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mark@chromium.org 7b24563e08 Update C++ style guide to 3.274: 
- Change formatting rules of braced initializers.
 - Permit use of constexpr and allow constexpr global variables.
 - Allow all C++11 features except for those that are specifically banned.
 - Fix/add C99 format specifiers for ptrdiff_t and ssize_t.
 - Add lambda expressions to the list of explicitly banned C++11 features.
 - Relax "return type is always on the same line as the function name" rule.
 - Allow unique_ptr, discourage ownership transfer. Allow noncopyable std::move.
 - Allow system-specific includes after other includes.
 - Add boost/math/distributions to the set of permitted Boost libraries.

Update Objective-C style guide to 2.59:
 - Use instancetype as return type for example init methods.
 - Remove invalid +stringWithInt: call.
 - Remove reference to pre-Objective-C 2.0 declaration requirements.
 - Remove reference to Objective-C exception macros.
 - Remove reference to informal protocols as an alternative to optional methods.
 - Class headers should include comments documenting non-trivial interfaces.
 - Don't specify that blocks are preferable to methods as callbacks.
 - Specify "strong" and "weak" as comments for non-Objective-C pointers.
 - Replace improper reference to ownership of a retained object.
 - Clarify some aspects of method ordering rules.
 - Prefixes are required for shared code and optional for applications.
 - Clarify that nil pointers are safe as receivers, not necessarily parameters.
 - Clarify that delegate pointers should typically be zeroing weak pointers.
 - Allow a 100-column limit, except for projects that choose to use 80.

Update Python style guide to 2.59:
 - Add more examples of bad code to the default arguments section.
 - Allow ''' when ' is used as the single quote within a file.
 - Remove references to pychecker. Recommend pylint.
 - Add more examples to the indentation section.

Update JavaScript style guide to 2.93:
 - Add @nocompile.
 - Fix a few typos.
 - When wrapping lines, indent more deeply for child expressions.
 - Document that @const can be used on a constructor.
 - Update eval section to discourage using eval for RPC.
 - Update an example to avoid encouraging using numbers as booleans.
 - Allow for no indentation of @desc jsdoc tags.
 - Add @public discussion.

Update shell style guide to 1.26:
 - Add a section on style for case statements.

Update Common Lisp style guide to 1.23:
 - fare-matcher was superseded by optima.
 - Clarify wording regarding DYNAMIC-EXTENT.
2013-09-25 21:16:00 +00:00

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<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="styleguide.xsl"?>
<GUIDE title="Google C++ Style Guide">
<p align="right">
Revision 3.274
</p>
<address>
Benjy Weinberger<br/>
Craig Silverstein<br/>
Gregory Eitzmann<br/>
Mark Mentovai<br/>
Tashana Landray
</address>
<OVERVIEW>
<CATEGORY title="Important Note">
<STYLEPOINT title="Displaying Hidden Details in this Guide">
<SUMMARY>
This style guide contains many details that are initially
hidden from view. They are marked by the triangle icon, which you
see here on your left. Click it now.
You should see "Hooray" appear below.
</SUMMARY>
<BODY>
<p>
Hooray! Now you know you can expand points to get more
details. Alternatively, there's an "expand all" at the
top of this document.
</p>
</BODY>
</STYLEPOINT>
</CATEGORY>
<CATEGORY title="Background">
<p>
C++ is the main development language
used by many of Google's open-source
projects.
As every C++ programmer knows, the language has many powerful features,
but this power brings with it complexity, which in turn can make code
more bug-prone and harder to read and maintain.
</p>
<p>
The goal of this guide is to manage this complexity by describing
in detail the dos and don'ts of writing C++
code. These rules exist to
keep
the
code base manageable while still allowing coders to use C++ language
features productively.
</p>
<p>
<em>Style</em>, also known as readability, is what we call the
conventions that govern our C++ code. The term Style is a bit of a
misnomer, since these conventions cover far more than just source
file formatting.
</p>
<p>
One way in which we keep the code base manageable is by enforcing
<em>consistency</em>.
It is very important that any
programmer
be able to look at another's code and quickly understand it.
Maintaining a uniform style and following conventions means that we can
more easily use "pattern-matching" to infer what various symbols are
and what invariants are true about them. Creating common, required
idioms and patterns makes code much easier to understand. In some
cases there might be good arguments for changing certain style
rules, but we nonetheless keep things as they are in order to
preserve consistency.
</p>
<p>
Another issue this guide addresses is that of C++ feature bloat.
C++ is a huge language with many advanced features. In some cases
we constrain, or even ban, use of certain features. We do this to
keep code simple and to avoid the various common errors and
problems that these features can cause. This guide lists these
features and explains why their use is restricted.
</p>
<p>
Open-source projects developed by Google
conform to the requirements in this guide.
</p>
<p>
Note that this guide is not a C++ tutorial: we assume that the
reader is familiar with the language.
</p>
</CATEGORY>
</OVERVIEW>
<CATEGORY title="Header Files">
<p>
In general, every <code>.cc</code> file should have an associated
<code>.h</code> file. There are some common exceptions, such as
unittests
and small <code>.cc</code> files containing just a <code>main()</code>
function.
</p>
<p>
Correct use of header files can make a huge difference to the
readability, size and performance of your code.
</p>
<p>
The following rules will guide you through the various pitfalls of
using header files.
</p>
<STYLEPOINT title="The #define Guard">
<SUMMARY>
All header files should have <code>#define</code> guards to
prevent multiple inclusion. The format of the symbol name
should be
<code><i>&lt;PROJECT&gt;</i>_<i>&lt;PATH&gt;</i>_<i>&lt;FILE&gt;</i>_H_</code>.
</SUMMARY>
<BODY>
<p>
To guarantee uniqueness, they should be based on the full path
in a project's source tree. For example, the file
<code>foo/src/bar/baz.h</code> in project <code>foo</code> should
have the following guard:
</p>
<CODE_SNIPPET>
#ifndef FOO_BAR_BAZ_H_
#define FOO_BAR_BAZ_H_
...
#endif // FOO_BAR_BAZ_H_
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Forward Declarations">
<SUMMARY>
You may forward declare ordinary classes in order to avoid
unnecessary <code>#include</code>s.
</SUMMARY>
<BODY>
<DEFINITION>
A "forward declaration" is a declaration of a class, function,
or template without an associated definition. <code>#include</code>
lines can often be replaced with forward declarations of whatever
symbols are actually used by the client code.
</DEFINITION>
<PROS>
<ul>
<li>Unnecessary <code>#include</code>s force the compiler to open
more files and process more input.</li>
<li>They can also force your code to be recompiled more often, due
to changes in the header.</li>
</ul>
</PROS>
<CONS>
<ul>
<li>It can be difficult to determine the correct form of a
forward declaration in the presence of features like templates,
typedefs, default parameters, and using declarations.</li>
<li>It can be difficult to determine whether a forward declaration
or a full <code>#include</code> is needed for a given piece of code,
particularly when implicit conversion operations are involved. In
extreme cases, replacing an <code>#include</code> with a forward
declaration can silently change the meaning of code.</li>
<li>Forward declaring multiple symbols from a header can be more
verbose than simply <code>#include</code>ing the header.</li>
<li>Forward declarations of functions and templates can prevent
the header owners from making otherwise-compatible changes to
their APIs; for example, widening a parameter type, or adding
a template parameter with a default value.</li>
<li>Forward declaring symbols from namespace <code>std::</code>
usually yields undefined behavior.</li>
<li>Structuring code to enable forward declarations (e.g.
using pointer members instead of object members) can make the
code slower and more complex.</li>
<li>The practical efficiency benefits of forward declarations are
unproven.</li>
</ul>
</CONS>
<DECISION>
<ul>
<li>When using a function declared in a header file, always
<code>#include</code> that header.</li>
<li>When using a class template, prefer to <code>#include</code> its
header file.</li>
<li>When using an ordinary class, relying on a forward declaration
is OK, but be wary of situations where a forward declaration may
be insufficient or incorrect; when in doubt, just
<code>#include</code> the appropriate header.</li>
<li>Do not replace data members with pointers just to avoid an
<code>#include</code>.</li>
</ul>
Always <code>#include</code> the file that actually provides the
declarations/definitions you need; do not rely on the symbol being
brought in transitively via headers not directly included. One
exception is that <code>myfile.cc</code> may rely on
<code>#include</code>s and forward declarations from its corresponding
header file <code>myfile.h</code>.
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Inline Functions">
<SUMMARY>
Define functions inline only when they are small, say, 10 lines
or less.
</SUMMARY>
<BODY>
<DEFINITION>
You can declare functions in a way that allows the compiler to
expand them inline rather than calling them through the usual
function call mechanism.
</DEFINITION>
<PROS>
Inlining a function can generate more efficient object code,
as long as the inlined function is small. Feel free to inline
accessors and mutators, and other short, performance-critical
functions.
</PROS>
<CONS>
Overuse of inlining can actually make programs slower.
Depending on a function's size, inlining it can cause the code
size to increase or decrease. Inlining a very small accessor
function will usually decrease code size while inlining a very
large function can dramatically increase code size. On modern
processors smaller code usually runs faster due to better use
of the instruction cache.
</CONS>
<DECISION>
<p>
A decent rule of thumb is to not inline a function if it is
more than 10 lines long. Beware of destructors, which are
often longer than they appear because of implicit member-
and base-destructor calls!
</p>
<p>
Another useful rule of thumb: it's typically not cost
effective to inline functions with loops or switch
statements (unless, in the common case, the loop or switch
statement is never executed).
</p>
<p>
It is important to know that functions are not always
inlined even if they are declared as such; for example,
virtual and recursive functions are not normally inlined.
Usually recursive functions should not be inline. The main
reason for making a virtual function inline is to place its
definition in the class, either for convenience or to
document its behavior, e.g., for accessors and mutators.
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="The -inl.h Files">
<SUMMARY>
You may use file names with a <code>-inl.h</code> suffix to define
complex inline functions when needed.
</SUMMARY>
<BODY>
<p>
The definition of an inline function needs to be in a header
file, so that the compiler has the definition available for
inlining at the call sites. However, implementation code
properly belongs in <code>.cc</code> files, and we do not like
to have much actual code in <code>.h</code> files unless there
is a readability or performance advantage.
</p>
<p>
If an inline function definition is short, with very little,
if any, logic in it, you should put the code in your
<code>.h</code> file. For example, accessors and mutators
should certainly be inside a class definition. More complex
inline functions may also be put in a <code>.h</code> file for
the convenience of the implementer and callers, though if this
makes the <code>.h</code> file too unwieldy you can instead
put that code in a separate <code>-inl.h</code> file.
This separates the implementation from the class definition,
while still allowing the implementation to be included where
necessary.
</p>
<p>
Another use of <code>-inl.h</code> files is for definitions of
function templates. This can be used to keep your template
definitions easy to read.
</p>
<p>
Do not forget that a <code>-inl.h</code> file requires a
<a href="#The__define_Guard"><code>#define</code> guard</a> just
like any other header file.
</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Function Parameter Ordering">
<SUMMARY>
When defining a function, parameter order is: inputs,
then outputs.
</SUMMARY>
<BODY>
<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 non-<code>const</code>
pointers. 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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Names and Order of Includes">
<SUMMARY>
Use standard order for readability and to avoid hidden
dependencies: C library, C++ library,
other libraries' <code>.h</code>, your
project's
<code>.h</code>.
