storage classes, e.g. automatic
|
|
variables, otherwise the usual variable naming rules apply.</p><p>
|
|
|
|
</p></div>
|
|
|
|
<h3 id="Function_Names">Function Names</h3>
|
|
|
|
<div class="summary">
|
|
<p>Regular functions have mixed case; accessors and mutators may be named
|
|
like variables.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Ordinarily, functions should start with a capital letter and have a
|
|
capital letter for each new word
|
|
(a.k.a. "<a href="https://en.wikipedia.org/wiki/Camel_case">Camel
|
|
Case</a>" or "Pascal case"). Such names should not have
|
|
underscores. Prefer to capitalize acronyms as single words
|
|
(i.e. <code>StartRpc()</code>, not <code>StartRPC()</code>).</p>
|
|
|
|
<pre>AddTableEntry()
|
|
DeleteUrl()
|
|
OpenFileOrDie()
|
|
</pre>
|
|
|
|
<p>(The same naming rule applies to class- and namespace-scope
|
|
constants that are exposed as part of an API and that are intended to look
|
|
like functions, because the fact that they're
|
|
objects rather than functions is an unimportant implementation detail.)</p>
|
|
|
|
<p>Accessors and mutators (get and set functions) may be named like
|
|
variables. These often correspond to actual member variables, but this is
|
|
not required. For example, <code>int count()</code> and <code>void
|
|
set_count(int count)</code>.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Namespace_Names">Namespace Names</h3>
|
|
|
|
<div class="summary">
|
|
Namespace names are all lower-case. Top-level namespace names are
|
|
based on the project name
|
|
. Avoid collisions
|
|
between nested namespaces and well-known top-level namespaces.
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
<p>The name of a top-level namespace should usually be the
|
|
name of the project or team whose code is contained in that
|
|
namespace. The code in that namespace should usually be in
|
|
a directory whose basename matches the namespace name (or
|
|
subdirectories thereof).</p>
|
|
|
|
|
|
|
|
|
|
|
|
<p>Keep in mind that the <a href="#General_Naming_Rules">rule
|
|
against abbreviated names</a> applies to namespaces just as much
|
|
as variable names. Code inside the namespace seldom needs to
|
|
mention the namespace name, so there's usually no particular need
|
|
for abbreviation anyway.</p>
|
|
|
|
<p>Avoid nested namespaces that match well-known top-level
|
|
namespaces. Collisions between namespace names can lead to surprising
|
|
build breaks because of name lookup rules. In particular, do not
|
|
create any nested <code>std</code> namespaces. Prefer unique project
|
|
identifiers
|
|
(<code>websearch::index</code>, <code>websearch::index_util</code>)
|
|
over collision-prone names like <code>websearch::util</code>.</p>
|
|
|
|
<p>For <code>internal</code> namespaces, be wary of other code being
|
|
added to the same <code>internal</code> namespace causing a collision
|
|
(internal helpers within a team tend to be related and may lead to
|
|
collisions). In such a situation, using the filename to make a unique
|
|
internal name is helpful
|
|
(<code>websearch::index::frobber_internal</code> for use
|
|
in <code>frobber.h</code>)</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Enumerator_Names">Enumerator Names</h3>
|
|
|
|
<div class="summary">
|
|
<p>Enumerators (for both scoped and unscoped enums) should be named <i>either</i> like
|
|
<a href="#Constant_Names">constants</a> or like
|
|
<a href="#Macro_Names">macros</a>: either <code>kEnumName</code> or
|
|
<code>ENUM_NAME</code>.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Preferably, the individual enumerators should be named
|
|
like <a href="#Constant_Names">constants</a>. However, it
|
|
is also acceptable to name them like
|
|
<a href="#Macro_Names">macros</a>. The enumeration name,
|
|
<code>UrlTableErrors</code> (and
|
|
<code>AlternateUrlTableErrors</code>), is a type, and
|
|
therefore mixed case.</p>
|
|
|
|
<pre>enum UrlTableErrors {
|
|
kOK = 0,
|
|
kErrorOutOfMemory,
|
|
kErrorMalformedInput,
|
|
};
|
|
enum AlternateUrlTableErrors {
|
|
OK = 0,
|
|
OUT_OF_MEMORY = 1,
|
|
MALFORMED_INPUT = 2,
|
|
};
|
|
</pre>
|
|
|
|
<p>Until January 2009, the style was to name enum values
|
|
like <a href="#Macro_Names">macros</a>. This caused
|
|
problems with name collisions between enum values and
|
|
macros. Hence, the change to prefer constant-style naming
|
|
was put in place. New code should prefer constant-style
|
|
naming if possible. However, there is no reason to change
|
|
old code to use constant-style names, unless the old
|
|
names are actually causing a compile-time problem.</p>
|
|
|
|
|
|
|
|
</div>
|
|
|
|
<h3 id="Macro_Names">Macro Names</h3>
|
|
|
|
<div class="summary">
|
|
<p>You're not really going to <a href="#Preprocessor_Macros">
|
|
define a macro</a>, are you? If you do, they're like this:
|
|
<code>MY_MACRO_THAT_SCARES_SMALL_CHILDREN</code>.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Please see the <a href="#Preprocessor_Macros">description
|
|
of macros</a>; in general macros should <em>not</em> be used.
|
|
However, if they are absolutely needed, then they should be
|
|
named with all capitals and underscores.</p>
|
|
|
|
<pre>#define ROUND(x) ...
|
|
#define PI_ROUNDED 3.0
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Exceptions_to_Naming_Rules">Exceptions to Naming Rules</h3>
|
|
|
|
<div class="summary">
|
|
<p>If you are naming something that is analogous to an
|
|
existing C or C++ entity then you can follow the existing
|
|
naming convention scheme.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<dl>
|
|
<dt><code>bigopen()</code></dt>
|
|
<dd>function name, follows form of <code>open()</code></dd>
|
|
|
|
<dt><code>uint</code></dt>
|
|
<dd><code>typedef</code></dd>
|
|
|
|
<dt><code>bigpos</code></dt>
|
|
<dd><code>struct</code> or <code>class</code>, follows
|
|
form of <code>pos</code></dd>
|
|
|
|
<dt><code>sparse_hash_map</code></dt>
|
|
<dd>STL-like entity; follows STL naming conventions</dd>
|
|
|
|
<dt><code>LONGLONG_MAX</code></dt>
|
|
<dd>a constant, as in <code>INT_MAX</code></dd>
|
|
</dl>
|
|
|
|
</div>
|
|
|
|
<h2 id="Comments">Comments</h2>
|
|
|
|
<p>Though a pain to write, comments are absolutely vital to
|
|
keeping our code readable. The following rules describe what
|
|
you should comment and where. But remember: while comments are
|
|
very important, the best code is self-documenting. Giving
|
|
sensible names to types and variables is much better than using
|
|
obscure names that you must then explain through comments.</p>
|
|
|
|
<p>When writing your comments, write for your audience: the
|
|
next
|
|
contributor who will need to
|
|
understand your code. Be generous — the next
|
|
one may be you!</p>
|
|
|
|
<h3 id="Comment_Style">Comment Style</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use either the <code>//</code> or <code>/* */</code>
|
|
syntax, as long as you are consistent.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>You can use either the <code>//</code> or the <code>/*
|
|
*/</code> syntax; however, <code>//</code> is
|
|
<em>much</em> more common. Be consistent with how you
|
|
comment and what style you use where.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="File_Comments">File Comments</h3>
|
|
|
|
<div class="summary">
|
|
<p>Start each file with license boilerplate.</p>
|
|
|
|
<p>File comments describe the contents of a file. If a file declares,
|
|
implements, or tests exactly one abstraction that is documented by a comment
|
|
at the point of declaration, file comments are not required. All other files
|
|
must have file comments.</p>
|
|
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<h4 class="stylepoint_subsection">Legal Notice and Author
|
|
Line</h4>
|
|
|
|
|
|
|
|
<p>Every file should contain license
|
|
boilerplate. Choose the appropriate boilerplate for the
|
|
license used by the project (for example, Apache 2.0,
|
|
BSD, LGPL, GPL).</p>
|
|
|
|
<p>If you make significant changes to a file with an
|
|
author line, consider deleting the author line.</p>
|
|
|
|
<h4 class="stylepoint_subsection">File Contents</h4>
|
|
|
|
<p>If a <code>.h</code> declares multiple abstractions, the file-level comment
|
|
should broadly describe the contents of the file, and how the abstractions are
|
|
related. A 1 or 2 sentence file-level comment may be sufficient. The detailed
|
|
documentation about individual abstractions belongs with those abstractions,
|
|
not at the file level.</p>
|
|
|
|
<p>Do not duplicate comments in both the <code>.h</code> and the
|
|
<code>.cc</code>. Duplicated comments diverge.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Class_Comments">Class Comments</h3>
|
|
|
|
<div class="summary">
|
|
<p>Every non-obvious class declaration should have an accompanying
|
|
comment that describes what it is for and how it should be used.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<pre>// Iterates over the contents of a GargantuanTable.
