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This notably includes changes for namespace naming and use of auto, as
well as clarification on when comments are necessary and attribute
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@ -15,7 +15,7 @@
<h2 class="ignoreLink" id="Background">Background</h2>
<p>C++ is the main development language used by
<p>C++ is one of the main development languages 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
@ -179,36 +179,37 @@ pitfalls of using header files.</p>
<h3 id="Self_contained_Headers">Self-contained Headers</h3>
<div class="summary">
<p>Header files should be self-contained and end in <code>.h</code>. Files that
are meant for textual inclusion, but are not headers, should end in
<code>.inc</code>. Separate <code>-inl.h</code> headers are disallowed.</p>
<p>Header files should be self-contained (compile on their own) and
end in <code>.h</code>. Non-header files that are meant for inclusion
should end in <code>.inc</code>. </p>
</div>
<div class="stylebody">
<p>All header files should be self-contained. In other
words, users and refactoring tools should not have to adhere to special
conditions in order to include the header. Specifically, a
header should have <a href="#The__define_Guard">header guards</a>,
should include all other headers it needs, and should not require any
particular symbols to be defined.</p>
<p>All header files should be self-contained. Users and refactoring
tools should not have to adhere to special conditions to include the
header. Specifically, a header should
have <a href="#The__define_Guard">header guards</a> and include all
other headers it needs.</p>
<p>There are rare cases where a file is not meant to be self-contained, but
instead is meant to be textually included at a specific point in the code.
Examples are files that need to be included multiple times or
platform-specific extensions that essentially are part of other headers. Such
files should use the file extension <code>.inc</code>.</p>
<p>If a template or inline function is declared in a <code>.h</code>
file, that same header should provide its definition. The definitions
of these constructs must be included into every <code>.cc</code> file
that uses them, or the program may fail to link in some build
configurations. Do not move these definitions to separately included
header files (<code>-inl.h</code>); this practice was common in the
past, but is no longer allowed.</p>
<p>If a template or inline function is declared in a <code>.h</code> file,
define it in that same file. The definitions of these constructs must
be included into every <code>.cc</code> file that uses them, or the
program may fail to link in some build configurations. Do not move these
definitions to separate <code>-inl.h</code> files.</p>
<p>As an exception, a template that is explicitly instantiated for
all relevant sets of template arguments, or that is a private
implementation detail of a class, is allowed to be defined in the one
and only <code>.cc</code> file that instantiates the template.</p>
<p>As an exception, a function template that is explicitly
instantiated for all relevant sets of template arguments, or
that is a private member of a class, may
be defined in the only <code>.cc</code> file that
instantiates the template.</p>
<p>There are rare cases where a file is not meant to be
self-contained, but it is meant to be included at a specific point in
the code. Examples are files that need to be included multiple times
or platform-specific implementation details that are essentially part
of other self-contained headers. Such files should use the file
extension <code>.inc</code>.</p>
</div>
@ -509,7 +510,8 @@ system-specific code small and localized. Example:</p>
<p>With few exceptions, place code in a namespace. Namespaces
should have unique names based on the project name, and possibly
its path. Unnamed namespaces in <code>.cc</code> files are
encouraged. Do not use <i>using-directives</i>. Do not use
encouraged. Do not use <i>using-directives</i> (e.g.
<code>using namespace foo</code>). Do not use
inline namespaces.</p>
</div>
@ -732,7 +734,7 @@ external resources or have significant dependencies.</p>
</div>
<div class="decision">
<p>Sometimes it is useful, or even necessary, to define a
<p>Sometimes it is useful to define a
function not bound to a class instance. Such a function
can be either a static member or a nonmember function.
Nonmember functions should not depend on external
@ -758,7 +760,7 @@ class FooBar {
}
</pre>
<p>If you must define a nonmember function and it is only
<p>If you 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()
@ -1097,10 +1099,10 @@ also omit <code>explicit</code>, in order to support copy-initialization
<h3 id="Copyable_Movable_Types">Copyable and Movable Types</h3>
<a id="Copy_Constructors"></a>
<div class="summary">
<p>Support copying and/or moving if it makes sense for your type.