</SUMMARY>
<BODY>
<p>
All of a project's header files should be
listed as descendants of the project's source directory
without use of UNIX directory shortcuts <code>.</code> (the current
directory) or <code>..</code> (the parent directory). For
example,
<code>google-awesome-project/src/base/logging.h</code>
should be included as
</p>
<CODE_SNIPPET>
#include "base/logging.h"
</CODE_SNIPPET>
<p>
In <code><var>dir/foo</var>.cc</code> or <code><var>dir/foo_test</var>.cc</code>,
whose main purpose is to implement or test the stuff in
<code><var>dir2/foo2</var>.h</code>, order your includes as
follows:
</p>
<ol>
<li> <code><var>dir2/foo2</var>.h</code> (preferred location
— see details below).</li>
<li> C system files.</li>
<li> C++ system files.</li>
<li> Other libraries' <code>.h</code> files.</li>
<li>
Your project's
<code>.h</code> files.</li>
</ol>
<p>
With the preferred ordering, if <code><var>dir2/foo2</var>.h</code>
omits any necessary includes, the build of
<code><var>dir/foo</var>.cc</code> or
<code><var>dir/foo</var>_test.cc</code> will break.
Thus, this rule ensures that build breaks show up first
for the people working on these files, not for innocent people
in other packages.
</p>
<p>
<code><var>dir/foo</var>.cc</code> and
<code><var>dir2/foo2</var>.h</code> are often in the same
directory (e.g. <code>base/basictypes_test.cc</code> and
<code>base/basictypes.h</code>), but can be in different
directories too.
</p>
<p>
Within each section the includes should be ordered alphabetically.
Note that older code might not conform to this rule and should be
fixed when convenient.
</p>
<p>
For example, the includes in
<code>google-awesome-project/src/foo/internal/fooserver.cc</code>
might look like this:
</p>
<CODE_SNIPPET>
#include "foo/public/fooserver.h" // Preferred location.
#include &lt;sys/types.h&gt;
#include &lt;unistd.h&gt;
#include &lt;hash_map&gt;
#include &lt;vector&gt;
#include "base/basictypes.h"
#include "base/commandlineflags.h"
#include "foo/public/bar.h"
</CODE_SNIPPET>
<p>
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>
<CODE_SNIPPET>
#include "foo/public/fooserver.h"
#include "base/port.h" // For LANG_CXX11.
#ifdef LANG_CXX11
#include &lt;initializer_list&gt;
#endif // LANG_CXX11
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
</CATEGORY>
<CATEGORY title="Scoping">
<STYLEPOINT title="Namespaces">
<SUMMARY>
Unnamed namespaces in <code>.cc</code> files are encouraged. With
named namespaces, choose the name based on the
project, and possibly its path.
Do not use a <SYNTAX>using-directive</SYNTAX>.
Do not use inline namespaces.
</SUMMARY>
<BODY>
<DEFINITION>
Namespaces subdivide the global scope into distinct, named
scopes, and so are useful for preventing name collisions in
the global scope.
</DEFINITION>
<PROS>
<p>
Namespaces provide a (hierarchical) axis of naming, in
addition to the (also hierarchical) name axis provided by
classes.
</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.
</p>
<p>
Inline namespaces automatically place their names in the
enclosing scope. Consider the following snippet, for example:
</p>
<CODE_SNIPPET>
namespace X {
inline namespace Y {
void foo();
}
}
</CODE_SNIPPET>
<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>
</PROS>
<CONS>
<p>
Namespaces can be confusing, because they provide an
additional (hierarchical) axis of naming, in addition to the
(also hierarchical) name axis provided by classes.
</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>
Use of unnamed namespaces in header files can easily cause
violations of the C++ One Definition Rule (ODR).
</p>
</CONS>
<DECISION>
<p>
Use namespaces according to the policy described below.
Terminate namespaces with comments as shown in the given examples.
</p>
<SUBSECTION title="Unnamed Namespaces">
<ul>
<li> Unnamed namespaces are allowed and even encouraged in
<code>.cc</code> files, to avoid runtime naming
conflicts:
<CODE_SNIPPET>
namespace { // This is in a .cc file.
// The content of a namespace is not indented
enum { kUnused, kEOF, kError }; // Commonly used tokens.
bool AtEof() { return pos_ == kEOF; } // Uses our namespace's EOF.
} // namespace
</CODE_SNIPPET>
<p>
However, file-scope declarations that are
associated with a particular class may be declared
in that class as types, static data members or
static member functions rather than as members of
an unnamed namespace.
</p>
</li>
<li> Do not use unnamed namespaces in <code>.h</code>
files.
</li>
</ul>
</SUBSECTION>
<SUBSECTION title="Named Namespaces">
<p>
Named namespaces should be used as follows:
</p>
<ul>
<li> Namespaces wrap the entire source file after includes,
<a href="http://google-gflags.googlecode.com/">gflags</a>
definitions/declarations, and forward declarations of classes
from other namespaces:
<CODE_SNIPPET>
// 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
</CODE_SNIPPET>
<CODE_SNIPPET>
// In the .cc file
namespace mynamespace {
// Definition of functions is within scope of the namespace.
void MyClass::Foo() {
...
}
} // namespace mynamespace
</CODE_SNIPPET>
<p>
The typical <code>.cc</code> file might have more
complex detail, including the need to reference classes
in other namespaces.
</p>
<CODE_SNIPPET>
#include "a.h"
DEFINE_bool(someflag, false, "dummy flag");
class C; // Forward declaration of class C in the global namespace.
namespace a { class A; } // Forward declaration of a::A.
namespace b {
...code for b... // Code goes against the left margin.
} // namespace b
</CODE_SNIPPET>
</li>
<li> Do not declare anything in namespace
<code>std</code>, not even 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> You may not use a <SYNTAX>using-directive</SYNTAX> to
make all names from a namespace available.
<BAD_CODE_SNIPPET>
// Forbidden -- This pollutes the namespace.
using namespace foo;
</BAD_CODE_SNIPPET>
</li>
<li> You may use a <SYNTAX>using-declaration</SYNTAX>
anywhere in a <code>.cc</code> file, and in functions,
methods or classes in <code>.h</code> files.
<CODE_SNIPPET>
// OK in .cc files.
// Must be in a function, method or class in .h files.
using ::foo::bar;
</CODE_SNIPPET>
</li>
<li> Namespace aliases are allowed anywhere in a
<code>.cc</code> file, anywhere inside the named
namespace that wraps an entire <code>.h</code> file,
and in functions and methods.
<CODE_SNIPPET>
// Shorten access to some commonly used names in .cc files.
namespace fbz = ::foo::bar::baz;
// Shorten access to some commonly used names (in a .h file).
namespace librarian {
// The following alias is available to all files including
// this header (in namespace librarian):
// alias names should therefore be chosen consistently
// within a project.
namespace pd_s = ::pipeline_diagnostics::sidetable;
inline void my_inline_function() {
// namespace alias local to a function (or method).
namespace fbz = ::foo::bar::baz;
...
}
} // namespace librarian
</CODE_SNIPPET>
<p>
Note that an alias in a .h file is visible to everyone
#including that file, so public headers (those available
outside a project) and headers transitively #included by them,
should avoid defining aliases, as part of the general
goal of keeping public APIs as small as possible.
</p>
</li>
<li> Do not use inline namespaces.
</li>
</ul>
</SUBSECTION>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Nested Classes">
<SUMMARY>
Although you may use public nested classes when they are part of
an interface, consider a <a HREF="#Namespaces">namespace</a> to
keep declarations out of the global scope.
</SUMMARY>
<BODY>
<DEFINITION>
A class can define another class within it; this is also
called a <SYNTAX>member class</SYNTAX>.
<CODE_SNIPPET>
class Foo {
private:
// Bar is a member class, nested within Foo.
class Bar {
...
};
};
</CODE_SNIPPET>
</DEFINITION>
<PROS>
This is useful when the nested (or member) class is only used
by the enclosing class; making it a member puts it in the
enclosing class scope rather than polluting the outer scope
with the class name. Nested classes can be forward declared
within the enclosing class and then defined in the
<code>.cc</code> file to avoid including the nested class
definition in the enclosing class declaration, since the
nested class definition is usually only relevant to the
implementation.
</PROS>
<CONS>
Nested classes can be forward-declared only within the
definition of the enclosing class. Thus, any header file
manipulating a <code>Foo::Bar*</code> pointer will have to
include the full class declaration for <code>Foo</code>.
</CONS>
<DECISION>
Do not make nested classes public unless they are actually
part of the interface, e.g., a class that holds a set of
options for some method.
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Nonmember, Static Member, and Global Functions">
<SUMMARY>
Prefer nonmember functions within a namespace or static member
functions to global functions; use completely global functions
rarely.
</SUMMARY>
<BODY>
<PROS>
Nonmember and static member functions can be useful in some
situations. Putting nonmember functions in a namespace avoids
polluting the global namespace.
</PROS>
<CONS>
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.
</CONS>
<DECISION>
<p>
Sometimes it is useful, or even necessary, 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.
</p>
<p>
Functions defined in the same compilation unit as production
classes may introduce unnecessary coupling and link-time
dependencies when directly called from other compilation
units; static member functions are particularly susceptible
to this. Consider extracting a new class, or placing the
functions in a namespace possibly in a separate library.
</p>
<p>
If you must define a nonmember function and it is only
needed in its <code>.cc</code> file, use an unnamed
<a HREF="#Namespaces">namespace</a> or <code>static</code>
linkage (eg <code>static int Foo() {...}</code>) to limit
its scope.
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Local Variables">
<SUMMARY>
Place a function's variables in the narrowest scope possible,
and initialize variables in the declaration.
</SUMMARY>
<BODY>
<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>
<BAD_CODE_SNIPPET>
int i;
i = f(); // Bad -- initialization separate from declaration.
</BAD_CODE_SNIPPET>
<CODE_SNIPPET>
int j = g(); // Good -- declaration has initialization.
</CODE_SNIPPET>
<BAD_CODE_SNIPPET>
vector&lt;int&gt; v;
v.push_back(1); // Prefer initializing using brace initialization.
v.push_back(2);
</BAD_CODE_SNIPPET>
<CODE_SNIPPET>
vector&lt;int&gt; v = {1, 2}; // Good -- v starts initialized.
</CODE_SNIPPET>
<p>
Note that gcc implements <code>for (int i = 0; i
&lt; 10; ++i)</code> correctly (the scope of <code>i</code> is
only the scope of the <code>for</code> loop), so you can then
reuse <code>i</code> in another <code>for</code> loop in the
same scope. It also correctly scopes declarations in
<code>if</code> and <code>while</code> statements, e.g.
</p>
<CODE_SNIPPET>
while (const char* p = strchr(str, '/')) str = p + 1;
</CODE_SNIPPET>
<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>
<BAD_CODE_SNIPPET>
// Inefficient implementation:
for (int i = 0; i &lt; 1000000; ++i) {
Foo f; // My ctor and dtor get called 1000000 times each.
f.DoSomething(i);
}
</BAD_CODE_SNIPPET>
<p>
It may be more efficient to declare such a variable used in a
loop outside that loop:
</p>
<CODE_SNIPPET>
Foo f; // My ctor and dtor get called once each.
for (int i = 0; i &lt; 1000000; ++i) {
f.DoSomething(i);
}
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Static and Global Variables">
<SUMMARY>
Static or global variables of class type 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.
</SUMMARY>
<BODY>
<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 static POD 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.
</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>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>
</BODY>
</STYLEPOINT>
</CATEGORY>
<CATEGORY title="Classes">
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.
<STYLEPOINT title="Doing Work in Constructors">
<SUMMARY>
Avoid doing complex initialization in constructors (in particular,
initialization that can fail or that requires virtual method calls).
</SUMMARY>
<BODY>
<DEFINITION>
It is possible to perform initialization in the body of the
constructor.