|
|
// Example:
|
|
// GargantuanTableIterator* iter = table->NewIterator();
|
|
// for (iter->Seek("foo"); !iter->done(); iter->Next()) {
|
|
// process(iter->key(), iter->value());
|
|
// }
|
|
// delete iter;
|
|
class GargantuanTableIterator {
|
|
...
|
|
};
|
|
</pre>
|
|
|
|
<p>The class comment should provide the reader with enough information to know
|
|
how and when to use the class, as well as any additional considerations
|
|
necessary to correctly use the class. Document the synchronization assumptions
|
|
the class makes, if any. If an instance of the class can be accessed by
|
|
multiple threads, take extra care to document the rules and invariants
|
|
surrounding multithreaded use.</p>
|
|
|
|
<p>The class comment is often a good place for a small example code snippet
|
|
demonstrating a simple and focused usage of the class.</p>
|
|
|
|
<p>When sufficiently separated (e.g. <code>.h</code> and <code>.cc</code>
|
|
files), comments describing the use of the class should go together with its
|
|
interface definition; comments about the class operation and implementation
|
|
should accompany the implementation of the class's methods.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Function_Comments">Function Comments</h3>
|
|
|
|
<div class="summary">
|
|
<p>Declaration comments describe use of the function (when it is
|
|
non-obvious); comments at the definition of a function describe
|
|
operation.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<h4 class="stylepoint_subsection">Function Declarations</h4>
|
|
|
|
<p>Almost every function declaration should have comments immediately
|
|
preceding it that describe what the function does and how to use
|
|
it. These comments may be omitted only if the function is simple and
|
|
obvious (e.g. simple accessors for obvious properties of the
|
|
class). These comments should be descriptive ("Opens the file")
|
|
rather than imperative ("Open the file"); the comment describes the
|
|
function, it does not tell the function what to do. In general, these
|
|
comments do not describe how the function performs its task. Instead,
|
|
that should be left to comments in the function definition.</p>
|
|
|
|
<p>Types of things to mention in comments at the function
|
|
declaration:</p>
|
|
|
|
<ul>
|
|
<li>What the inputs and outputs are.</li>
|
|
|
|
<li>For class member functions: whether the object
|
|
remembers reference arguments beyond the duration of
|
|
the method call, and whether it will free them or
|
|
not.</li>
|
|
|
|
<li>If the function allocates memory that the caller
|
|
must free.</li>
|
|
|
|
<li>Whether any of the arguments can be a null
|
|
pointer.</li>
|
|
|
|
<li>If there are any performance implications of how a
|
|
function is used.</li>
|
|
|
|
<li>If the function is re-entrant. What are its
|
|
synchronization assumptions?</li>
|
|
</ul>
|
|
|
|
<p>Here is an example:</p>
|
|
|
|
<pre>// Returns an iterator for this table. It is the client's
|
|
// responsibility to delete the iterator when it is done with it,
|
|
// and it must not use the iterator once the GargantuanTable object
|
|
// on which the iterator was created has been deleted.
|
|
//
|
|
// The iterator is initially positioned at the beginning of the table.
|
|
//
|
|
// This method is equivalent to:
|
|
// Iterator* iter = table->NewIterator();
|
|
// iter->Seek("");
|
|
// return iter;
|
|
// If you are going to immediately seek to another place in the
|
|
// returned iterator, it will be faster to use NewIterator()
|
|
// and avoid the extra seek.
|
|
Iterator* GetIterator() const;
|
|
</pre>
|
|
|
|
<p>However, do not be unnecessarily verbose or state the
|
|
completely obvious. Notice below that it is not necessary
|
|
to say "returns false otherwise" because this is
|
|
implied.</p>
|
|
|
|
<pre>// Returns true if the table cannot hold any more entries.
|
|
bool IsTableFull();
|
|
</pre>
|
|
|
|
<p>When documenting function overrides, focus on the
|
|
specifics of the override itself, rather than repeating
|
|
the comment from the overridden function. In many of these
|
|
cases, the override needs no additional documentation and
|
|
thus no comment is required.</p>
|
|
|
|
<p>When commenting constructors and destructors, remember
|
|
that the person reading your code knows what constructors
|
|
and destructors are for, so comments that just say
|
|
something like "destroys this object" are not useful.
|
|
Document what constructors do with their arguments (for
|
|
example, if they take ownership of pointers), and what
|
|
cleanup the destructor does. If this is trivial, just
|
|
skip the comment. It is quite common for destructors not
|
|
to have a header comment.</p>
|
|
|
|
<h4 class="stylepoint_subsection">Function Definitions</h4>
|
|
|
|
<p>If there is anything tricky about how a function does
|
|
its job, the function definition should have an
|
|
explanatory comment. For example, in the definition
|
|
comment you might describe any coding tricks you use,
|
|
give an overview of the steps you go through, or explain
|
|
why you chose to implement the function in the way you
|
|
did rather than using a viable alternative. For instance,
|
|
you might mention why it must acquire a lock for the
|
|
first half of the function but why it is not needed for
|
|
the second half.</p>
|
|
|
|
<p>Note you should <em>not</em> just repeat the comments
|
|
given with the function declaration, in the
|
|
<code>.h</code> file or wherever. It's okay to
|
|
recapitulate briefly what the function does, but the
|
|
focus of the comments should be on how it does it.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Variable_Comments">Variable Comments</h3>
|
|
|
|
<div class="summary">
|
|
<p>In general the actual name of the variable should be
|
|
descriptive enough to give a good idea of what the variable
|
|
is used for. In certain cases, more comments are required.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<h4 class="stylepoint_subsection">Class Data Members</h4>
|
|
|
|
<p>The purpose of each class data member (also called an instance
|
|
variable or member variable) must be clear. If there are any
|
|
invariants (special values, relationships between members, lifetime
|
|
requirements) not clearly expressed by the type and name, they must be
|
|
commented. However, if the type and name suffice (<code>int
|
|
num_events_;</code>), no comment is needed.</p>
|
|
|
|
<p>In particular, add comments to describe the existence and meaning
|
|
of sentinel values, such as nullptr or -1, when they are not
|
|
obvious. For example:</p>
|
|
|
|
<pre>private:
|
|
// Used to bounds-check table accesses. -1 means
|
|
// that we don't yet know how many entries the table has.
|
|
int num_total_entries_;
|
|
</pre>
|
|
|
|
<h4 class="stylepoint_subsection">Global Variables</h4>
|
|
|
|
<p>All global variables should have a comment describing what they
|
|
are, what they are used for, and (if unclear) why it needs to be
|
|
global. For example:</p>
|
|
|
|
<pre>// The total number of tests cases that we run through in this regression test.
|
|
const int kNumTestCases = 6;
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Implementation_Comments">Implementation Comments</h3>
|
|
|
|
<div class="summary">
|
|
<p>In your implementation you should have comments in tricky,
|
|
non-obvious, interesting, or important parts of your code.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<h4 class="stylepoint_subsection">Explanatory Comments</h4>
|
|
|
|
<p>Tricky or complicated code blocks should have comments
|
|
before them. Example:</p>
|
|
|
|
<pre>// Divide result by two, taking into account that x
|
|
// contains the carry from the add.