Otherwise, disable the implicitly generated special
functions that perform copies and moves.</p>
</div>
<p>Support copying and/or moving if these operations are clear and meaningful
for your type. Otherwise, disable the implicitly generated special functions
that perform copies and moves.
</p></div>
<div class="stylebody">
@ -1122,15 +1124,15 @@ in some situations, e.g. when passing objects by value.</p>
</div>
<div class="pros">
<p>Objects of copyable and movable types can be passed and returned
by value, which makes APIs simpler, safer, and more general.
Unlike when passing pointers or references, there's no risk of
confusion over ownership, lifetime, mutability, and similar
issues, and no need to specify them in the contract. It also
prevents non-local interactions between the client and the
implementation, which makes them easier to understand and
maintain. Such objects can be used with generic
APIs that require pass-by-value, such as most containers.</p>
<p>Objects of copyable and movable types can be passed and returned by value,
which makes APIs simpler, safer, and more general. Unlike when passing objects
by pointer or reference, there's no risk of confusion over ownership,
lifetime, mutability, and similar issues, and no need to specify them in the
contract. It also prevents non-local interactions between the client and the
implementation, which makes them easier to understand, maintain, and optimize by
the compiler. Further, such objects can be used with generic APIs that
require pass-by-value, such as most containers, and they allow for additional
flexibility in e.g., type composition.</p>
<p>Copy/move constructors and assignment operators are usually
easier to define correctly than alternatives
@ -1151,54 +1153,39 @@ in some cases.</p>
</div>
<div class="cons">
<p>Many types do not need to be copyable, and providing copy
operations for them can be confusing, nonsensical, or outright
incorrect. Copy/assigment operations for base class types are
hazardous, because use of them can lead to
<a href="https://en.wikipedia.org/wiki/Object_slicing">object
slicing</a>. Defaulted or carelessly-implemented copy operations
can be incorrect, and the resulting bugs can be confusing and
difficult to diagnose.</p>
<p>Some types do not need to be copyable, and providing copy
operations for such types can be confusing, nonsensical, or outright
incorrect. Types representing singleton objects (<code>Registerer</code>),
objects tied to a specific scope (<code>Cleanup</code>), or closely coupled to
object identity (<code>Mutex</code>) cannot be copied meaningfully.
Copy operations for base class types that are to be used
polymorphically are hazardous, because use of them can lead to
<a href="http://en.wikipedia.org/wiki/Object_slicing">object slicing</a>.
Defaulted or carelessly-implemented copy operations can be incorrect, and the
resulting bugs can be confusing and difficult to diagnose.</p>
<p>Copy constructors are invoked implicitly, which makes the
invocation easy to miss. This may cause confusion, particularly
for programmers used to languages where pass-by-reference is
conventional or mandatory. It may also encourage excessive
copying, which can cause performance problems.</p>
invocation easy to miss. This may cause confusion for programmers used to
languages where pass-by-reference is conventional or mandatory. It may also
encourage excessive copying, which can cause performance problems.</p>
</div>
<div class="decision">
<p>Make your type copyable/movable if it will be useful, and if it makes sense
in the context of the rest of the API. As a rule of thumb, if the behavior
(including computational complexity) of a copy isn't immediately obvious to
users of your type, your type shouldn't be copyable. If you define a copy or
move constructor, define the corresponding assignment operator, and vice-versa.
If your type is copyable, do not define move operations unless they are
significantly more efficient than the corresponding copy operations. If your
<p>Provide the copy and move operations if their meaning is clear to a casual
user and the copying/moving does not incur unexpected costs. If you define a
copy or move constructor, define the corresponding assignment operator, and
vice-versa. If your type is copyable, do not define move operations unless they
are significantly more efficient than the corresponding copy operations. If your
type is not copyable, but the correctness of a move is obvious to users of the
type, you may make the type move-only by defining both of the move operations.