</DEFINITION>
<PROS>
Convenience in typing. No need to worry about whether the
class has been initialized or not.
</PROS>
<CONS>
The problems with doing work in constructors are:
<ul>
<li> There is no easy way for constructors to signal errors,
short of 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
indeterminate state.
</li>
<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> If someone creates a global variable of this type
(which is against the rules, but still), the
constructor code will be called before
<code>main()</code>, possibly breaking some implicit
assumptions in the constructor code. For instance,
<a href="http://google-gflags.googlecode.com/">gflags</a>
will not yet have been initialized.
</li>
</ul>
</CONS>
<DECISION>
Constructors should never call virtual functions or attempt to raise
non-fatal failures. If your object requires non-trivial
initialization, consider using a factory function or <code>Init()</code> method.
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Initialization">
<SUMMARY>
If your class defines member variables, you must provide an
in-class initializer for every member variable or write a constructor
(which can be a default constructor). If you do not declare
any constructors yourself then the compiler will generate a default
constructor for you, which may leave some fields uninitialized or
initialized to inappropriate values.
</SUMMARY>
<BODY>
<DEFINITION>
The default constructor is called when we <code>new</code> a class
object with no arguments. It is always called when calling
<code>new[]</code> (for arrays). In-class member initialization means
declaring a member variable using a construction like <code>int count_
= 17;</code> or <code>string name_{"abc"};</code>, as opposed to just
<code>int count_;</code> or <code>string name_;</code>.
</DEFINITION>
<PROS>
<p>
A user defined default constructor is used to initialize an object
if no initializer is provided. It can ensure that an object is
always in a valid and usable state as soon as it's constructed; it
can also ensure that an object is initially created in an obviously
"impossible" state, to aid debugging.
</p>
<p>
In-class member initialization ensures that a member variable will
be initialized appropriately without having to duplicate the
initialization code in multiple constructors. This can reduce bugs
where you add a new member variable, initialize it in one
constructor, and forget to put that initialization code in another
constructor.
</p>
</PROS>
<CONS>
<p>
Explicitly defining a default constructor is extra work for
you, the code writer.
</p>
<p>
In-class member initialization is potentially confusing if a member
variable is initialized as part of its declaration and also
initialized in a constructor, since the value in the constructor
will override the value in the declaration.
</p>
</CONS>
<DECISION>
<p>
Use in-class member initialization for simple initializations,
especially when a member variable must be initialized the same way
in more than one constructor.
</p>
<p>
If your class defines member variables that aren't
initialized in-class, and if it has no other constructors,
you must define a default constructor (one that takes no
arguments). It should preferably initialize the object in
such a way that its internal state is consistent and valid.
</p>
<p>
The reason for this is that if you have no other
constructors and do not define a default constructor, the
compiler will generate one for you. This compiler
generated constructor may not initialize your object
sensibly.
</p>
<p>
If your class inherits from an existing class but you add no
new member variables, you are not required to have a default
constructor.
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Explicit Constructors">
<SUMMARY>
Use the C++ keyword <code>explicit</code> for constructors with
one argument.
</SUMMARY>
<BODY>
<DEFINITION>
Normally, if a constructor takes one argument, it can be used
as a conversion. For instance, if you define
<code>Foo::Foo(string name)</code> and then pass a string to a
function that expects a <code>Foo</code>, the constructor will
be called to convert the string into a <code>Foo</code> and
will pass the <code>Foo</code> to your function for you. This
can be convenient but is also a source of trouble when things
get converted and new objects created without you meaning them
to. Declaring a constructor <code>explicit</code> prevents it
from being invoked implicitly as a conversion.
</DEFINITION>
<PROS>
Avoids undesirable conversions.
</PROS>
<CONS>
None.
</CONS>
<DECISION>
<p>
We require all single argument constructors to be
explicit. Always put <code>explicit</code> in front of
one-argument constructors in the class definition:
<code>explicit Foo(string name);</code>
</p>
<p>
The exception is copy constructors, which, in the rare
cases when we allow them, should probably not be
<code>explicit</code>.
Classes that are intended to be
transparent wrappers around other classes are also
exceptions.
Such exceptions should be clearly marked with comments.
</p>
<p>
Finally, constructors that take only an initializer_list may be
non-explicit. This is to permit construction of your type using the
assigment form for brace init lists (i.e. <code>MyType m = {1, 2}
</code>).
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Copy Constructors">
<SUMMARY>
Provide a copy constructor and assignment operator only when necessary.
Otherwise, disable them with <code>DISALLOW_COPY_AND_ASSIGN</code>.
</SUMMARY>
<BODY>
<DEFINITION>
The copy constructor and assignment operator are used to create copies
of objects. The copy constructor is implicitly invoked by the
compiler in some situations, e.g. passing objects by value.
</DEFINITION>
<PROS>
Copy constructors make it easy to copy objects. STL
containers require that all contents be copyable and
assignable. Copy constructors can be more efficient than
<code>CopyFrom()</code>-style workarounds because they combine
construction with copying, the compiler can elide them in some
contexts, and they make it easier to avoid heap allocation.
</PROS>
<CONS>
Implicit copying of objects in C++ is a rich source of bugs
and of performance problems. It also reduces readability, as
it becomes hard to track which objects are being passed around
by value as opposed to by reference, and therefore where
changes to an object are reflected.
</CONS>
<DECISION>
<p>
Few classes need to be copyable. Most should have neither a
copy constructor nor an assignment operator. In many situations,
a pointer or reference will work just as well as a copied value,
with better performance. For example, you can pass function
parameters by reference or pointer instead of by value, and you can
store pointers rather than objects in an STL container.
</p>
<p>
If your class needs to be copyable, prefer providing a copy method,
such as <code>CopyFrom()</code> or <code>Clone()</code>, rather than
a copy constructor, because such methods cannot be invoked
implicitly. If a copy method is insufficient in your situation
(e.g. for performance reasons, or because your class needs to be
stored by value in an STL container), provide both a copy
constructor and assignment operator.
</p>
<p>
If your class does not need a copy constructor or assignment
operator, you must explicitly disable them.
To do so, add dummy declarations for the copy constructor and
assignment operator in the <code>private:</code> section of your
class, but do not provide any corresponding definition (so that
any attempt to use them results in a link error).
</p>
<p>
For convenience, a <code>DISALLOW_COPY_AND_ASSIGN</code> macro
can be used:
</p>
<CODE_SNIPPET>
// A macro to disallow the copy constructor and operator= functions
// This should be used in the private: declarations for a class
#define DISALLOW_COPY_AND_ASSIGN(TypeName) \
TypeName(const TypeName&amp;); \
void operator=(const TypeName&amp;)
</CODE_SNIPPET>
<p>
Then, in <code>class Foo</code>:
</p>
<CODE_SNIPPET>
class Foo {
public:
Foo(int f);
~Foo();
private:
DISALLOW_COPY_AND_ASSIGN(Foo);
};
</CODE_SNIPPET>
<p>
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Delegating and inheriting constructors">
<SUMMARY>
Use delegating and inheriting constructors
when they reduce code duplication.
</SUMMARY>
<BODY>
<DEFINITION>
<p>
Delegating and inheriting constructors are two different features,
both introduced in C++11, for reducing code duplication in
constructors. Delegating constructors allow one of a class's
constructors to forward work to one of the class's other
constructors, using a special variant of the initialization list
syntax. For example:
</p>
<CODE_SNIPPET>
X::X(const string&amp; name) : name_(name) {
...
}
X::X() : X("") { }
</CODE_SNIPPET>
<p>
Inheriting constructors allow a derived class to have its base
class's constructors available directly, just as with any of the
base class's other member functions, instead of having to redeclare
them. This is especially useful if the base has multiple
constructors. For example:
</p>
<CODE_SNIPPET>
class Base {
public:
Base();
Base(int n);
Base(const string&amp; s);
...
};
class Derived : public Base {
public:
using Base::Base; // Base's constructors are redeclared here.
};
</CODE_SNIPPET>
<p>
This is especially useful when <code>Derived</code>'s constructors
don't have to do anything more than calling <code>Base</code>'s
constructors.
</p>
</DEFINITION>
<PROS>
<p>
Delegating and inheriting constructors reduce verbosity
and boilerplate, which can improve readability.
</p>
<p>
Delegating constructors are familiar to Java programmers.
</p>
</PROS>
<CONS>
<p>
It's possible to approximate the behavior of delegating constructors
by using a helper function.
</p>
<p>
Inheriting constructors may be confusing if a derived class
introduces new member variables, since the base class constructor
doesn't know about them.
</p>
</CONS>
<DECISION>
<p>
Use delegating and inheriting
constructors when they reduce boilerplate and improve
readability. Be cautious about inheriting
constructors when your derived class has new member variables.
Inheriting constructors may still be appropriate in that case
if you can use in-class member initialization for the derived
class's member variables.
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Structs vs. Classes">
<SUMMARY>
Use a <code>struct</code> only for passive objects that carry data;
everything else is a <code>class</code>.
</SUMMARY>
<BODY>
<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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Inheritance">
<SUMMARY>
Composition is often more appropriate than inheritance. When
using inheritance, make it <code>public</code>.
</SUMMARY>
<BODY>
<DEFINITION>
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.
</DEFINITION>
<PROS>
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.
</PROS>
<CONS>
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.
</CONS>
<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>
When redefining an inherited virtual function, explicitly
declare it <code>virtual</code> in the declaration of the
derived class. Rationale: If <code>virtual</code> is
omitted, the reader has to check all ancestors of the
class in question to determine if the function is virtual
or not.
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Multiple Inheritance">
<SUMMARY>
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.
</SUMMARY>
<BODY>
<DEFINITION>
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>.
</DEFINITION>
<PROS>
Multiple implementation inheritance may let you re-use even more code
than single inheritance (see <a HREF="#Inheritance">Inheritance</a>).
</PROS>
<CONS>
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.
</CONS>
<DECISION>
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.
<SUBSECTION title="Note:">
There is an <a HREF="#Windows_Code">exception</a> to this
rule on Windows.
</SUBSECTION>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Interfaces">
<SUMMARY>
Classes that satisfy certain conditions are allowed, but not required, to
end with an <code>Interface</code> suffix.
</SUMMARY>
<BODY>
<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>
</DEFINITION>
<PROS>
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.
</PROS>
<CONS>
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.
</CONS>
<DECISION>
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>.
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Operator Overloading">
<SUMMARY>
Do not overload operators except in rare, special circumstances.
Do not create user-defined literals.
</SUMMARY>
<BODY>
<DEFINITION>
A class can define that operators such as <code>+</code> and
<code>/</code> operate on the class as if it were a built-in
type. An overload of <code>operator""</code> allows
the built-in literal syntax to be used to create objects of
class types.
</DEFINITION>
<PROS>
<p>
Operator overloading can make code appear more intuitive because a
class will behave in the same way as built-in types (such as
<code>int</code>). Overloaded operators are more playful names for
functions that are less-colorfully named, such as
<code>Equals()</code> or <code>Add()</code>.
</p>
<p>
For some template functions to work correctly, you may need to
define operators.
</p>
<p>
User-defined literals are a very concise notation for creating
objects of user-defined types.
</p>
</PROS>
<CONS>
While operator overloading can make code more intuitive, it
has several drawbacks:
<ul>
<li> It can fool our intuition into thinking that expensive
operations are cheap, built-in operations.
</li>
<li> It is much harder to find the call sites for overloaded
operators. Searching for <code>Equals()</code> is much
easier than searching for relevant invocations of
<code>==</code>.