|
|
for (int i = 0; i < result->size(); i++) {
|
|
x = (x << 8) + (*result)[i];
|
|
(*result)[i] = x >> 1;
|
|
x &= 1;
|
|
}
|
|
</pre>
|
|
|
|
<h4 class="stylepoint_subsection">Line Comments</h4>
|
|
|
|
<p>Also, lines that are non-obvious should get a comment
|
|
at the end of the line. These end-of-line comments should
|
|
be separated from the code by 2 spaces. Example:</p>
|
|
|
|
<pre>// If we have enough memory, mmap the data portion too.
|
|
mmap_budget = max<int64>(0, mmap_budget - index_->length());
|
|
if (mmap_budget >= data_size_ && !MmapData(mmap_chunk_bytes, mlock))
|
|
return; // Error already logged.
|
|
</pre>
|
|
|
|
<p>Note that there are both comments that describe what
|
|
the code is doing, and comments that mention that an
|
|
error has already been logged when the function
|
|
returns.</p>
|
|
|
|
<p>If you have several comments on subsequent lines, it
|
|
can often be more readable to line them up:</p>
|
|
|
|
<pre>DoSomething(); // Comment here so the comments line up.
|
|
DoSomethingElseThatIsLonger(); // Two spaces between the code and the comment.
|
|
{ // One space before comment when opening a new scope is allowed,
|
|
// thus the comment lines up with the following comments and code.
|
|
DoSomethingElse(); // Two spaces before line comments normally.
|
|
}
|
|
std::vector<string> list{
|
|
// Comments in braced lists describe the next element...
|
|
"First item",
|
|
// .. and should be aligned appropriately.
|
|
"Second item"};
|
|
DoSomething(); /* For trailing block comments, one space is fine. */
|
|
</pre>
|
|
|
|
<h4 class="stylepoint_subsection">Function Argument Comments</h4>
|
|
|
|
<p>When the meaning of a function argument is nonobvious, consider
|
|
one of the following remedies:</p>
|
|
|
|
<ul>
|
|
<li>If the argument is a literal constant, and the same constant is
|
|
used in multiple function calls in a way that tacitly assumes they're
|
|
the same, you should use a named constant to make that constraint
|
|
explicit, and to guarantee that it holds.</li>
|
|
|
|
<li>Consider changing the function signature to replace a <code>bool</code>
|
|
argument with an <code>enum</code> argument. This will make the argument
|
|
values self-describing.</li>
|
|
|
|
<li>For functions that have several configuration options, consider
|
|
defining a single class or struct to hold all the options
|
|
,
|
|
and pass an instance of that.
|
|
This approach has several advantages. Options are referenced by name
|
|
at the call site, which clarifies their meaning. It also reduces
|
|
function argument count, which makes function calls easier to read and
|
|
write. As an added benefit, you don't have to change call sites when
|
|
you add another option.
|
|
</li>
|
|
|
|
<li>Replace large or complex nested expressions with named variables.</li>
|
|
|
|
<li>As a last resort, use comments to clarify argument meanings at the
|
|
call site.</li>
|
|
</ul>
|
|
|
|
Consider the following example:
|
|
|
|
<pre class="badcode">// What are these arguments?
|
|
const DecimalNumber product = CalculateProduct(values, 7, false, nullptr);
|
|
</pre>
|
|
|
|
<p>versus:</p>
|
|
|
|
<pre>ProductOptions options;
|
|
options.set_precision_decimals(7);
|
|
options.set_use_cache(ProductOptions::kDontUseCache);
|
|
const DecimalNumber product =
|
|
CalculateProduct(values, options, /*completion_callback=*/nullptr);
|
|
</pre>
|
|
|
|
<h4 class="stylepoint_subsection">Don'ts</h4>
|
|
|
|
<p>Do not state the obvious. In particular, don't literally describe what
|
|
code does, unless the behavior is nonobvious to a reader who understands
|
|
C++ well. Instead, provide higher level comments that describe <i>why</i>
|
|
the code does what it does, or make the code self describing.</p>
|
|
|
|
Compare this:
|
|
|
|
<pre class="badcode">// Find the element in the vector. <-- Bad: obvious!
|
|
auto iter = std::find(v.begin(), v.end(), element);
|
|
if (iter != v.end()) {
|
|
Process(element);
|
|
}
|
|
</pre>
|
|
|
|
To this:
|
|
|
|
<pre>// Process "element" unless it was already processed.
|
|
auto iter = std::find(v.begin(), v.end(), element);
|
|
if (iter != v.end()) {
|
|
Process(element);
|
|
}
|
|
</pre>
|
|
|
|
Self-describing code doesn't need a comment. The comment from
|
|
the example above would be obvious:
|
|
|
|
<pre>if (!IsAlreadyProcessed(element)) {
|
|
Process(element);
|
|
}
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Punctuation,_Spelling_and_Grammar">Punctuation, Spelling and Grammar</h3>
|
|
|
|
<div class="summary">
|
|
<p>Pay attention to punctuation, spelling, and grammar; it is
|
|
easier to read well-written comments than badly written
|
|
ones.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Comments should be as readable as narrative text, with
|
|
proper capitalization and punctuation. In many cases,
|
|
complete sentences are more readable than sentence
|
|
fragments. Shorter comments, such as comments at the end
|
|
of a line of code, can sometimes be less formal, but you
|
|
should be consistent with your style.</p>
|
|
|
|
<p>Although it can be frustrating to have a code reviewer
|
|
point out that you are using a comma when you should be
|
|
using a semicolon, it is very important that source code
|
|
maintain a high level of clarity and readability. Proper
|
|
punctuation, spelling, and grammar help with that
|
|
goal.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="TODO_Comments">TODO Comments</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use <code>TODO</code> comments for code that is temporary,
|
|
a short-term solution, or good-enough but not perfect.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p><code>TODO</code>s should include the string
|
|
<code>TODO</code> in all caps, followed by the
|
|
|
|
name, e-mail address, bug ID, or other
|
|
identifier
|
|
of the person or issue with the best context
|
|
about the problem referenced by the <code>TODO</code>. The
|
|
main purpose is to have a consistent <code>TODO</code> that
|
|
can be searched to find out how to get more details upon
|
|
request. A <code>TODO</code> is not a commitment that the
|
|
person referenced will fix the problem. Thus when you create
|
|
a <code>TODO</code> with a name, it is almost always your
|
|
name that is given.</p>
|
|
|
|
|
|
|
|
<div>
|
|
<pre>// TODO(kl@gmail.com): Use a "*" here for concatenation operator.
|
|
// TODO(Zeke) change this to use relations.
|
|
// TODO(bug 12345): remove the "Last visitors" feature
|
|
</pre>
|
|
</div>
|
|
|
|
<p>If your <code>TODO</code> is of the form "At a future
|
|
date do something" make sure that you either include a
|
|
very specific date ("Fix by November 2005") or a very
|
|
specific event ("Remove this code when all clients can
|
|
handle XML responses.").</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Deprecation_Comments">Deprecation Comments</h3>
|
|
|
|
<div class="summary">
|
|
<p>Mark deprecated interface points with <code>DEPRECATED</code>
|
|
comments.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>You can mark an interface as deprecated by writing a
|
|
comment containing the word <code>DEPRECATED</code> in
|
|
all caps. The comment goes either before the declaration
|
|
of the interface or on the same line as the
|
|
declaration.</p>
|
|
|
|
|
|
|
|
<p>After the word
|
|
<code>DEPRECATED</code>, write your name, e-mail address,
|
|
or other identifier in parentheses.</p>
|
|
|
|
<p>A deprecation comment must include simple, clear
|
|
directions for people to fix their callsites. In C++, you
|
|
can implement a deprecated function as an inline function
|
|
that calls the new interface point.</p>
|
|
|
|
<p>Marking an interface point <code>DEPRECATED</code>
|
|
will not magically cause any callsites to change. If you
|
|
want people to actually stop using the deprecated
|
|
facility, you will have to fix the callsites yourself or
|
|
recruit a crew to help you.</p>
|
|
|
|
<p>New code should not contain calls to deprecated
|
|
interface points. Use the new interface point instead. If
|
|
you cannot understand the directions, find the person who
|
|
created the deprecation and ask them for help using the
|
|
new interface point.</p>
|
|
|
|
|
|
|
|
</div>
|
|
|
|
<h2 id="Formatting">Formatting</h2>
|
|
|
|
<p>Coding style and formatting are pretty arbitrary, but a
|
|
|
|
project is much easier to follow
|
|
if everyone uses the same style. Individuals may not agree with every
|
|
aspect of the formatting rules, and some of the rules may take
|
|
some getting used to, but it is important that all
|
|
|
|
project contributors follow the
|
|
style rules so that
|
|
they can all read and understand
|
|
everyone's code easily.</p>
|
|
|
|
|
|
|
|
<p>To help you format code correctly, we've
|
|
created a
|
|
<a href="https://raw.githubusercontent.com/google/styleguide/gh-pages/google-c-style.el">
|
|
settings file for emacs</a>.</p>
|
|
|
|
<h3 id="Line_Length">Line Length</h3>
|
|
|
|
<div class="summary">
|
|
<p>Each line of text in your code should be at most 80
|
|
characters long.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
|
|
|
|
<p>We recognize that this rule is
|
|
controversial, but so much existing code already adheres
|
|
to it, and we feel that consistency is important.</p>
|
|
|
|
<div class="pros">
|
|
<p>Those who favor this rule
|
|
argue that it is rude to force them to resize
|
|
their windows and there is no need for anything longer.