</p>
<p>If your type provides copy operations, it is recommended that you design
your class so that the default implementation of those operations is correct
and that you explicitly define them with <code>= default</code>. Remember to
review the correctness of any defaulted operations as you would any other
code.</p>
<pre>class Foo { // Copyable and movable type.
public:
Foo(Foo&amp;&amp; other) = default;
Foo(const Foo&amp; other) = default;
Foo&amp; operator=(Foo&amp;&amp; other) = default;
Foo&amp; operator=(const Foo&amp; other) = default;
private:
string field_;
};
</pre>
your class so that the default implementation of those operations is correct.
Remember to review the correctness of any defaulted operations as you would any
other code, and to document that your class is copyable and/or cheaply movable
if that's an API guarantee.</p>
<pre class="badcode">class Foo {
public:
@ -1218,93 +1205,11 @@ copyable, provide a public virtual <code>Clone()</code>
method, and a protected copy constructor that derived classes
can use to implement it.</p>
<p>If you do not want to support copy/move operations on
your type, explicitly disable them using <code>= delete</code> or
whatever
other mechanism your project uses.
<p>If you do not want to support copy/move operations on your type,
explicitly disable them using <code>= delete</code> in
the <code>public:</code> section. </p>
</p></div>
</div>
<h3 id="Delegating_and_inheriting_constructors">Delegating and Inheriting Constructors</h3>
<div class="summary">
<p> Use delegating and inheriting
constructors when they reduce code duplication.</p>
</div>
<div class="stylebody">
<div class="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>
<pre>X::X(const string&amp; name) : name_(name) {
...
}
X::X() : X("") {}
</pre>
<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>
<pre>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.
};
</pre>
<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>
</div>
<div class="pros">
<p>Delegating and inheriting constructors reduce
verbosity and boilerplate, which can improve
readability.</p>
<p>Delegating constructors are familiar to Java
programmers.</p>
</div>
<div class="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>
</div>
<div class="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>
</div>
</div>
<h3 id="Structs_vs._Classes">Structs vs. Classes</h3>
@ -1718,7 +1623,7 @@ be in the following order:</p>
<li>Constants (<code>static const</code> data
members)</li>
<li>Constructors</li>
<li>Constructors and assignment operators</li>
<li>Destructor</li>
@ -2190,8 +2095,6 @@ do use shared ownership, prefer to use
you need to be compatible with older versions of C++.
Never use <code>std::auto_ptr</code>. Instead, use
<code>std::unique_ptr</code>.</p>
</div>
</div>
@ -2628,9 +2531,10 @@ them.</p>
explicit type conversion is necessary. </p>
<ul>
<li>Convert arithmetic types using brace initialization. This is
the safest approach because code will not compile if conversion can
result in information loss. The syntax is also concise.</li>
<li>Use brace initialization to convert arithmetic types
(e.g. <code>int64{x}</code>). This is the safest approach because code
will not compile if conversion can result in information loss. The
syntax is also concise.</li>
@ -2715,7 +2619,7 @@ may select a different overload than you expect).</li>
<li>The practice of building up output through chains
of <code>&lt;&lt;</code> operators interferes with
internationalization, because it bakes word order into the
code, and streams' support for localization is <a href="https://www.boost.org/doc/libs/1_48_0/libs/locale/doc/html/rationale.html#rationale_why">
code, and streams' support for localization is <a href="http://www.boost.org/doc/libs/1_48_0/libs/locale/doc/html/rationale.html#rationale_why">
flawed</a>.</li>
@ -3227,7 +3131,8 @@ uint64_t my_mask = 3ULL &lt;&lt; 48;
<div class="summary">
<p>Be very cautious with macros. Prefer inline functions,
enums, and <code>const</code> variables to macros.</p>
enums, and <code>const</code> variables to macros. Don't
use macros to define pieces of a C++ API.</p>
</div>
<div class="stylebody">
@ -3236,6 +3141,26 @@ enums, and <code>const</code> variables to macros.</p>
the code the compiler sees. This can introduce unexpected
behavior, especially since macros have global scope.</p>
<p>The problems introduced by macros are especially severe
when they are used to define pieces of a C++ API,
and still more so for public APIs. Every error message from
the compiler when developers incorrectly use that interface
now must explain how the macros formed the interface.