</li>
<li> Some operators work on pointers too, making it easy to
introduce bugs. <code>Foo + 4</code> may do one thing,
while <code>&amp;Foo + 4</code> does something totally
different. The compiler does not complain for either of
these, making this very hard to debug.
</li>
<li> User-defined literals allow creating new syntactic forms
that are unfamiliar even to experienced C++ programmers.
</li>
</ul>
Overloading also has surprising ramifications. For instance,
if a class overloads unary <code>operator&amp;</code>, it
cannot safely be forward-declared.
</CONS>
<DECISION>
<p>
In general, do not overload operators. The assignment operator
(<code>operator=</code>), in particular, is insidious and
should be avoided. You can define functions like
<code>Equals()</code> and <code>CopyFrom()</code> if you
need them. Likewise, avoid the dangerous
unary <code>operator&amp;</code> at all costs, if there's
any possibility the class might be forward-declared.
</p>
<p>
Do not overload <code>operator""</code>, i.e.
do not introduce user-defined literals.
</p>
<p>
However, there may be rare cases where you need to overload
an operator to interoperate with templates or "standard" C++
classes (such as <code>operator&lt;&lt;(ostream&amp;, const
T&amp;)</code> for logging). These are acceptable if fully
justified, but you should try to avoid these whenever
possible. In particular, do not overload <code>operator==</code>
or <code>operator&lt;</code> just so that your class can be
used as a key in an STL container; instead, you should
create equality and comparison functor types when declaring
the container.
</p>
<p>
Some of the STL algorithms do require you to overload
<code>operator==</code>, and you may do so in these cases,
provided you document why.
</p>
<p>
See also <a HREF="#Copy_Constructors">Copy Constructors</a>
and <a HREF="#Function_Overloading">Function
Overloading</a>.
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Access Control">
<SUMMARY>
Make data members <code>private</code>, and provide
access to them through accessor functions as needed (for
technical reasons, we allow data members of a test fixture class
to be <code>protected</code> when using
<A HREF="http://code.google.com/p/googletest/">
Google Test</A>). Typically a variable would be
called <code>foo_</code> and the accessor function
<code>foo()</code>. You may also want a mutator function
<code>set_foo()</code>.
Exception: <code>static const</code> data members (typically
called <code>kFoo</code>) need not be <code>private</code>.
</SUMMARY>
<BODY>
<p>
The definitions of accessors are usually inlined in the header
file.
</p>
<p>
See also <a HREF="#Inheritance">Inheritance</a> and <a HREF="#Function_Names">Function Names</a>.
</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Declaration Order">
<SUMMARY>
Use the specified order of declarations within a class:
<code>public:</code> before <code>private:</code>, methods
before data members (variables), etc.
</SUMMARY>
<BODY>
<p>
Your class definition should start with its <code>public:</code>
section, followed by its <code>protected:</code> section and
then its <code>private:</code> section. If any of these sections
are empty, omit them.
</p>
<p>
Within each section, the declarations generally should be in
the following order:
</p>
<ul>
<li> Typedefs and Enums</li>
<li> Constants (<code>static const</code> data members)</li>
<li> Constructors</li>
<li> Destructor</li>
<li> Methods, including static methods</li>
<li> Data Members (except <code>static const</code> data members)</li>
</ul>
<p>
Friend declarations should always be in the private section, and
the <code>DISALLOW_COPY_AND_ASSIGN</code> macro invocation
should be at the end of the <code>private:</code> section. It
should be the last thing in the class. See <a HREF="#Copy_Constructors">Copy Constructors</a>.
</p>
<p>
Method definitions in the corresponding <code>.cc</code> file
should be the same as the declaration order, as much as possible.
</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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Write Short Functions">
<SUMMARY>
Prefer small and focused functions.
</SUMMARY>
<BODY>
<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>
</BODY>
</STYLEPOINT>
</CATEGORY>
<CATEGORY title="Google-Specific Magic">
<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>
<STYLEPOINT title="Ownership and Smart Pointers">
<SUMMARY>
Prefer to have single, fixed owners for dynamically allocated objects.
Prefer to transfer ownership with smart pointers.
</SUMMARY>
<BODY>
<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>-&gt;</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.
<code>shared_ptr</code> is a smart pointer type which expresses
shared ownership of a dynamically allocated object.
<code>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>shared_ptr</code> is destroyed.
</p>
</DEFINITION>
<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>
</PROS>
<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
<a href="#Rvalue_references">generally forbidden</a> in Google
code, 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>
</CONS>
<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:
<CODE_SNIPPET>
std::unique_ptr&lt;Foo&gt; FooFactory();
void FooConsumer(std::unique_ptr&lt;Foo&gt; ptr);
</CODE_SNIPPET>
</p>
<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>shared_ptr&lt;const Foo&gt;</code>). If you do use shared
ownership, prefer to use <code>shared_ptr</code>.
</p>
<p>
Do not use <code>scoped_ptr</code> in new code unless you need to be
compatible with older versions of C++. Never use
<code>linked_ptr</code> or <code>std::auto_ptr</code>. In all three
cases, use <code>std::unique_ptr</code> instead.
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="cpplint">
<SUMMARY>
Use
<code>cpplint.py</code>
to detect style errors.
</SUMMARY>
<BODY>
<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.
</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="http://google-styleguide.googlecode.com/svn/trunk/cpplint/cpplint.py"><code>cpplint.py</code></A> separately.
</p>
</BODY>
</STYLEPOINT>
</CATEGORY>
<CATEGORY title="Other C++ Features">
<STYLEPOINT title="Reference Arguments">
<SUMMARY>
All parameters passed by reference must be labeled
<code>const</code>.
</SUMMARY>
<BODY>
<DEFINITION>
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
&amp;val)</code>.
</DEFINITION>
<PROS>
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.
</PROS>
<CONS>
References can be confusing, as they have value syntax but
pointer semantics.
</CONS>
<DECISION>
<p>
Within function parameter lists all references must be
<code>const</code>:
</p>
<CODE_SNIPPET>
void Foo(const string &amp;in, string *out);
</CODE_SNIPPET>
<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&amp;</code> for input parameters. For
example:
<ul>
<li>You want to pass in a null pointer.</li>
<li>The function saves a pointer or reference to the input.</li>
</ul>
Remember that most of the time input parameters are going to be
specified as <code>const T&amp;</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&amp;</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>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Rvalue references">
<SUMMARY>
Do not use rvalue references, <code>std::forward</code>,
<code>std::move_iterator</code>, or <code>std::move_if_noexcept</code>.
Use the single-argument form of <code>std::move</code> only with
non-copyable arguments.
</SUMMARY>
<BODY>
<DEFINITION>
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&amp;&amp; s);</code> declares a
function whose argument is an rvalue reference to a string.
</DEFINITION>
<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>vector&lt;string&gt;</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.
</li>
<li>
Rvalue references make it possible to implement types that are
moveable 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>
</PROS>
<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>
<li>Rvalue references encourage a programming style that makes heavier
use of value semantics. This style is
unfamiliar to many developers, and its performance characteristics
can be hard to reason about.
</li>
</ul>
</CONS>
<DECISION>
<p>
Do not use rvalue references, and do not use the
<code>std::forward</code> or <code>std::move_if_noexcept</code>
utility functions (which are essentially just casts to rvalue
reference types), or <code>std::move_iterator</code>. Use
single-argument <code>std::move</code> only with objects that are
not copyable (e.g. <code>std::unique_ptr</code>), or in templated
code with objects that might not be copyable.
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Function Overloading">
<SUMMARY>
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.
</SUMMARY>
<BODY>
<DEFINITION>
<p>
You may write a function that takes a
<code>const string&amp;</code> and overload it with another that
takes <code>const char*</code>.
</p>
<CODE_SNIPPET>
class MyClass {
public:
void Analyze(const string &amp;text);
void Analyze(const char *text, size_t textlen);
};
</CODE_SNIPPET>
</DEFINITION>
<PROS>
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.
</PROS>
<CONS>
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.
</CONS>
<DECISION>
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>.
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Default Arguments">
<SUMMARY>
We do not allow default function parameters, except in limited
situations as explained below. Simulate them with function
overloading instead, if appropriate.
</SUMMARY>
<BODY>
<PROS>
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.
</PROS>
<CONS>
Function pointers are confusing in the presence of default
arguments, since the function signature often doesn't match
the call signature. Adding a default argument to an existing
function changes its type, which can cause problems with code
taking its address. Adding function overloads avoids these
problems. In addition, default parameters may result in
bulkier code since they are replicated at every call-site --
as opposed to overloaded functions, where "the default"
appears only in the function definition.
</CONS>
<DECISION>
<p>
While the cons above are not that onerous, they still
outweigh the (small) benefits of default arguments over
function overloading. So except as described below, we
require all arguments to be explicitly specified.
</p>
<p>
One specific exception is when the function is a static
function (or in an unnamed namespace) in a .cc file. In
this case, the cons don't apply since the function's use is
so localized.
</p>
<p>
Another specific exception is when default arguments are
used to simulate variable-length argument lists.
</p>
<CODE_SNIPPET>
// Support up to 4 params by using a default empty AlphaNum.
string StrCat(const AlphaNum &amp;a,
const AlphaNum &amp;b = gEmptyAlphaNum,
const AlphaNum &amp;c = gEmptyAlphaNum,
const AlphaNum &amp;d = gEmptyAlphaNum);
</CODE_SNIPPET>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Variable-Length Arrays and alloca()">
<SUMMARY>
We do not allow variable-length arrays or <code>alloca()</code>.
</SUMMARY>
<BODY>
<PROS>
Variable-length arrays have natural-looking syntax. Both
variable-length arrays and <code>alloca()</code> are very
efficient.
</PROS>
<CONS>
Variable-length arrays and alloca are not part of Standard
C++. More importantly, they allocate a data-dependent amount
of stack space that can trigger difficult-to-find memory
overwriting bugs: "It ran fine on my machine, but dies
mysteriously in production".
</CONS>
<DECISION>
Use a safe allocator instead, such as
<code>scoped_ptr</code>/<code>scoped_array</code>.
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Friends">
<SUMMARY>
We allow use of <code>friend</code> classes and functions,
within reason.
</SUMMARY>
<BODY>
<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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Exceptions">
<SUMMARY>
We do not use C++ exceptions.
</SUMMARY>
<BODY>
<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>
</PROS>
<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>
</CONS>
<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>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Run-Time Type Information (RTTI)">
<SUMMARY>
Avoid using Run Time Type Information (RTTI).
</SUMMARY>
<BODY>
<DEFINITION>
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>.
</DEFINITION>
<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>
</CONS>
<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
<CODE_SNIPPET>
bool Base::Equal(Base* other) = 0;
bool Derived::Equal(Base* other) {
Derived* that = dynamic_cast&lt;Derived*&gt;(other);
if (that == NULL)
return false;
...
}
</CODE_SNIPPET>
</p>
</PROS>
<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:
<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>
<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.
<BAD_CODE_SNIPPET>
if (typeid(*data) == typeid(D1)) {
...
} else if (typeid(*data) == typeid(D2)) {
...
} else if (typeid(*data) == typeid(D3)) {
...
</BAD_CODE_SNIPPET>
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>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Casting">
<SUMMARY>
Use C++ casts like <code>static_cast&lt;&gt;()</code>. Do not use
other cast formats like <code>int y = (int)x;</code> or
<code>int y = int(x);</code>.
</SUMMARY>
<BODY>
<DEFINITION>
C++ introduced a different cast system from C that
distinguishes the types of cast operations.
</DEFINITION>
<PROS>
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>); C++ casts
avoid this. Additionally C++ casts are more visible when
searching for them.
</PROS>
<CONS>
The syntax is nasty.
</CONS>
<DECISION>
<p>
Do not use C-style casts. Instead, use these C++-style
casts.