|
|
Some folks are used to having several code windows
|
|
side-by-side, and thus don't have room to widen their
|
|
windows in any case. People set up their work environment
|
|
assuming a particular maximum window width, and 80
|
|
columns has been the traditional standard. Why change
|
|
it?</p>
|
|
</div>
|
|
|
|
<div class="cons">
|
|
<p>Proponents of change argue that a wider line can make
|
|
code more readable. The 80-column limit is an hidebound
|
|
throwback to 1960s mainframes; modern equipment has wide screens that
|
|
can easily show longer lines.</p>
|
|
</div>
|
|
|
|
<div class="decision">
|
|
<p> 80 characters is the maximum.</p>
|
|
|
|
<p class="exception">Comment lines can be longer than 80
|
|
characters if it is not feasible to split them without
|
|
harming readability, ease of cut and paste or auto-linking
|
|
-- e.g. if a line contains an example command or a literal
|
|
URL longer than 80 characters.</p>
|
|
|
|
<p class="exception">A raw-string literal may have content
|
|
that exceeds 80 characters. Except for test code, such literals
|
|
should appear near the top of a file.</p>
|
|
|
|
<p class="exception">An <code>#include</code> statement with a
|
|
long path may exceed 80 columns.</p>
|
|
|
|
<p class="exception">You needn't be concerned about
|
|
<a href="#The__define_Guard">header guards</a> that exceed
|
|
the maximum length. </p>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<h3 id="Non-ASCII_Characters">Non-ASCII Characters</h3>
|
|
|
|
<div class="summary">
|
|
<p>Non-ASCII characters should be rare, and must use UTF-8
|
|
formatting.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>You shouldn't hard-code user-facing text in source,
|
|
even English, so use of non-ASCII characters should be
|
|
rare. However, in certain cases it is appropriate to
|
|
include such words in your code. For example, if your
|
|
code parses data files from foreign sources, it may be
|
|
appropriate to hard-code the non-ASCII string(s) used in
|
|
those data files as delimiters. More commonly, unittest
|
|
code (which does not need to be localized) might
|
|
contain non-ASCII strings. In such cases, you should use
|
|
UTF-8, since that is an encoding
|
|
understood by most tools able to handle more than just
|
|
ASCII.</p>
|
|
|
|
<p>Hex encoding is also OK, and encouraged where it
|
|
enhances readability — for example,
|
|
<code>"\xEF\xBB\xBF"</code>, or, even more simply,
|
|
<code>u8"\uFEFF"</code>, is the Unicode zero-width
|
|
no-break space character, which would be invisible if
|
|
included in the source as straight UTF-8.</p>
|
|
|
|
<p>Use the <code>u8</code> prefix
|
|
to guarantee that a string literal containing
|
|
<code>\uXXXX</code> escape sequences is encoded as UTF-8.
|
|
Do not use it for strings containing non-ASCII characters
|
|
encoded as UTF-8, because that will produce incorrect
|
|
output if the compiler does not interpret the source file
|
|
as UTF-8. </p>
|
|
|
|
<p>You shouldn't use the C++11 <code>char16_t</code> and
|
|
<code>char32_t</code> character types, since they're for
|
|
non-UTF-8 text. For similar reasons you also shouldn't
|
|
use <code>wchar_t</code> (unless you're writing code that
|
|
interacts with the Windows API, which uses
|
|
<code>wchar_t</code> extensively).</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Spaces_vs._Tabs">Spaces vs. Tabs</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use only spaces, and indent 2 spaces at a time.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>We use spaces for indentation. Do not use tabs in your
|
|
code. You should set your editor to emit spaces when you
|
|
hit the tab key.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Function_Declarations_and_Definitions">Function Declarations and Definitions</h3>
|
|
|
|
<div class="summary">
|
|
<p>Return type on the same line as function name, parameters
|
|
on the same line if they fit. Wrap parameter lists which do
|
|
not fit on a single line as you would wrap arguments in a
|
|
<a href="#Function_Calls">function call</a>.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Functions look like this:</p>
|
|
|
|
|
|
<pre>ReturnType ClassName::FunctionName(Type par_name1, Type par_name2) {
|
|
DoSomething();
|
|
...
|
|
}
|
|
</pre>
|
|
|
|
<p>If you have too much text to fit on one line:</p>
|
|
|
|
<pre>ReturnType ClassName::ReallyLongFunctionName(Type par_name1, Type par_name2,
|
|
Type par_name3) {
|
|
DoSomething();
|
|
...
|
|
}
|
|
</pre>
|
|
|
|
<p>or if you cannot fit even the first parameter:</p>
|
|
|
|
<pre>ReturnType LongClassName::ReallyReallyReallyLongFunctionName(
|
|
Type par_name1, // 4 space indent
|
|
Type par_name2,
|
|
Type par_name3) {
|
|
DoSomething(); // 2 space indent
|
|
...
|
|
}
|
|
</pre>
|
|
|
|
<p>Some points to note:</p>
|
|
|
|
<ul>
|
|
<li>Choose good parameter names.</li>
|
|
|
|
<li>Parameter names may be omitted only if the parameter is unused and its
|
|
purpose is obvious.</li>
|
|
|
|
<li>If you cannot fit the return type and the function
|
|
name on a single line, break between them.</li>
|
|
|
|
<li>If you break after the return type of a function
|
|
declaration or definition, do not indent.</li>
|
|
|
|
<li>The open parenthesis is always on the same line as
|
|
the function name.</li>
|
|
|
|
<li>There is never a space between the function name
|
|
and the open parenthesis.</li>
|
|
|
|
<li>There is never a space between the parentheses and
|
|
the parameters.</li>
|
|
|
|
<li>The open curly brace is always on the end of the last line of the function
|
|
declaration, not the start of the next line.</li>
|
|
|
|
<li>The close curly brace is either on the last line by
|
|
itself or on the same line as the open curly brace.</li>
|
|
|
|
<li>There should be a space between the close
|
|
parenthesis and the open curly brace.</li>
|
|
|
|
<li>All parameters should be aligned if possible.</li>
|
|
|
|
<li>Default indentation is 2 spaces.</li>
|
|
|
|
<li>Wrapped parameters have a 4 space indent.</li>
|
|
</ul>
|
|
|
|
<p>Unused parameters that are obvious from context may be omitted:</p>
|
|
|
|
<pre>class Foo {
|
|
public:
|
|
Foo(Foo&&);
|
|
Foo(const Foo&);
|
|
Foo& operator=(Foo&&);
|
|
Foo& operator=(const Foo&);
|
|
};
|
|
</pre>
|
|
|
|
<p>Unused parameters that might not be obvious should comment out the variable
|
|
name in the function definition:</p>
|
|
|
|
<pre>class Shape {
|
|
public:
|
|
virtual void Rotate(double radians) = 0;
|
|
};
|
|
|
|
class Circle : public Shape {
|
|
public:
|
|
void Rotate(double radians) override;
|
|
};
|
|
|
|
void Circle::Rotate(double /*radians*/) {}
|
|
</pre>
|
|
|
|
<pre class="badcode">// Bad - if someone wants to implement later, it's not clear what the
|
|
// variable means.