Refactoring and analysis tools have a dramatically harder
time updating the interface. As a consequence, we
specifically disallow using macros in this way.
For example, avoid patterns like:</p>
<pre class="badcode">class WOMBAT_TYPE(Foo) {
// ...
public:
EXPAND_PUBLIC_WOMBAT_API(Foo)
EXPAND_WOMBAT_COMPARISONS(Foo, ==, &lt;)
};
</pre>
<p>Luckily, macros are not nearly as necessary in C++ as
they are in C. Instead of using a macro to inline
performance-critical code, use an inline function.
@ -3254,7 +3179,10 @@ 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>
to achieve the same result. If you need to use a macro to
define an interface, contact
your project leads to request
a waiver of this rule.</p>
<p>The following usage pattern will avoid many problems
with macros; if you use macros, follow it whenever
@ -3347,96 +3275,47 @@ memset(&amp;data, 0, sizeof(data));
<h3 id="auto">auto</h3>
<div class="summary">
<p>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.</p>
<p>Use <code>auto</code> to avoid type names that are noisy, obvious,
or unimportant - cases where the type doesn't aid in clarity for the
reader. Continue to use manifest type declarations when it helps
readability, and never use <code>auto</code> for anything but local
variables.</p>
</div>
<div class="stylebody">
<div class="definition">
<p> 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.</p>
<pre>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].
</pre>
</div>
<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
<a href="#trailing_return">trailing return type</a>.</p>
<div class="pros">
<p>C++ type names can sometimes be long and cumbersome,
especially when they involve templates or namespaces. In
a statement like:</p>
<pre>sparse_hash_map&lt;string, int&gt;::iterator iter = m.find(val);
</pre>
<p>
</p><ul>
<li>C++ type names can be long and cumbersome, especially when they
involve templates or namespaces.</li>
<li>When a C++ type name is repeated within a single declaration or a
small code region, the repetition may not be aiding readability.</li>
<li>It is sometimes safer to let the type be specified by the type of
the initialization expression, since that avoids the possibility of
unintended copies or type conversions.</li>
</ul>
<p></p>
</div>
<p>the return type is hard to read, and obscures the
primary purpose of the statement. Changing it to:</p>
<pre>auto iter = m.find(val);
</pre>
<p>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:</p>
<pre>diagnostics::ErrorStatus* status = new diagnostics::ErrorStatus("xyz");
</pre>
<p>Using <code>auto</code> makes it easier to use
intermediate variables when appropriate, by reducing the
burden of writing their types explicitly.</p>
<div class="cons">
<p>Sometimes code is clearer when types are manifest,
especially when a variable's initialization depends on
things that were declared far away. In an expression
things that were declared far away. In expressions
like:</p>
<pre>auto i = x.Lookup(key);
<pre class="badcode">auto foo = x.add_foo();
auto i = y.Find(key);
</pre>
<p>it may not be obvious what <code>i</code>'s type is,
if <code>x</code> was declared hundreds of lines earlier.</p>
<p>It may not be obvious what the resulting types are if the type
of <code>y</code> isn't very well known, or if <code>y</code> was
declared many lines earlier.</p>
<p>Programmers have to understand the difference between
<code>auto</code> and <code>const auto&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 exact
rules have been in flux, and compilers don't all
implement the final rules yet.
The declarations:</p>
<pre>auto x{3};
auto y = {3};
</pre>
<p>mean different things &#8212;
<code>x</code> is an <code>int</code>, while
<code>y</code> is a <code>std::initializer_list&lt;int&gt;</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
@ -3446,12 +3325,47 @@ than intended.</p>
<div class="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
initialize an <code>auto</code>-typed variable with
a braced initializer list.