</p>
<ul>
<li> Use <code>static_cast</code> as the equivalent of a
C-style cast that does value conversion, or when you need to explicitly up-cast
a pointer from a class to its superclass.
</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>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Streams">
<SUMMARY>
Use streams only for logging.
</SUMMARY>
<BODY>
<DEFINITION>
Streams are a replacement for <code>printf()</code> and
<code>scanf()</code>.
</DEFINITION>
<PROS>
With streams, you do not need to know the type of the object
you are printing. You do not have problems with format
strings not matching the argument list. (Though with gcc, you
do not have that problem with <code>printf</code> either.) Streams
have automatic constructors and destructors that open and close the
relevant files.
</PROS>
<CONS>
Streams make it difficult to do functionality like
<code>pread()</code>. Some formatting (particularly the common
format string idiom <code>%.*s</code>) is difficult if not
impossible to do efficiently using streams without using
<code>printf</code>-like hacks. Streams do not support operator
reordering (the <code>%1s</code> directive), which is helpful for
internationalization.
</CONS>
<DECISION>
<p>
Do not use streams, except where required by a logging interface.
Use <code>printf</code>-like routines instead.
</p>
<p>
There are various pros and cons to using streams, but in
this case, as in many other cases, consistency trumps the
debate. Do not use streams in your code.
</p>
<SUBSECTION title="Extended Discussion">
<p>
There has been debate on this issue, so this explains the
reasoning in greater depth. Recall the Only One Way
guiding principle: we want to make sure that whenever we
do a certain type of I/O, the code looks the same in all
those places. Because of this, we do not want to allow
users to decide between using streams or using
<code>printf</code> plus Read/Write/etc. Instead, we should
settle on one or the other. We made an exception for logging
because it is a pretty specialized application, and for
historical reasons.
</p>
<p>
Proponents of streams have argued that streams are the obvious
choice of the two, but the issue is not actually so clear. For
every advantage of streams they point out, there is an
equivalent disadvantage. The biggest advantage is that
you do not need to know the type of the object to be
printing. This is a fair point. But, there is a
downside: you can easily use the wrong type, and the
compiler will not warn you. It is easy to make this
kind of mistake without knowing when using streams.
</p>
<CODE_SNIPPET>
cout &lt;&lt; this; // Prints the address
cout &lt;&lt; *this; // Prints the contents
</CODE_SNIPPET>
<p>
The compiler does not generate an error because
<code>&lt;&lt;</code> has been overloaded. We discourage
overloading for just this reason.
</p>
<p>
Some say <code>printf</code> formatting is ugly and hard to
read, but streams are often no better. Consider the following
two fragments, both with the same typo. Which is easier to
discover?
</p>
<CODE_SNIPPET>
cerr &lt;&lt; "Error connecting to '" &lt;&lt; foo-&gt;bar()-&gt;hostname.first
&lt;&lt; ":" &lt;&lt; foo-&gt;bar()-&gt;hostname.second &lt;&lt; ": " &lt;&lt; strerror(errno);
fprintf(stderr, "Error connecting to '%s:%u: %s",
foo-&gt;bar()-&gt;hostname.first, foo-&gt;bar()-&gt;hostname.second,
strerror(errno));
</CODE_SNIPPET>
<p>
And so on and so forth for any issue you might bring up.
(You could argue, "Things would be better with the right
wrappers," but if it is true for one scheme, is it not
also true for the other? Also, remember the goal is to
make the language smaller, not add yet more machinery that
someone has to learn.)
</p>
<p>
Either path would yield different advantages and
disadvantages, and there is not a clearly superior
solution. The simplicity doctrine mandates we settle on
one of them though, and the majority decision was on
<code>printf</code> + <code>read</code>/<code>write</code>.
</p>
</SUBSECTION>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Preincrement and Predecrement">
<SUMMARY>
Use prefix form (<code>++i</code>) of the increment and
decrement operators with iterators and other template objects.
</SUMMARY>
<BODY>
<DEFINITION>
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).
</DEFINITION>
<PROS>
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?
</PROS>
<CONS>
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.
</CONS>
<DECISION>
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.
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Use of const">
<SUMMARY>
Use <code>const</code> whenever it makes sense.
With C++11,
<code>constexpr</code> is a better choice for some uses of const.
</SUMMARY>
<BODY>
<DEFINITION>
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>).
</DEFINITION>
<PROS>
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.
</PROS>
<CONS>
<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.
</CONS>
<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 does not modify an argument passed by
reference or by pointer, that argument should be
<code>const</code>.
</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>
</DECISION>
<SUBSECTION title="Where to put the const">
<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>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Use of constexpr">
<SUMMARY>
In C++11, use <code>constexpr</code>
to define true constants or to ensure constant initialization.
</SUMMARY>
<BODY>
<DEFINITION>
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.
</DEFINITION>
<PROS>
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.
</PROS>
<CONS>
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.
</CONS>
<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>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Integer Types">
<SUMMARY>
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>&lt;stdint.h&gt;</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.
</SUMMARY>
<BODY>
<DEFINITION>
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.
</DEFINITION>
<PROS>
Uniformity of declaration.
</PROS>
<CONS>
The sizes of integral types in C++ can vary based on compiler
and architecture.
</CONS>
<DECISION>
<p>
<code>&lt;stdint.h&gt;</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>
</DECISION>
<SUBSECTION title="On Unsigned Integers">
<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>
<CODE_SNIPPET>
for (unsigned int i = foo.Length()-1; i &gt;= 0; --i) ...
</CODE_SNIPPET>
<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>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="64-bit Portability">
<SUMMARY>
Code should be 64-bit and 32-bit friendly. Bear in mind problems of
printing, comparisons, and structure alignment.
</SUMMARY>
<BODY>
<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>
<CODE_SNIPPET>
// 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"
</CODE_SNIPPET>
<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/>
</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/>
</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. 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>
Use the <code>LL</code> or <code>ULL</code> suffixes as
needed to create 64-bit constants. For example:
<CODE_SNIPPET>
int64_t my_value = 0x123456789LL;
uint64_t my_mask = 3ULL &lt;&lt; 48;
</CODE_SNIPPET>
</li>
<li> If you really need different code on 32-bit and 64-bit
systems, use <code>#ifdef _LP64</code> to choose between
the code variants. (But please avoid this if
possible, and keep any such changes localized.)
</li>
</ul>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Preprocessor Macros">
<SUMMARY>
Be very cautious with macros. Prefer inline functions, enums,
and <code>const</code> variables to macros.
</SUMMARY>
<BODY>
<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>
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.
</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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="0 and nullptr/NULL">
<SUMMARY>
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.
</SUMMARY>
<BODY>
<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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="sizeof">
<SUMMARY>
Prefer <code>sizeof(<var>varname</var>)</code> to
<code>sizeof(<var>type</var>)</code>.
</SUMMARY>
<BODY>
<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>
<p>
<CODE_SNIPPET>
Struct data;
memset(&amp;data, 0, sizeof(data));
</CODE_SNIPPET>
<BAD_CODE_SNIPPET>
memset(&amp;data, 0, sizeof(Struct));
</BAD_CODE_SNIPPET>
<CODE_SNIPPET>
if (raw_size &lt; sizeof(int)) {
LOG(ERROR) &lt;&lt; "compressed record not big enough for count: " &lt;&lt; raw_size;
return false;
}
</CODE_SNIPPET>
</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="auto">
<SUMMARY>
Use <code>auto</code> to avoid type names that are just clutter.
Continue to use manifest type declarations when it helps readability,
and never use <code>auto</code> for anything but local variables.
</SUMMARY>
<BODY>
<DEFINITION>
In C++11, a variable whose type is given as <code>auto</code> will be given
a type that matches that of the expression used to initialize
it. You can use <code>auto</code> either to initialize a
variable by copying, or to bind a reference.
<CODE_SNIPPET>
vector&lt;string&gt; v;
...
auto s1 = v[0]; // Makes a copy of v[0].
const auto&amp; s2 = v[0]; // s2 is a reference to v[0].
</CODE_SNIPPET>
</DEFINITION>
<PROS>
<p>
C++ type names can sometimes be long and cumbersome,
especially when they involve templates or namespaces. In a statement like
<CODE_SNIPPET>
sparse_hash_map&lt;string, int&gt;::iterator iter = m.find(val);
</CODE_SNIPPET>
the return type is hard to read, and obscures the primary
purpose of the statement. Changing it to
<CODE_SNIPPET>
auto iter = m.find(val);
</CODE_SNIPPET>
makes it more readable.
</p>
<p>
Without <code>auto</code> we are sometimes forced to write a
type name twice in the same expression, adding no value
for the reader, as in
<CODE_SNIPPET>
diagnostics::ErrorStatus* status = new diagnostics::ErrorStatus("xyz");
</CODE_SNIPPET>
</p>
<p>
Using <code>auto</code> makes it easier to use intermediate
variables when appropriate, by reducing the burden of writing
their types explicitly.
</p>
</PROS>
<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 an expression like
<CODE_SNIPPET>
auto i = x.Lookup(key);
</CODE_SNIPPET>
it may not be obvious what <code>i</code>'s type is, if <code>x</code>
was declared hundreds of lines earlier.
</p>
<p>Programmers have to understand the difference between <code>auto</code>
and <code>const auto&amp;</code> or they'll get copies when
they didn't mean to.
</p>
<p>The interaction between <code>auto</code> and C++11
brace-initialization can be confusing. The declarations
<CODE_SNIPPET>
auto x(3); // Note: parentheses.
auto y{3}; // Note: curly braces.
</CODE_SNIPPET>
mean different things — <code>x</code> is
an <code>int</code>, while <code>y</code> is
an <code>initializer_list</code>. The same applies to other
normally-invisible proxy types.
</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>
</CONS>
<DECISION>
<p><code>auto</code> is permitted, for local variables only.
Do not use <code>auto</code> for file-scope or namespace-scope
variables, or for class members. Never assign a braced initializer list
to an <code>auto</code>-typed variable.</p>
<p>The <code>auto</code> keyword is also used in an unrelated
C++11 feature: it's part of the syntax for a new kind of
function declaration with a trailing return type. Function
declarations with trailing return types are not permitted.</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Brace Initialization">
<SUMMARY>
You may use brace initialization.
</SUMMARY>
<BODY>
<p>In C++03, aggregate types (arrays and structs with no constructor) could
be initialized using braces.
<CODE_SNIPPET>
struct Point { int x; int y; };
Point p = {1, 2};
</CODE_SNIPPET></p>
<p>In C++11, this syntax has been expanded for use with all other datatypes.
The brace initialization form is called <i>braced-init-list</i>. Here are
a few examples of its use.
<CODE_SNIPPET>
// Vector takes lists of elements.
vector&lt;string&gt; v{"foo", "bar"};
// The same, except this form cannot be used if the initializer_list
// constructor is explicit. You may choose to use either form.
vector&lt;string&gt; v = {"foo", "bar"};
// Maps take lists of pairs. Nested braced-init-lists work.
map&lt;int, string&gt; m = {{1, "one"}, {2, "2"}};
// braced-init-lists can be implicitly converted to return types.
vector&lt;int&gt; 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 test_function2(vector&lt;int&gt; v) {}
test_function2({1, 2, 3});
</CODE_SNIPPET></p>
<p>User data types can also define constructors that take
<code>initializer_list</code>, which is automatically created from
<i>braced-init-list</i>:
<CODE_SNIPPET>
class MyType {
public:
// initializer_list is a reference to the underlying init list,
// so it can be passed by value.