|
|
void Circle::Rotate(double) {}
|
|
</pre>
|
|
|
|
<p>Attributes, and macros that expand to attributes, appear at the very
|
|
beginning of the function declaration or definition, before the
|
|
return type:</p>
|
|
<pre>MUST_USE_RESULT bool IsOK();
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Formatting_Lambda_Expressions">Lambda Expressions</h3>
|
|
|
|
<div class="summary">
|
|
<p>Format parameters and bodies as for any other function, and capture
|
|
lists like other comma-separated lists.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
<p>For by-reference captures, do not leave a space between the
|
|
ampersand (&) and the variable name.</p>
|
|
<pre>int x = 0;
|
|
auto x_plus_n = [&x](int n) -> int { return x + n; }
|
|
</pre>
|
|
<p>Short lambdas may be written inline as function arguments.</p>
|
|
<pre>std::set<int> blacklist = {7, 8, 9};
|
|
std::vector<int> digits = {3, 9, 1, 8, 4, 7, 1};
|
|
digits.erase(std::remove_if(digits.begin(), digits.end(), [&blacklist](int i) {
|
|
return blacklist.find(i) != blacklist.end();
|
|
}),
|
|
digits.end());
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Function_Calls">Function Calls</h3>
|
|
|
|
<div class="summary">
|
|
<p>Either write the call all on a single line, wrap the
|
|
arguments at the parenthesis, or start the arguments on a new
|
|
line indented by four spaces and continue at that 4 space
|
|
indent. In the absence of other considerations, use the
|
|
minimum number of lines, including placing multiple arguments
|
|
on each line where appropriate.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Function calls have the following format:</p>
|
|
<pre>bool result = DoSomething(argument1, argument2, argument3);
|
|
</pre>
|
|
|
|
<p>If the arguments do not all fit on one line, they
|
|
should be broken up onto multiple lines, with each
|
|
subsequent line aligned with the first argument. Do not
|
|
add spaces after the open paren or before the close
|
|
paren:</p>
|
|
<pre>bool result = DoSomething(averyveryveryverylongargument1,
|
|
argument2, argument3);
|
|
</pre>
|
|
|
|
<p>Arguments may optionally all be placed on subsequent
|
|
lines with a four space indent:</p>
|
|
<pre>if (...) {
|
|
...
|
|
...
|
|
if (...) {
|
|
bool result = DoSomething(
|
|
argument1, argument2, // 4 space indent
|
|
argument3, argument4);
|
|
...
|
|
}
|
|
</pre>
|
|
|
|
<p>Put multiple arguments on a single line to reduce the
|
|
number of lines necessary for calling a function unless
|
|
there is a specific readability problem. Some find that
|
|
formatting with strictly one argument on each line is
|
|
more readable and simplifies editing of the arguments.
|
|
However, we prioritize for the reader over the ease of
|
|
editing arguments, and most readability problems are
|
|
better addressed with the following techniques.</p>
|
|
|
|
<p>If having multiple arguments in a single line decreases
|
|
readability due to the complexity or confusing nature of the
|
|
expressions that make up some arguments, try creating
|
|
variables that capture those arguments in a descriptive name:</p>
|
|
<pre>int my_heuristic = scores[x] * y + bases[x];
|
|
bool result = DoSomething(my_heuristic, x, y, z);
|
|
</pre>
|
|
|
|
<p>Or put the confusing argument on its own line with
|
|
an explanatory comment:</p>
|
|
<pre>bool result = DoSomething(scores[x] * y + bases[x], // Score heuristic.
|
|
x, y, z);
|
|
</pre>
|
|
|
|
<p>If there is still a case where one argument is
|
|
significantly more readable on its own line, then put it on
|
|
its own line. The decision should be specific to the argument
|
|
which is made more readable rather than a general policy.</p>
|
|
|
|
<p>Sometimes arguments form a structure that is important
|
|
for readability. In those cases, feel free to format the
|
|
arguments according to that structure:</p>
|
|
<pre>// Transform the widget by a 3x3 matrix.
|
|
my_widget.Transform(x1, x2, x3,
|
|
y1, y2, y3,
|
|
z1, z2, z3);
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Braced_Initializer_List_Format">Braced Initializer List Format</h3>
|
|
|
|
<div class="summary">
|
|
<p>Format a <a href="#Braced_Initializer_List">braced initializer list</a>
|
|
exactly like you would format a function call in its place.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>If the braced list follows a name (e.g. a type or
|
|
variable name), format as if the <code>{}</code> were the
|
|
parentheses of a function call with that name. If there
|
|
is no name, assume a zero-length name.</p>
|
|
|
|
<pre>// Examples of braced init list on a single line.
|
|
return {foo, bar};
|
|
functioncall({foo, bar});
|
|
std::pair<int, int> p{foo, bar};
|
|
|
|
// When you have to wrap.
|
|
SomeFunction(
|
|
{"assume a zero-length name before {"},
|
|
some_other_function_parameter);
|
|
SomeType variable{
|
|
some, other, values,
|
|
{"assume a zero-length name before {"},
|
|
SomeOtherType{
|
|
"Very long string requiring the surrounding breaks.",
|
|
some, other values},
|
|
SomeOtherType{"Slightly shorter string",
|
|
some, other, values}};
|
|
SomeType variable{
|
|
"This is too long to fit all in one line"};
|
|
MyType m = { // Here, you could also break before {.
|
|
superlongvariablename1,
|
|
superlongvariablename2,
|
|
{short, interior, list},
|
|
{interiorwrappinglist,
|
|
interiorwrappinglist2}};
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Conditionals">Conditionals</h3>
|
|
|
|
<div class="summary">
|
|
<p>Prefer no spaces inside parentheses. The <code>if</code>
|
|
and <code>else</code> keywords belong on separate lines.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>There are two acceptable formats for a basic
|
|
conditional statement. One includes spaces between the
|
|
parentheses and the condition, and one does not.</p>
|
|
|
|
<p>The most common form is without spaces. Either is
|
|
fine, but <em>be consistent</em>. If you are modifying a
|
|
file, use the format that is already present. If you are
|
|
writing new code, use the format that the other files in
|
|
that directory or project use. If in doubt and you have
|
|
no personal preference, do not add the spaces.</p>
|
|
|
|
<pre>if (condition) { // no spaces inside parentheses
|
|
... // 2 space indent.
|
|
} else if (...) { // The else goes on the same line as the closing brace.
|
|
...
|
|
} else {
|
|
...
|
|
}
|
|
</pre>
|
|
|
|
<p>If you prefer you may add spaces inside the
|
|
parentheses:</p>
|
|
|
|
<pre>if ( condition ) { // spaces inside parentheses - rare
|
|
... // 2 space indent.
|
|
} else { // The else goes on the same line as the closing brace.
|
|
...
|
|
}
|
|
</pre>
|
|
|
|
<p>Note that in all cases you must have a space between
|
|
the <code>if</code> and the open parenthesis. You must
|
|
also have a space between the close parenthesis and the
|
|
curly brace, if you're using one.</p>
|
|
|
|
<pre class="badcode">if(condition) { // Bad - space missing after IF.
|
|
if (condition){ // Bad - space missing before {.
|
|
if(condition){ // Doubly bad.
|
|
</pre>
|
|
|
|
<pre>if (condition) { // Good - proper space after IF and before {.