</p></div>
<p><code>auto</code> is permitted, for local variables only, when it
increases readability, particularly as described below. Do not
use <code>auto</code> for file-scope or namespace-scope variables, or
for class members. Never initialize an <code>auto</code>-typed
variable with a braced initializer list.</p>
<p>Specific cases where <code>auto</code> is allowed or encouraged:
</p><ul>
<li>(Encouraged) For iterators and other long/cluttery type names, particularly
when the type is clear from context (calls
to <code>find</code>, <code>begin</code>, or <code>end</code> for
instance).</li>
<li>(Allowed) When the type is clear from local context (in the same expression
or within a few lines). Initialization of a pointer or smart pointer
with calls
to <code>new</code>
commonly falls into this category, as does use of <code>auto</code> in
a range-based loop over a container whose type is spelled out
nearby.</li>
<li>(Allowed) When the type doesn't matter because it isn't being used for
anything other than equality comparison.</li>
<li>(Encouraged) When iterating over a map with a range-based loop
(because it is often assumed that the correct type
is <code>pair&lt;KeyType,ValueType&gt;</code> whereas it is actually
<code>pair&lt;const KeyType,ValueType&gt;</code>). This is
particularly well paired with local <code>key</code>
and <code>value</code> aliases for <code>.first</code>
and <code>.second</code> (often const-ref).
<pre class="code">for (const auto&amp; item : some_map) {
const KeyType&amp; key = item.first;
const ValType&amp; value = item.second;
// The rest of the loop can now just refer to key and value,
// a reader can see the types in question, and we've avoided
// the too-common case of extra copies in this iteration.
}
</pre>
</li>
</ul>
<p></p>
</div>
</div>
@ -3553,9 +3467,8 @@ means can be confusing.</p>
<h3 id="Lambda_expressions">Lambda expressions</h3>
<div class="summary">
<p>Use lambda expressions where appropriate. Avoid
default lambda captures when capturing <code>this</code> or
if the lambda will escape the current scope.</p>
<p>Use lambda expressions where appropriate. Prefer explicit captures
when the lambda will escape the current scope.</p>
</div>
<div class="stylebody">
@ -3571,22 +3484,32 @@ functions as arguments. For example:</p>
});
</pre>
They further allow capturing variables from the enclosing scope either
explicitly by name, or implicitly using a default capture. Default captures
implicitly capture any variable referenced in the lambda body, including
<code>this</code> if any members are used. The default capture must come
first and be one of <code>&amp;</code> for capture by reference or
<code>=</code> for capture by value, followed by any explicit captures which
differ from the default:
<p> They further allow capturing variables from the enclosing scope either
explicitly by name, or implicitly using a default capture. Explicit captures
require each variable to be listed, as
either a value or reference capture:</p>
<pre>int weight = 3;
int sum = 0;
// Captures `weight` (implicitly) by value and `sum` by reference.
std::for_each(v.begin(), v.end(), [=, &amp;sum](int x) {
// Captures `weight` by value and `sum` by reference.
std::for_each(v.begin(), v.end(), [weight, &amp;sum](int x) {
sum += weight * x;
});
</pre>
Default captures implicitly capture any variable referenced in the
lambda body, including <code>this</code> if any members are used:
<pre>const std::vector&lt;int&gt; lookup_table = ...;
std::vector&lt;int&gt; indices = ...;
// Captures `lookup_table` by reference, sorts `indices` by the value
// of the associated element in `lookup_table`.
std::sort(indices.begin(), indices.end(), [&amp;](int a, int b) {
return lookup_table[a] &lt; lookup_table[b];
});
</pre>
<p>Lambdas were introduced in C++11 along with a set of utilities
for working with function objects, such as the polymorphic
wrapper <code>std::function</code>.