MyType(initializer_list&lt;int&gt; init_list) {
for (int element : init_list) {}
}
};
MyType m{2, 3, 5, 7};
</CODE_SNIPPET></p>
<p>Finally, brace initialization can also call ordinary constructors of
data types that do not have <code>initializer_list</code> constructors.
<CODE_SNIPPET>
double d{1.23};
// Calls ordinary constructor as long as MyOtherType has no
// 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"};
</CODE_SNIPPET></p>
<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.
<BAD_CODE_SNIPPET>
auto d = {1.23}; // d is an initializer_list&lt;double&gt;
</BAD_CODE_SNIPPET>
<CODE_SNIPPET>
auto d = double{1.23}; // Good -- d is a double, not an initializer_list.
</CODE_SNIPPET>
</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Lambda expressions">
<SUMMARY>
Do not use lambda expressions, or the related <code>std::function</code>
or <code>std::bind</code> utilities.
</SUMMARY>
<BODY>
<DEFINITION>
Lambda expressions are a concise way of creating anonymous function
objects. They're often useful when passing functions as arguments.
For example: <code>std::sort(v.begin(), v.end(),
[](string x, string y) { return x[1] &lt; y[1]; });</code> 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>.
</DEFINITION>
<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>
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>
</PROS>
<CONS>
<ul>
<li>
Variable capture in lambdas can be tricky, and might be a new
source of dangling-pointer bugs.
</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>
</CONS>
<DECISION>
<p>
Do not use lambda expressions, <code>std::function</code> or
<code>std::bind</code>.
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Boost">
<SUMMARY>
Use only approved libraries from the Boost library collection.
</SUMMARY>
<BODY>
<DEFINITION>
The <a href="http://www.boost.org/">Boost library collection</a> is
a popular collection of peer-reviewed, free, open-source C++ libraries.
</DEFINITION>
<PROS>
Boost code is generally very high-quality, is widely portable, and fills
many important gaps in the C++ standard library, such as type traits,
better binders, and better smart pointers. It also provides an
implementation of the TR1 extension to the standard library.
</PROS>
<CONS>
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.
</CONS>
<DECISION>
<div>
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:
<ul>
<li> <a href="http://www.boost.org/libs/utility/call_traits.htm">
Call Traits</a> from <code>boost/call_traits.hpp</code>
</li>
<li> <a href="http://www.boost.org/libs/utility/compressed_pair.htm">
Compressed Pair</a> from <code>boost/compressed_pair.hpp</code>
</li>
<li> <a href="http://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="http://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> The part of
<a href="http://www.boost.org/libs/iterator/">
Iterator</a> that deals with defining iterators:
<code>boost/iterator/iterator_adaptor.hpp</code>,
<code>boost/iterator/iterator_facade.hpp</code>, and
<code>boost/function_output_iterator.hpp</code></li>
<li> The part of
<a href="http://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="http://www.boost.org/libs/bimap/">
Bimap</a> from <code>boost/bimap</code>
</li>
<li> <a href="http://www.boost.org/libs/math/doc/html/dist.html">
Statistical Distributions and Functions</a> from
<code>boost/math/distributions</code>
</li>
</ul>
We are actively considering adding other Boost features to the list, so
this list may be expanded in the future.
</div>
<p>
The following libraries are permitted, but their use is discouraged
because they've been superseded by standard libraries in C++11:
<ul>
<li> <a href="http://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="http://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>
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="C++11">
<SUMMARY>
Use libraries and language extensions from C++11 (formerly
known as C++0x) when appropriate.
Consider portability to other environments before
using C++11 features in your project.
</SUMMARY>
<BODY>
<DEFINITION>
C++11 is the latest ISO C++ standard.
It contains
<a href="http://en.wikipedia.org/wiki/C%2B%2B11">significant
changes</a> both to the language and libraries.
</DEFINITION>
<PROS>
C++11 has become the official standard, and eventually will
be 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.
</PROS>
<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>
</CONS>
<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>
Functions with trailing return types, e.g. writing
<code>auto foo() -&gt; int;</code> instead of
<code>int foo();</code>, because of a desire to preserve
stylistic consistency with the many existing function
declarations.
</li>
<li>
Compile-time rational numbers (<code>&lt;ratio&gt;</code>),
because of concerns that it's tied to a more template-heavy
interface style.
</li>
<li>
The <code>&lt;cfenv&gt;</code> and <code>&lt;fenv.h&gt;</code>
headers, because many compilers do not support those
features reliably.
</li>
<li>
Lambda expressions, or the related <code>std::function</code> or
<code>std::bind</code> utilities.
</li>
</ul>
</DECISION>
</BODY>
</STYLEPOINT>
</CATEGORY>
<CATEGORY title="Naming">
<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>
<STYLEPOINT title="General Naming Rules">
<SUMMARY>
Function names, variable names, and filenames should be
descriptive; eschew abbreviation.
</SUMMARY>
<BODY>
<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>
<CODE_SNIPPET>
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.
</CODE_SNIPPET>
<BAD_CODE_SNIPPET>
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.
</BAD_CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="File Names">
<SUMMARY>
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 "_".
</SUMMARY>
<BODY>
<p>
Examples of acceptable file names:
</p>
<p>
<code>
my_useful_class.cc<br/>
my-useful-class.cc<br/>
myusefulclass.cc<br/>
myusefulclass_test.cc // _unittest and _regtest are deprecated.<br/>
</code>
</p>
<p>
C++ files should end in <code>.cc</code> and header files
should end in <code>.h</code>.
</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. However, if your inline functions
include a lot of code, they may go into a third file that
ends in <code>-inl.h</code>. In a class with a lot of inline
code, your class could have three files:
</p>
<CODE_SNIPPET>
url_table.h // The class declaration.
url_table.cc // The class definition.
url_table-inl.h // Inline functions that include lots of code.
</CODE_SNIPPET>
<p>
See also the section <a href="#The_-inl.h_Files">-inl.h Files</a>
</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Type Names">
<SUMMARY>
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>.
</SUMMARY>
<BODY>
<p>
The names of all types — classes, structs, typedefs, and enums
— 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>
<CODE_SNIPPET>
// classes and structs
class UrlTable { ...
class UrlTableTester { ...
struct UrlTableProperties { ...
// typedefs
typedef hash_map&lt;UrlTableProperties *, string&gt; PropertiesMap;
// enums
enum UrlTableErrors { ...
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Variable Names">
<SUMMARY>
Variable names are all lowercase, with underscores between
words. Class member variables have trailing underscores. For
instance: <code>my_exciting_local_variable</code>,
<code>my_exciting_member_variable_</code>.
</SUMMARY>
<BODY>
<SUBSECTION title="Common Variable names">
<p>
For example:
</p>
<CODE_SNIPPET>
string table_name; // OK - uses underscore.
string tablename; // OK - all lowercase.
</CODE_SNIPPET>
<BAD_CODE_SNIPPET>
string tableName; // Bad - mixed case.
</BAD_CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Class Data Members">
<p>
Data members (also called instance variables or member
variables) are lowercase with optional underscores like
regular variable names, but always end with a trailing
underscore.
</p>
<CODE_SNIPPET>
string table_name_; // OK - underscore at end.
string tablename_; // OK.
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Struct Variables">
<p>
Data members in structs should be named like regular
variables without the trailing underscores that data members
in classes have.
</p>
<CODE_SNIPPET>
struct UrlTableProperties {
string name;
int num_entries;
}
</CODE_SNIPPET>
<p>
See <a HREF="#Structs_vs._Classes">Structs vs. Classes</a> for a
discussion of when to use a struct versus a class.
</p>
</SUBSECTION>
<SUBSECTION title="Global Variables">
<p>
There are no special requirements for global variables,
which should be rare in any case, but if you use one,
consider prefixing it with <code>g_</code> or some other
marker to easily distinguish it from local variables.
</p>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Constant Names">
<SUMMARY>
Use a <code>k</code> followed by mixed case:
<code>kDaysInAWeek</code>.
</SUMMARY>
<BODY>
<p>
All compile-time constants, whether they are declared locally,
globally, or as part of a class, follow a slightly different
naming convention from other variables. Use a <code>k</code>
followed by words with uppercase first letters:
</p>
<CODE_SNIPPET>
const int kDaysInAWeek = 7;
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Function Names">
<SUMMARY>
Regular functions have mixed case; accessors and mutators match
the name of the variable: <code>MyExcitingFunction()</code>,
<code>MyExcitingMethod()</code>,
<code>my_exciting_member_variable()</code>,
<code>set_my_exciting_member_variable()</code>.
</SUMMARY>
<BODY>
<SUBSECTION title="Regular Functions">
<p>
Functions should start with a capital letter and have a
capital letter for each new word. No underscores.
</p>
<p>
If your function crashes upon an error, you should append OrDie to
the function name. This only applies to functions which could be
used by production code and to errors that are reasonably
likely to occur during normal operation.
</p>
<CODE_SNIPPET>
AddTableEntry()
DeleteUrl()
OpenFileOrDie()
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Accessors and Mutators">
<p>
Accessors and mutators (get and set functions) should match
the name of the variable they are getting and setting. This
shows an excerpt of a class whose instance variable is
<code>num_entries_</code>.
</p>
<CODE_SNIPPET>
class MyClass {
public:
...
int num_entries() const { return num_entries_; }
void set_num_entries(int num_entries) { num_entries_ = num_entries; }
private:
int num_entries_;
};
</CODE_SNIPPET>
<p>
You may also use lowercase letters for other very short
inlined functions. For example if a function were so cheap
you would not cache the value if you were calling it in a
loop, then lowercase naming would be acceptable.
</p>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Namespace Names">
<SUMMARY>
Namespace names are all lower-case, and based on project names and
possibly their directory structure:
<code>google_awesome_project</code>.
</SUMMARY>
<BODY>
<p>
See <a HREF="#Namespaces">Namespaces</a> for a discussion of
namespaces and how to name them.
</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Enumerator Names">
<SUMMARY>
Enumerators 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>.
</SUMMARY>
<BODY>
<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>
<CODE_SNIPPET>
enum UrlTableErrors {
kOK = 0,
kErrorOutOfMemory,
kErrorMalformedInput,
};
enum AlternateUrlTableErrors {
OK = 0,
OUT_OF_MEMORY = 1,
MALFORMED_INPUT = 2,
};
</CODE_SNIPPET>
<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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Macro Names">
<SUMMARY>
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>.
</SUMMARY>
<BODY>
<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>
<CODE_SNIPPET>
#define ROUND(x) ...
#define PI_ROUNDED 3.0
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Exceptions to Naming Rules">
<SUMMARY>
If you are naming something that is analogous to an existing C
or C++ entity then you can follow the existing naming convention
scheme.
</SUMMARY>
<BODY>
<p>
<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>
</p>
</BODY>
</STYLEPOINT>
</CATEGORY>
<CATEGORY title="Comments">
<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>
<STYLEPOINT title="Comment Style">
<SUMMARY>
Use either the <code>//</code> or <code>/* */</code> syntax, as long
as you are consistent.
</SUMMARY>
<BODY>
<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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="File Comments">
<SUMMARY>
Start each file with license boilerplate,
followed by a description of its contents.
</SUMMARY>
<BODY>
<SUBSECTION title="Legal Notice and Author Line">
<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>
</SUBSECTION>
<SUBSECTION title="File Contents">
<p>
Every file should have a comment at the top describing its contents.
</p>
<p>
Generally a <code>.h</code> file will describe the classes
that are declared in the file with an overview of what they
are for and how they are used. A <code>.cc</code> file
should contain more information about implementation details
or discussions of tricky algorithms. If you feel the
implementation details or a discussion of the algorithms
would be useful for someone reading the <code>.h</code>,
feel free to put it there instead, but mention in the
<code>.cc</code> that the documentation is in the
<code>.h</code> file.