|
|
</pre>
|
|
|
|
<p>Short conditional statements may be written on one
|
|
line if this enhances readability. You may use this only
|
|
when the line is brief and the statement does not use the
|
|
<code>else</code> clause.</p>
|
|
|
|
<pre>if (x == kFoo) return new Foo();
|
|
if (x == kBar) return new Bar();
|
|
</pre>
|
|
|
|
<p>This is not allowed when the if statement has an
|
|
<code>else</code>:</p>
|
|
|
|
<pre class="badcode">// Not allowed - IF statement on one line when there is an ELSE clause
|
|
if (x) DoThis();
|
|
else DoThat();
|
|
</pre>
|
|
|
|
<p>In general, curly braces are not required for
|
|
single-line statements, but they are allowed if you like
|
|
them; conditional or loop statements with complex
|
|
conditions or statements may be more readable with curly
|
|
braces. Some
|
|
projects require that an
|
|
<code>if</code> must always always have an accompanying
|
|
brace.</p>
|
|
|
|
<pre>if (condition)
|
|
DoSomething(); // 2 space indent.
|
|
|
|
if (condition) {
|
|
DoSomething(); // 2 space indent.
|
|
}
|
|
</pre>
|
|
|
|
<p>However, if one part of an
|
|
<code>if</code>-<code>else</code> statement uses curly
|
|
braces, the other part must too:</p>
|
|
|
|
<pre class="badcode">// Not allowed - curly on IF but not ELSE
|
|
if (condition) {
|
|
foo;
|
|
} else
|
|
bar;
|
|
|
|
// Not allowed - curly on ELSE but not IF
|
|
if (condition)
|
|
foo;
|
|
else {
|
|
bar;
|
|
}
|
|
</pre>
|
|
|
|
<pre>// Curly braces around both IF and ELSE required because
|
|
// one of the clauses used braces.
|
|
if (condition) {
|
|
foo;
|
|
} else {
|
|
bar;
|
|
}
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Loops_and_Switch_Statements">Loops and Switch Statements</h3>
|
|
|
|
<div class="summary">
|
|
<p>Switch statements may use braces for blocks. Annotate
|
|
non-trivial fall-through between cases.
|
|
Braces are optional for single-statement loops.
|
|
Empty loop bodies should use empty braces or <code>continue</code>.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p><code>case</code> blocks in <code>switch</code>
|
|
statements can have curly braces or not, depending on
|
|
your preference. If you do include curly braces they
|
|
should be placed as shown below.</p>
|
|
|
|
<p>If not conditional on an enumerated value, switch
|
|
statements should always have a <code>default</code> case
|
|
(in the case of an enumerated value, the compiler will
|
|
warn you if any values are not handled). If the default
|
|
case should never execute, simply
|
|
<code>assert</code>:</p>
|
|
|
|
|
|
|
|
<div>
|
|
<pre>switch (var) {
|
|
case 0: { // 2 space indent
|
|
... // 4 space indent
|
|
break;
|
|
}
|
|
case 1: {
|
|
...
|
|
break;
|
|
}
|
|
default: {
|
|
assert(false);
|
|
}
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
|
<p> Braces are optional for single-statement loops.</p>
|
|
|
|
<pre>for (int i = 0; i < kSomeNumber; ++i)
|
|
printf("I love you\n");
|
|
|
|
for (int i = 0; i < kSomeNumber; ++i) {
|
|
printf("I take it back\n");
|
|
}
|
|
</pre>
|
|
|
|
|
|
<p>Empty loop bodies should use an empty pair of braces or <code>continue</code>,
|
|
but not a single semicolon.</p>
|
|
|
|
<pre>while (condition) {
|
|
// Repeat test until it returns false.
|
|
}
|
|
for (int i = 0; i < kSomeNumber; ++i) {} // Good - one newline is also OK.
|
|
while (condition) continue; // Good - continue indicates no logic.
|
|
</pre>
|
|
|
|
<pre class="badcode">while (condition); // Bad - looks like part of do/while loop.
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Pointer_and_Reference_Expressions">Pointer and Reference Expressions</h3>
|
|
|
|
<div class="summary">
|
|
<p>No spaces around period or arrow. Pointer operators do not
|
|
have trailing spaces.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>The following are examples of correctly-formatted
|
|
pointer and reference expressions:</p>
|
|
|
|
<pre>x = *p;
|
|
p = &x;
|
|
x = r.y;
|
|
x = r->y;
|
|
</pre>
|
|
|
|
<p>Note that:</p>
|
|
|
|
<ul>
|
|
<li>There are no spaces around the period or arrow when
|
|
accessing a member.</li>
|
|
|
|
<li>Pointer operators have no space after the
|
|
<code>*</code> or <code>&</code>.</li>
|
|
</ul>
|
|
|
|
<p>When declaring a pointer variable or argument, you may
|
|
place the asterisk adjacent to either the type or to the
|
|
variable name:</p>
|
|
|
|
<pre>// These are fine, space preceding.
|
|
char *c;
|
|
const string &str;
|
|
|
|
// These are fine, space following.
|
|
char* c;
|
|
const string& str;
|
|
</pre>
|
|
|
|
It is allowed (if unusual) to declare multiple variables in the same
|
|
declaration, but it is disallowed if any of those have pointer or
|
|
reference decorations. Such declarations are easily misread.
|
|
<pre>// Fine if helpful for readability.
|
|
int x, y;
|
|
</pre>
|
|
<pre class="badcode">int x, *y; // Disallowed - no & or * in multiple declaration
|
|
char * c; // Bad - spaces on both sides of *
|
|
const string & str; // Bad - spaces on both sides of &
|
|
</pre>
|
|
|
|
<p>You should do this consistently within a single
|
|
file,
|
|
so, when modifying an existing file, use the style in
|
|
that file.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Boolean_Expressions">Boolean Expressions</h3>
|
|
|
|
<div class="summary">
|
|
<p>When you have a boolean expression that is longer than the
|
|
<a href="#Line_Length">standard line length</a>, be
|
|
consistent in how you break up the lines.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>In this example, the logical AND operator is always at
|
|
the end of the lines:</p>
|
|
|
|
<pre>if (this_one_thing > this_other_thing &&
|
|
a_third_thing == a_fourth_thing &&
|
|
yet_another && last_one) {
|
|
...
|
|
}
|
|
</pre>
|
|
|
|
<p>Note that when the code wraps in this example, both of
|
|
the <code>&&</code> logical AND operators are at
|
|
the end of the line. This is more common in Google code,
|
|
though wrapping all operators at the beginning of the
|
|
line is also allowed. Feel free to insert extra
|
|
parentheses judiciously because they can be very helpful
|
|
in increasing readability when used
|
|
appropriately. Also note that you should always use
|
|
the punctuation operators, such as
|
|
<code>&&</code> and <code>~</code>, rather than
|
|
the word operators, such as <code>and</code> and
|
|
<code>compl</code>.</p>
|
|
|
|
</div>
|
|
|
|
<h3 id="Return_Values">Return Values</h3>
|
|
|
|
<div class="summary">
|
|
<p>Do not needlessly surround the <code>return</code>
|
|
expression with parentheses.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Use parentheses in <code>return expr;</code> only
|
|
where you would use them in <code>x = expr;</code>.</p>
|
|
|
|
<pre>return result; // No parentheses in the simple case.
|
|
// Parentheses OK to make a complex expression more readable.
|
|
return (some_long_condition &&
|
|
another_condition);
|
|
</pre>
|
|
|
|
<pre class="badcode">return (value); // You wouldn't write var = (value);
|
|
return(result); // return is not a function!
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
|
|
|
|
<h3 id="Variable_and_Array_Initialization">Variable and Array Initialization</h3>
|
|
|
|
<div class="summary">
|
|
<p>Your choice of <code>=</code>, <code>()</code>, or
|
|
<code>{}</code>.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>You may choose between <code>=</code>,
|
|
<code>()</code>, and <code>{}</code>; the following are
|
|
all correct:</p>
|
|
|
|
<pre>int x = 3;
|
|
int x(3);
|
|
int x{3};
|
|
string name = "Some Name";
|
|
string name("Some Name");
|
|
string name{"Some Name"};
|
|
</pre>
|
|
|
|
<p>Be careful when using a braced initialization list <code>{...}</code>
|
|
on a type with an <code>std::initializer_list</code> constructor.
|
|
A nonempty <i>braced-init-list</i> prefers the
|
|
<code>std::initializer_list</code> constructor whenever
|
|
possible. Note that empty braces <code>{}</code> are special, and
|
|
will call a default constructor if available. To force the
|
|
non-<code>std::initializer_list</code> constructor, use parentheses
|
|
instead of braces.</p>
|
|
|
|
<pre>std::vector<int> v(100, 1); // A vector of 100 1s.
|
|
std::vector<int> v{100, 1}; // A vector of 100, 1.
|
|
</pre>
|
|
|
|
<p>Also, the brace form prevents narrowing of integral
|
|
types. This can prevent some types of programming
|
|
errors.</p>
|
|
|
|
<pre>int pi(3.14); // OK -- pi == 3.
|
|
int pi{3.14}; // Compile error: narrowing conversion.