</p>
<p>
Do not duplicate comments in both the <code>.h</code> and
the <code>.cc</code>. Duplicated comments diverge.
</p>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Class Comments">
<SUMMARY>
Every class definition should have an accompanying comment that
describes what it is for and how it should be used.
</SUMMARY>
<BODY>
<CODE_SNIPPET>
// Iterates over the contents of a GargantuanTable. Sample usage:
// GargantuanTableIterator* iter = table-&gt;NewIterator();
// for (iter-&gt;Seek("foo"); !iter-&gt;done(); iter-&gt;Next()) {
// process(iter-&gt;key(), iter-&gt;value());
// }
// delete iter;
class GargantuanTableIterator {
...
};
</CODE_SNIPPET>
<p>
If you have already described a class in detail in the
comments at the top of your file feel free to simply state
"See comment at top of file for a complete description", but
be sure to have some sort of comment.
</p>
<p>
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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Function Comments">
<SUMMARY>
Declaration comments describe use of the function; comments at
the definition of a function describe operation.
</SUMMARY>
<BODY>
<SUBSECTION title="Function Declarations">
<p>
Every function declaration should have comments immediately
preceding it that describe what the function does and how to
use it. 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>
<CODE_SNIPPET>
// 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-&gt;NewIterator();
// iter-&gt;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;
</CODE_SNIPPET>
<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>
<CODE_SNIPPET>
// Returns true if the table cannot hold any more entries.
bool IsTableFull();
</CODE_SNIPPET>
<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>
</SUBSECTION>
<SUBSECTION title="Function Definitions">
<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>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Variable Comments">
<SUMMARY>
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.
</SUMMARY>
<BODY>
<SUBSECTION title="Class Data Members">
<p>
Each class data member (also called an instance variable or
member variable) should have a comment describing what it is
used for. If the variable can take sentinel values with
special meanings, such as a null pointer or -1, document this.
For example:
</p>
<CODE_SNIPPET>
private:
// Keeps track of the total number of entries in the table.
// Used to ensure we do not go over the limit. -1 means
// that we don't yet know how many entries the table has.
int num_total_entries_;
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Global Variables">
<p>
As with data members, all global variables should have a
comment describing what they are and what they are used for.
For example:
</p>
<CODE_SNIPPET>
// The total number of tests cases that we run through in this regression test.
const int kNumTestCases = 6;
</CODE_SNIPPET>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Implementation Comments">
<SUMMARY>
In your implementation you should have comments in tricky,
non-obvious, interesting, or important parts of your code.
</SUMMARY>
<BODY>
<SUBSECTION title="Class Data Members">
<p>
Tricky or complicated code blocks should have comments
before them. Example:
</p>
<CODE_SNIPPET>
// Divide result by two, taking into account that x
// contains the carry from the add.
for (int i = 0; i &lt; result-&gt;size(); i++) {
x = (x &lt;&lt; 8) + (*result)[i];
(*result)[i] = x &gt;&gt; 1;
x &amp;= 1;
}
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Line Comments">
<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>
<CODE_SNIPPET>
// If we have enough memory, mmap the data portion too.
mmap_budget = max&lt;int64&gt;(0, mmap_budget - index_-&gt;length());
if (mmap_budget &gt;= data_size_ &amp;&amp; !MmapData(mmap_chunk_bytes, mlock))
return; // Error already logged.
</CODE_SNIPPET>
<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>
<CODE_SNIPPET>
DoSomething(); // Comment here so the comments line up.
DoSomethingElseThatIsLonger(); // Comment here so there are 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.
}
DoSomething(); /* For trailing block comments, one space is fine. */
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="nullptr/NULL, true/false, 1, 2, 3...">
<p>
When you pass in a null pointer, boolean, or literal integer
values to functions, you should consider adding a comment about
what they are, or make your code self-documenting by using
constants. For example, compare:
</p>
<BAD_CODE_SNIPPET>
bool success = CalculateSomething(interesting_value,
10,
false,
NULL); // What are these arguments??
</BAD_CODE_SNIPPET>
<p>
versus:
</p>
<CODE_SNIPPET>
bool success = CalculateSomething(interesting_value,
10, // Default base value.
false, // Not the first time we're calling this.
NULL); // No callback.
</CODE_SNIPPET>
<p>
Or alternatively, constants or self-describing variables:
</p>
<CODE_SNIPPET>
const int kDefaultBaseValue = 10;
const bool kFirstTimeCalling = false;
Callback *null_callback = NULL;
bool success = CalculateSomething(interesting_value,
kDefaultBaseValue,
kFirstTimeCalling,
null_callback);
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Don'ts">
<p>
Note that you should <em>never</em> describe the code
itself. Assume that the person reading the code knows C++
better than you do, even though he or she does not know what
you are trying to do:
</p>
<BAD_CODE_SNIPPET>
// Now go through the b array and make sure that if i occurs,
// the next element is i+1.
... // Geez. What a useless comment.
</BAD_CODE_SNIPPET>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Punctuation, Spelling and Grammar">
<SUMMARY>
Pay attention to punctuation, spelling, and grammar; it is
easier to read well-written comments than badly written ones.
</SUMMARY>
<BODY>
<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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="TODO Comments">
<SUMMARY>
Use <code>TODO</code> comments for code that is temporary, a
short-term solution, or good-enough but not perfect.
</SUMMARY>
<BODY>
<p>
<code>TODO</code>s should include the string <code>TODO</code> in
all caps, followed by the
name, e-mail address, or other
identifier
of the person who can best provide context about the problem
referenced by the <code>TODO</code>. A colon is optional. The main
purpose is to have a consistent <code>TODO</code> format that can be
searched to find the person who can provide 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>, it is
almost always your
name
that is given.
</p>
<CODE_SNIPPET>
// TODO(kl@gmail.com): Use a "*" here for concatenation operator.
// TODO(Zeke) change this to use relations.
</CODE_SNIPPET>
<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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Deprecation Comments">
<SUMMARY>
Mark deprecated interface points with <code>DEPRECATED</code> comments.
</SUMMARY>
<BODY>
<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>
</BODY>
</STYLEPOINT>
</CATEGORY>
<CATEGORY title="Formatting">
<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="http://google-styleguide.googlecode.com/svn/trunk/google-c-style.el">settings
file for emacs</A>.
</p>
<STYLEPOINT title="Line Length">
<SUMMARY>
Each line of text in your code should be at most 80 characters
long.
</SUMMARY>
<BODY>
<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>
<PROS>
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?
</PROS>
<CONS>
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.
</CONS>
<DECISION>
<p>
80 characters is the maximum.
</p>
<p>
Exception: if a comment line contains an example command or
a literal URL longer than 80 characters, that line may be
longer than 80 characters for ease of cut and paste.
</p>
<p>
Exception: an <code>#include</code> statement with a long
path may exceed 80 columns. Try to avoid situations where this
becomes necessary.
</p>
<p>
Exception: you needn't be concerned about
<a href="#The__define_Guard">header guards</a>
that exceed the maximum length.
</p>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Non-ASCII Characters">
<SUMMARY>
Non-ASCII characters should be rare, and must use UTF-8 formatting.
</SUMMARY>
<BODY>
<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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Spaces vs. Tabs">
<SUMMARY>
Use only spaces, and indent 2 spaces at a time.
</SUMMARY>
<BODY>
<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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Function Declarations and Definitions">
<SUMMARY>
Return type on the same line as function name, parameters on the
same line if they fit.
</SUMMARY>
<BODY>
<p>
Functions look like this:
</p>
<CODE_SNIPPET>
ReturnType ClassName::FunctionName(Type par_name1, Type par_name2) {
DoSomething();
...
}
</CODE_SNIPPET>
<p>
If you have too much text to fit on one line:
</p>
<CODE_SNIPPET>
ReturnType ClassName::ReallyLongFunctionName(Type par_name1, Type par_name2,
Type par_name3) {
DoSomething();
...
}
</CODE_SNIPPET>
<p>
or if you cannot fit even the first parameter:
</p>
<CODE_SNIPPET>
ReturnType LongClassName::ReallyReallyReallyLongFunctionName(
Type par_name1, // 4 space indent
Type par_name2,
Type par_name3) {
DoSomething(); // 2 space indent
...
}
</CODE_SNIPPET>
<p>
Some points to note:
</p>
<ul>
<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 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 at the end of the same
line as the last parameter.
</li>
<li> The close curly brace is either on the last line by itself
or (if other style rules permit) 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 named, with identical names in the
declaration and implementation.
</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>
If some parameters are unused, comment out the variable name in the
function definition:
</p>
<CODE_SNIPPET>
// Always have named parameters in interfaces.
class Shape {
public:
virtual void Rotate(double radians) = 0;
}
// Always have named parameters in the declaration.
class Circle : public Shape {
public:
virtual void Rotate(double radians);
}
// Comment out unused named parameters in definitions.
void Circle::Rotate(double /*radians*/) {}
</CODE_SNIPPET>
<BAD_CODE_SNIPPET>
// Bad - if someone wants to implement later, it's not clear what the
// variable means.
void Circle::Rotate(double) {}
</BAD_CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Function Calls">
<SUMMARY>
On one line if it fits; otherwise, wrap arguments at the
parenthesis.
</SUMMARY>
<BODY>
<p>
Function calls have the following format:
</p>
<CODE_SNIPPET>
bool retval = DoSomething(argument1, argument2, argument3);
</CODE_SNIPPET>
<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>
<CODE_SNIPPET>
bool retval = DoSomething(averyveryveryverylongargument1,
argument2, argument3);
</CODE_SNIPPET>
<p>
If the function has many arguments, consider having one per
line if this makes the code more readable:
</p>
<CODE_SNIPPET>
bool retval = DoSomething(argument1,
argument2,
argument3,
argument4);
</CODE_SNIPPET>
<p>
Arguments may optionally all be placed on subsequent lines, with one
line per argument:
</p>
<CODE_SNIPPET>
if (...) {
...
...
if (...) {
DoSomething(
argument1, // 4 space indent
argument2,
argument3,
argument4);
}
</CODE_SNIPPET>
<p>
In particular, this should be done if the function signature is so long
that it cannot fit within the maximum <a href="#Line_Length">line
length</a>.
</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Braced Initializer Lists">
<SUMMARY>
Format a braced list exactly like you would format a function call in its
place.
</SUMMARY>
<BODY>
<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>
<CODE_SNIPPET>
// Examples of braced init list on a single line.
return {foo, bar};
functioncall({foo, bar});
pair&lt;int, int&gt; 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}};
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Conditionals">
<SUMMARY>
Prefer no spaces inside parentheses. The <code>else</code>
keyword belongs on a new line.
</SUMMARY>
<BODY>
<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>
<CODE_SNIPPET>
if (condition) { // no spaces inside parentheses
... // 2 space indent.
} else if (...) { // The else goes on the same line as the closing brace.
...
} else {
...
}
</CODE_SNIPPET>
<p>
If you prefer you may add spaces inside the
parentheses:
</p>
<CODE_SNIPPET>
if ( condition ) { // spaces inside parentheses - rare
... // 2 space indent.
} else { // The else goes on the same line as the closing brace.
...
}
</CODE_SNIPPET>
<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>
<BAD_CODE_SNIPPET>
if(condition) // Bad - space missing after IF.
if (condition){ // Bad - space missing before {.
if(condition){ // Doubly bad.
</BAD_CODE_SNIPPET>
<CODE_SNIPPET>
if (condition) { // Good - proper space after IF and before {.
</CODE_SNIPPET>
<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>
<CODE_SNIPPET>
if (x == kFoo) return new Foo();
if (x == kBar) return new Bar();
</CODE_SNIPPET>
<p>
This is not allowed when the if statement has an
<code>else</code>:
</p>
<BAD_CODE_SNIPPET>
// Not allowed - IF statement on one line when there is an ELSE clause
if (x) DoThis();
else DoThat();
</BAD_CODE_SNIPPET>
<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>
<CODE_SNIPPET>
if (condition)
DoSomething(); // 2 space indent.
if (condition) {
DoSomething(); // 2 space indent.
}
</CODE_SNIPPET>
<p>
However, if one part of an <code>if</code>-<code>else</code>
statement uses curly braces, the other part must too:
</p>
<BAD_CODE_SNIPPET>
// 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;
}
</BAD_CODE_SNIPPET>
<CODE_SNIPPET>
// Curly braces around both IF and ELSE required because
// one of the clauses used braces.
if (condition) {
foo;
} else {
bar;
}
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Loops and Switch Statements">
<SUMMARY>
Switch statements may use braces for blocks. Annotate non-trivial
fall-through between cases. Empty loop bodies should use <code>{}</code>
or <code>continue</code>.
</SUMMARY>
<BODY>
<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>
<CODE_SNIPPET>
switch (var) {
case 0: { // 2 space indent
... // 4 space indent
break;
}
case 1: {
...
break;
}
default: {
assert(false);
}
}
</CODE_SNIPPET>
<p>
Empty loop bodies should use <code>{}</code> or
<code>continue</code>, but not a single semicolon.
</p>
<CODE_SNIPPET>
while (condition) {
// Repeat test until it returns false.
}
for (int i = 0; i &lt; kSomeNumber; ++i) {} // Good - empty body.
while (condition) continue; // Good - continue indicates no logic.
</CODE_SNIPPET>
<BAD_CODE_SNIPPET>
while (condition); // Bad - looks like part of do/while loop.
</BAD_CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Pointer and Reference Expressions">
<SUMMARY>
No spaces around period or arrow. Pointer operators do not have
trailing spaces.
</SUMMARY>
<BODY>
<p>
The following are examples of correctly-formatted pointer and
reference expressions:
</p>
<CODE_SNIPPET>
x = *p;
p = &amp;x;
x = r.y;
x = r-&gt;y;
</CODE_SNIPPET>
<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>&amp;</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>
<CODE_SNIPPET>
// These are fine, space preceding.
char *c;
const string &amp;str;
// These are fine, space following.
char* c; // but remember to do "char* c, *d, *e, ...;"!
const string&amp; str;
</CODE_SNIPPET>
<BAD_CODE_SNIPPET>
char * c; // Bad - spaces on both sides of *
const string &amp; str; // Bad - spaces on both sides of &amp;
</BAD_CODE_SNIPPET>
<p>
You should do this consistently within a single
file,
so, when modifying an existing file, use the style in that
file.
</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Boolean Expressions">
<SUMMARY>
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.
</SUMMARY>
<BODY>
<p>
In this example, the logical AND operator is always at the end
of the lines:
</p>
<CODE_SNIPPET>
if (this_one_thing &gt; this_other_thing &amp;&amp;
a_third_thing == a_fourth_thing &amp;&amp;
yet_another &amp;&amp; last_one) {
...
}
</CODE_SNIPPET>
<p>
Note that when the code wraps in this example, both of
the <code>&amp;&amp;</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>&amp;&amp;</code> and
<code>~</code>, rather than the word operators, such as <code>and</code>
and <code>compl</code>.
</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Return Values">
<SUMMARY>
Do not needlessly surround the <code>return</code> expression with
parentheses.
</SUMMARY>
<BODY>
<p>
Use parentheses in <code>return expr;</code> only where you would use
them in <code>x = expr;</code>.
</p>
<CODE_SNIPPET>
return result; // No parentheses in the simple case.
return (some_long_condition &amp;&amp; // Parentheses ok to make a complex
another_condition); // expression more readable.
</CODE_SNIPPET>
<BAD_CODE_SNIPPET>
return (value); // You wouldn't write var = (value);
return(result); // return is not a function!
</BAD_CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Variable and Array Initialization">
<SUMMARY>
Your choice of <code>=</code>, <code>()</code>, or <code>{}</code>.
</SUMMARY>
<BODY>
<p>
You may choose between <code>=</code>, <code>()</code>, and
<code>{}</code>; the following are all correct:
</p>
<CODE_SNIPPET>
int x = 3;
int x(3);
int x{3};
string name = "Some Name";
string name("Some Name");
string name{"Some Name"};
</CODE_SNIPPET>
<p>
Be careful when using the <code>{}</code> on a type that takes an
<code>initializer_list</code> in one of its constructors. The
<code>{}</code> syntax prefers the <code>initializer_list</code>
constructor whenever possible. To get the non-
<code>initializer_list</code> constructor, use <code>()</code>.
</p>
<CODE_SNIPPET>
vector&lt;int&gt; v(100, 1); // A vector of 100 1s.
vector&lt;int&gt; v{100, 1}; // A vector of 100, 1.
</CODE_SNIPPET>
<p>
Also, the brace form prevents narrowing of integral types. This can
prevent some types of programming errors.
</p>
<CODE_SNIPPET>
int pi(3.14); // OK -- pi == 3.
int pi{3.14}; // Compile error: narrowing conversion.
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Preprocessor Directives">
<SUMMARY>
The hash mark that starts a preprocessor directive should
always be at the beginning of the line.
</SUMMARY>
<BODY>
<p>
Even when preprocessor directives are within the body of
indented code, the directives should start at the beginning of
the line.
</p>
<CODE_SNIPPET>
// 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();
}
</CODE_SNIPPET>
<BAD_CODE_SNIPPET>
// 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();
}
</BAD_CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Class Format">
<SUMMARY>
Sections in <code>public</code>, <code>protected</code> and
<code>private</code> order, each indented one space.
</SUMMARY>
<BODY>
<p>
The basic format for a class declaration (lacking the
comments, see <a HREF="#Class_Comments">Class Comments</a> for
a discussion of what comments are needed) is:
</p>
<CODE_SNIPPET>
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_;
DISALLOW_COPY_AND_ASSIGN(MyClass);
};
</CODE_SNIPPET>
<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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Constructor Initializer Lists">
<SUMMARY>
Constructor initializer lists can be all on one line or with
subsequent lines indented four spaces.
</SUMMARY>
<BODY>
<p>
There are two acceptable formats for initializer lists:
</p>
<CODE_SNIPPET>
// When it all fits on one line:
MyClass::MyClass(int var) : some_var_(var), some_other_var_(var + 1) {}
</CODE_SNIPPET>
<p>
or
</p>
<CODE_SNIPPET>
// When it requires multiple lines, indent 4 spaces, putting the colon on
// the first initializer line:
MyClass::MyClass(int var)
: some_var_(var), // 4 space indent
some_other_var_(var + 1) { // lined up
...
DoSomething();
...
}
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Namespace Formatting">
<SUMMARY>
The contents of namespaces are not indented.
</SUMMARY>
<BODY>
<p>
<a href="#Namespaces">Namespaces</a> do not add an extra level of
indentation. For example, use:
</p>
<CODE_SNIPPET>
namespace {
void foo() { // Correct. No extra indentation within namespace.
...
}
} // namespace
</CODE_SNIPPET>
<p>
Do not indent within a namespace:
</p>
<BAD_CODE_SNIPPET>
namespace {
// Wrong. Indented when it should not be.
void foo() {
...
}
} // namespace
</BAD_CODE_SNIPPET>
<p>
When declaring nested namespaces, put each namespace on its own line.
</p>
<CODE_SNIPPET>
namespace foo {
namespace bar {
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Horizontal Whitespace">
<SUMMARY>
Use of horizontal whitespace depends on location. Never put trailing
whitespace at the end of a line.
</SUMMARY>
<BODY>
<SUBSECTION title="General">
<CODE_SNIPPET>
void f(bool b) { // Open braces should always have a space before them.
...
int i = 0; // Semicolons usually have no space before them.
int x[] = { 0 }; // Spaces inside braces for braced-init-list are
int x[] = {0}; // optional. If you use them, put them on both sides!
// 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.
...
</CODE_SNIPPET>
<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>
</SUBSECTION>
<SUBSECTION title="Loops and Conditionals">
<CODE_SNIPPET>
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 &lt; 5; ++i) {
switch ( i ) { // Loops and conditions may have spaces inside
if ( test ) { // parentheses, but this is rare. Be consistent.
for ( int i = 0; i &lt; 5; ++i ) {
for ( ; i &lt; 5 ; ++i) { // For loops always have a space after the
... // semicolon, and may have a space before the
// semicolon.
for (auto x : counts) { // Range-based for loops always have a
... // space before and after the colon.
}
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.
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Operators">
<CODE_SNIPPET>
x = 0; // Assignment operators always have spaces around
// them.
x = -5; // No spaces separating unary operators and their
++x; // arguments.
if (x &amp;&amp; !y)
...
v = w * x + y / z; // Binary operators usually have spaces around them,
v = w*x + y/z; // but it's okay to remove spaces around factors.
v = w * (x + z); // Parentheses should have no spaces inside them.
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Templates and Casts">
<CODE_SNIPPET>
vector&lt;string&gt; x; // No spaces inside the angle
y = static_cast&lt;char*&gt;(x); // brackets (&lt; and &gt;), before
// &lt;, or between &gt;( in a cast.
vector&lt;char *&gt; x; // Spaces between type and pointer are
// okay, but be consistent.
set&lt;list&lt;string&gt;&gt; x; // Permitted in C++11 code.
set&lt;list&lt;string&gt; &gt; x; // C++03 required a space in &gt; &gt;.
set&lt; list&lt;string&gt; &gt; x; // You may optionally use
// symmetric spacing in &lt; &lt;.
</CODE_SNIPPET>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Vertical Whitespace">
<SUMMARY>
Minimize use of vertical whitespace.
</SUMMARY>
<BODY>
<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>
</BODY>
</STYLEPOINT>
</CATEGORY>
<CATEGORY title="Exceptions to the Rules">
<p>
The coding conventions described above are mandatory. However,
like all good rules, these sometimes have exceptions, which we
discuss here.
</p>
<STYLEPOINT title="Existing Non-conformant Code">
<SUMMARY>
You may diverge from the rules when dealing with code that does not
conform to this style guide.
</SUMMARY>
<BODY>
<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>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Windows Code">
<SUMMARY>
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.
</SUMMARY>
<BODY>
<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>
</BODY>
</STYLEPOINT>
</CATEGORY>
<PARTING_WORDS>
<p>
Use common sense and <em>BE CONSISTENT</em>.
</p>
<p>
If you are editing code, take a few minutes to look at the
code around you and determine its style. If they use spaces
around their <code>if</code> clauses, you should, too. If
their comments have little boxes of stars around them, make
your comments have little boxes of stars around them too.
</p>
<p>
The point of having style guidelines is to have a common
vocabulary of coding so people can concentrate on what you are
saying, rather than on how you are saying it. We present
global style rules here so people know the vocabulary. But
local style is also important. If code you add to a file
looks drastically different from the existing code around it,
the discontinuity throws readers out of their rhythm when they
go to read it. Try to avoid this.
</p>
<p>
OK, enough writing about writing code; the code itself is much
more interesting. Have fun!
</p>
</PARTING_WORDS>
<HR/>
<p align="right">
Revision 3.274
</p>
<address>
Benjy Weinberger<br/>
Craig Silverstein<br/>
Gregory Eitzmann<br/>
Mark Mentovai<br/>
Tashana Landray
</address>
</GUIDE>
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