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Preprocessor_Directives">Preprocessor Directives</h3>
|
|
|
|
<div class="summary">
|
|
<p>The hash mark that starts a preprocessor directive should
|
|
always be at the beginning of the line.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>Even when preprocessor directives are within the body
|
|
of indented code, the directives should start at the
|
|
beginning of the line.</p>
|
|
|
|
<pre>// Good - directives at beginning of line
|
|
if (lopsided_score) {
|
|
#if DISASTER_PENDING // Correct -- Starts at beginning of line
|
|
DropEverything();
|
|
# if NOTIFY // OK but not required -- Spaces after #
|
|
NotifyClient();
|
|
# endif
|
|
#endif
|
|
BackToNormal();
|
|
}
|
|
</pre>
|
|
|
|
<pre class="badcode">// Bad - indented directives
|
|
if (lopsided_score) {
|
|
#if DISASTER_PENDING // Wrong! The "#if" should be at beginning of line
|
|
DropEverything();
|
|
#endif // Wrong! Do not indent "#endif"
|
|
BackToNormal();
|
|
}
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Class_Format">Class Format</h3>
|
|
|
|
<div class="summary">
|
|
<p>Sections in <code>public</code>, <code>protected</code> and
|
|
<code>private</code> order, each indented one space.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>The basic format for a class definition (lacking the
|
|
comments, see <a href="#Class_Comments">Class
|
|
Comments</a> for a discussion of what comments are
|
|
needed) is:</p>
|
|
|
|
<pre>class MyClass : public OtherClass {
|
|
public: // Note the 1 space indent!
|
|
MyClass(); // Regular 2 space indent.
|
|
explicit MyClass(int var);
|
|
~MyClass() {}
|
|
|
|
void SomeFunction();
|
|
void SomeFunctionThatDoesNothing() {
|
|
}
|
|
|
|
void set_some_var(int var) { some_var_ = var; }
|
|
int some_var() const { return some_var_; }
|
|
|
|
private:
|
|
bool SomeInternalFunction();
|
|
|
|
int some_var_;
|
|
int some_other_var_;
|
|
};
|
|
</pre>
|
|
|
|
<p>Things to note:</p>
|
|
|
|
<ul>
|
|
<li>Any base class name should be on the same line as
|
|
the subclass name, subject to the 80-column limit.</li>
|
|
|
|
<li>The <code>public:</code>, <code>protected:</code>,
|
|
and <code>private:</code> keywords should be indented
|
|
one space.</li>
|
|
|
|
<li>Except for the first instance, these keywords
|
|
should be preceded by a blank line. This rule is
|
|
optional in small classes.</li>
|
|
|
|
<li>Do not leave a blank line after these
|
|
keywords.</li>
|
|
|
|
<li>The <code>public</code> section should be first,
|
|
followed by the <code>protected</code> and finally the
|
|
<code>private</code> section.</li>
|
|
|
|
<li>See <a href="#Declaration_Order">Declaration
|
|
Order</a> for rules on ordering declarations within
|
|
each of these sections.</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<h3 id="Constructor_Initializer_Lists">Constructor Initializer Lists</h3>
|
|
|
|
<div class="summary">
|
|
<p>Constructor initializer lists can be all on one line or
|
|
with subsequent lines indented four spaces.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>The acceptable formats for initializer lists are:</p>
|
|
|
|
<pre>// When everything fits on one line:
|
|
MyClass::MyClass(int var) : some_var_(var) {
|
|
DoSomething();
|
|
}
|
|
|
|
// If the signature and initializer list are not all on one line,
|
|
// you must wrap before the colon and indent 4 spaces:
|
|
MyClass::MyClass(int var)
|
|
: some_var_(var), some_other_var_(var + 1) {
|
|
DoSomething();
|
|
}
|
|
|
|
// When the list spans multiple lines, put each member on its own line
|
|
// and align them:
|
|
MyClass::MyClass(int var)
|
|
: some_var_(var), // 4 space indent
|
|
some_other_var_(var + 1) { // lined up
|
|
DoSomething();
|
|
}
|
|
|
|
// As with any other code block, the close curly can be on the same
|
|
// line as the open curly, if it fits.
|
|
MyClass::MyClass(int var)
|
|
: some_var_(var) {}
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Namespace_Formatting">Namespace Formatting</h3>
|
|
|
|
<div class="summary">
|
|
<p>The contents of namespaces are not indented.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p><a href="#Namespaces">Namespaces</a> do not add an
|
|
extra level of indentation. For example, use:</p>
|
|
|
|
<pre>namespace {
|
|
|
|
void foo() { // Correct. No extra indentation within namespace.
|
|
...
|
|
}
|
|
|
|
} // namespace
|
|
</pre>
|
|
|
|
<p>Do not indent within a namespace:</p>
|
|
|
|
<pre class="badcode">namespace {
|
|
|
|
// Wrong. Indented when it should not be.
|
|
void foo() {
|
|
...
|
|
}
|
|
|
|
} // namespace
|
|
</pre>
|
|
|
|
<p>When declaring nested namespaces, put each namespace
|
|
on its own line.</p>
|
|
|
|
<pre>namespace foo {
|
|
namespace bar {
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Horizontal_Whitespace">Horizontal Whitespace</h3>
|
|
|
|
<div class="summary">
|
|
<p>Use of horizontal whitespace depends on location. Never put
|
|
trailing whitespace at the end of a line.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<h4 class="stylepoint_subsection">General</h4>
|
|
|
|
<pre>void f(bool b) { // Open braces should always have a space before them.
|
|
...
|
|
int i = 0; // Semicolons usually have no space before them.
|
|
// Spaces inside braces for braced-init-list are optional. If you use them,
|
|
// put them on both sides!
|
|
int x[] = { 0 };
|
|
int x[] = {0};
|
|
|
|
// Spaces around the colon in inheritance and initializer lists.
|
|
class Foo : public Bar {
|
|
public:
|
|
// For inline function implementations, put spaces between the braces
|
|
// and the implementation itself.
|
|
Foo(int b) : Bar(), baz_(b) {} // No spaces inside empty braces.
|
|
void Reset() { baz_ = 0; } // Spaces separating braces from implementation.
|
|
...
|
|
</pre>
|
|
|
|
<p>Adding trailing whitespace can cause extra work for
|
|
others editing the same file, when they merge, as can
|
|
removing existing trailing whitespace. So: Don't
|
|
introduce trailing whitespace. Remove it if you're
|
|
already changing that line, or do it in a separate
|
|
clean-up
|
|
operation (preferably when no-one
|
|
else is working on the file).</p>
|
|
|
|
<h4 class="stylepoint_subsection">Loops and Conditionals</h4>
|
|
|
|
<pre>if (b) { // Space after the keyword in conditions and loops.
|
|
} else { // Spaces around else.
|
|
}
|
|
while (test) {} // There is usually no space inside parentheses.
|
|
switch (i) {
|
|
for (int i = 0; i < 5; ++i) {
|
|
// Loops and conditions may have spaces inside parentheses, but this
|
|
// is rare. Be consistent.
|
|
switch ( i ) {
|
|
if ( test ) {
|
|
for ( int i = 0; i < 5; ++i ) {
|
|
// For loops always have a space after the semicolon. They may have a space
|
|
// before the semicolon, but this is rare.
|
|
for ( ; i < 5 ; ++i) {
|
|
...
|
|
|
|
// Range-based for loops always have a space before and after the colon.
|
|
for (auto x : counts) {
|
|
...
|
|
}
|
|
switch (i) {
|
|
case 1: // No space before colon in a switch case.
|
|
...
|
|
case 2: break; // Use a space after a colon if there's code after it.
|
|
</pre>
|
|
|
|
<h4 class="stylepoint_subsection">Operators</h4>
|
|
|
|
<pre>// Assignment operators always have spaces around them.
|
|
x = 0;
|
|
|
|
// Other binary operators usually have spaces around them, but it's
|
|
// OK to remove spaces around factors. Parentheses should have no
|
|
// internal padding.
|
|
v = w * x + y / z;
|
|
v = w*x + y/z;
|
|
v = w * (x + z);
|
|
|
|
// No spaces separating unary operators and their arguments.
|
|
x = -5;
|
|
++x;
|
|
if (x && !y)
|
|
...
|
|
</pre>
|
|
|
|
<h4 class="stylepoint_subsection">Templates and Casts</h4>
|
|
|
|
<pre>// No spaces inside the angle brackets (< and >), before
|
|
// <, or between >( in a cast
|
|
std::vector<string> x;
|
|
y = static_cast<char*>(x);
|
|
|
|
// Spaces between type and pointer are OK, but be consistent.
|
|
std::vector<char *> x;
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<h3 id="Vertical_Whitespace">Vertical Whitespace</h3>
|
|
|
|
<div class="summary">
|
|
<p>Minimize use of vertical whitespace.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>This is more a principle than a rule: don't use blank
|
|
lines when you don't have to. In particular, don't put
|
|
more than one or two blank lines between functions,
|
|
resist starting functions with a blank line, don't end
|
|
functions with a blank line, and be discriminating with
|
|
your use of blank lines inside functions.</p>
|
|
|
|
<p>The basic principle is: The more code that fits on one
|
|
screen, the easier it is to follow and understand the
|
|
control flow of the program. Of course, readability can
|
|
suffer from code being too dense as well as too spread
|
|
out, so use your judgement. But in general, minimize use
|
|
of vertical whitespace.</p>
|
|
|
|
<p>Some rules of thumb to help when blank lines may be
|
|
useful:</p>
|
|
|
|
<ul>
|
|
<li>Blank lines at the beginning or end of a function
|
|
very rarely help readability.</li>
|
|
|
|
<li>Blank lines inside a chain of if-else blocks may
|
|
well help readability.</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<h2 id="Exceptions_to_the_Rules">Exceptions to the Rules</h2>
|
|
|
|
<p>The coding conventions described above are mandatory.
|
|
However, like all good rules, these sometimes have exceptions,
|
|
which we discuss here.</p>
|
|
|
|
|
|
|
|
<div>
|
|
<h3 id="Existing_Non-conformant_Code">Existing Non-conformant Code</h3>
|
|
|
|
<div class="summary">
|
|
<p>You may diverge from the rules when dealing with code that
|
|
does not conform to this style guide.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
|
|
<p>If you find yourself modifying code that was written
|
|
to specifications other than those presented by this
|
|
guide, you may have to diverge from these rules in order
|
|
to stay consistent with the local conventions in that
|
|
code. If you are in doubt about how to do this, ask the
|
|
original author or the person currently responsible for
|
|
the code. Remember that <em>consistency</em> includes
|
|
local consistency, too.</p>
|
|
|
|
</div>
|
|
</div>
|
|
|
|
|
|
|
|
<h3 id="Windows_Code">Windows Code</h3>
|
|
|
|
<div class="summary">
|
|
<p> Windows
|
|
programmers have developed their own set of coding
|
|
conventions, mainly derived from the conventions in Windows
|
|
headers and other Microsoft code. We want to make it easy
|
|
for anyone to understand your code, so we have a single set
|
|
of guidelines for everyone writing C++ on any platform.</p>
|
|
</div>
|
|
|
|
<div class="stylebody">
|
|
<p>It is worth reiterating a few of the guidelines that
|
|
you might forget if you are used to the prevalent Windows
|
|
style:</p>
|
|
|
|
<ul>
|
|
<li>Do not use Hungarian notation (for example, naming
|
|
an integer <code>iNum</code>). Use the Google naming
|
|
conventions, including the <code>.cc</code> extension
|
|
for source files.</li>
|
|
|
|
<li>Windows defines many of its own synonyms for
|
|
primitive types, such as <code>DWORD</code>,
|
|
<code>HANDLE</code>, etc. It is perfectly acceptable,
|
|
and encouraged, that you use these types when calling
|
|
Windows API functions. Even so, keep as close as you
|
|
can to the underlying C++ types. For example, use
|
|
<code>const TCHAR *</code> instead of
|
|
<code>LPCTSTR</code>.</li>
|
|
|
|
<li>When compiling with Microsoft Visual C++, set the
|
|
compiler to warning level 3 or higher, and treat all
|
|
warnings as errors.</li>
|
|
|
|
<li>Do not use <code>#pragma once</code>; instead use
|
|
the standard Google include guards. The path in the
|
|
include guards should be relative to the top of your
|
|
project tree.</li>
|
|
|
|
<li>In fact, do not use any nonstandard extensions,
|
|
like <code>#pragma</code> and <code>__declspec</code>,
|
|
unless you absolutely must. Using
|
|
<code>__declspec(dllimport)</code> and
|
|
<code>__declspec(dllexport)</code> is allowed; however,
|
|
you must use them through macros such as
|
|
<code>DLLIMPORT</code> and <code>DLLEXPORT</code>, so
|
|
that someone can easily disable the extensions if they
|
|
share the code.</li>
|
|
</ul>
|
|
|
|
<p>However, there are just a few rules that we
|
|
occasionally need to break on Windows:</p>
|
|
|
|
<ul>
|
|
<li>Normally we <a href="#Multiple_Inheritance">forbid
|
|
the use of multiple implementation inheritance</a>;
|
|
however, it is required when using COM and some ATL/WTL
|
|
classes. You may use multiple implementation
|
|
inheritance to implement COM or ATL/WTL classes and
|
|
interfaces.</li>
|
|
|
|
<li>Although you should not use exceptions in your own
|
|
code, they are used extensively in the ATL and some
|
|
STLs, including the one that comes with Visual C++.
|
|
When using the ATL, you should define
|
|
<code>_ATL_NO_EXCEPTIONS</code> to disable exceptions.
|
|
You should investigate whether you can also disable
|
|
exceptions in your STL, but if not, it is OK to turn on
|
|
exceptions in the compiler. (Note that this is only to
|
|
get the STL to compile. You should still not write
|
|
exception handling code yourself.)</li>
|
|
|
|
<li>The usual way of working with precompiled headers
|
|
is to include a header file at the top of each source
|
|
file, typically with a name like <code>StdAfx.h</code>
|
|
or <code>precompile.h</code>. To make your code easier
|
|
to share with other projects, avoid including this file
|
|
explicitly (except in <code>precompile.cc</code>), and
|
|
use the <code>/FI</code> compiler option to include the
|
|
file automatically.</li>
|
|
|
|
<li>Resource headers, which are usually named
|
|
<code>resource.h</code> and contain only macros, do not
|
|
need to conform to these style guidelines.</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<h2 class="ignoreLink">Parting Words</h2>
|
|
|
|
<p>Use common sense and <em>BE CONSISTENT</em>.</p>
|
|
|
|
<p>If you are editing code, take a few minutes to look at the
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code around you and determine its style. If they use spaces
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around their <code>if</code> clauses, you should, too. If their
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comments have little boxes of stars around them, make your
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comments have little boxes of stars around them too.</p>
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<p>The point of having style guidelines is to have a common
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vocabulary of coding so people can concentrate on what you are
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saying, rather than on how you are saying it. We present global
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style rules here so people know the vocabulary. But local style
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is also important. If code you add to a file looks drastically
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different from the existing code around it, the discontinuity
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throws readers out of their rhythm when they go to read it. Try
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to avoid this.</p>
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<p>OK, enough writing about writing code; the code itself is much
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more interesting. Have fun!</p>
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