@ -3614,19 +3537,20 @@ wrapper <code>std::function</code>.
<div class="cons">
<ul>
<li>Default captures make it easy to overlook an
implicit capture and use of <code>this</code>.
</li>
<li>Variable capture in lambdas can be a source of dangling-pointer
bugs, particularly if a lambda escapes the current scope.</li>
<li>Variable capture in lambdas can be tricky, and
might be a new source of dangling-pointer bugs,
particularly if a lambda escapes the current scope.</li>
<li>Default captures by value can be misleading because they do not prevent
dangling-pointer bugs. Capturing a pointer by value doesn't cause a deep
copy, so it often has the same lifetime issues as capture by reference.
This is especially confusing when capturing 'this' by value, since the use
of 'this' is often implicit.</li>
<li>It's possible for use of lambdas to get out of
hand; very long nested anonymous functions can make
code harder to understand.</li>
</ul>
</div>
@ -3634,10 +3558,8 @@ wrapper <code>std::function</code>.
<ul>
<li>Use lambda expressions where appropriate, with formatting as
described <a href="#Formatting_Lambda_Expressions">below</a>.</li>
<li>Use default captures judiciously, when it aids readability.
In particular, prefer to write lambda captures explicitly when
capturing <code>this</code> or if the lambda will escape the
current scope. For example, instead of:
<li>Prefer explicit captures if the lambda may escape the current scope.
For example, instead of:
<pre class="badcode">{
Foo foo;
...
@ -3662,6 +3584,15 @@ prefer to write:
// reference.
</pre>
</li>
<li>Use default capture by reference ([&amp;]) only when the
lifetime of the lambda is obviously shorter than any potential
captures.
</li>
<li>Use default capture by value ([=]) only as a means of binding a
few variables for a short lambda, where the set of captured
variables is obvious at a glance. Prefer not to write long or
complex lambdas with default capture by value.
</li>
<li>Keep unnamed lambdas short. If a lambda body is more than
maybe five lines long, prefer to give the lambda a name, or to
use a named function instead of a lambda.</li>
@ -4029,10 +3960,59 @@ guide, the following C++11 features may not be used:</p>
</ul>
</div>
</div>
<h3 id="Nonstandard_Extensions">Nonstandard Extensions</h3>
<div class="summary">
<p>Nonstandard extensions to C++ may not be used unless otherwise specified.</p>
</div>
<div class="stylebody">
<div class="definition">
<p>Compilers support various extensions that are not part of standard C++. Such
extensions include GCC's <code>__attribute__</code>, intrinsic functions such
as <code>__builtin_prefetch</code>, designated initializers (e.g.
<code>Foo f = {.field = 3}</code>), inline assembly, <code>__COUNTER__</code>,
<code>__PRETTY_FUNCTION__</code>, compound statement expressions (e.g.
<code>foo = ({ int x; Bar(&amp;x); x })</code>, and the <code>a?:b</code>
syntax.</p>
</div>
<div class="pros">
<ul>
<li>Nonstandard extensions may provide useful features that do not exist
in standard C++. For example, some people think that designated
initializers are more readable than standard C++ features like
constructors.</li>
<li>Important performance guidance to the compiler can only be specified
using extensions.</li>
</ul>
</div>
<div class="cons">
<ul>
<li>Nonstandard extensions do not work in all compilers. Use of nonstandard
extensions reduces portability of code.</li>
<li>Even if they are supported in all targeted compilers, the extensions
are often not well-specified, and there may be subtle behavior differences
between compilers.</li>
<li>Nonstandard extensions add to the language features that a reader must
know to understand the code.</li>
</ul>
</div>
<div class="decision">
<p>Do not use nonstandard extensions. You may use portability wrappers that
are implemented using nonstandard extensions, so long as those wrappers
are provided by a designated project-wide
portability header.</p>
</div>
</div>
<h3 id="Aliases">Aliases</h3>
<div class="summary">
@ -4141,7 +4121,7 @@ the rules are the rules.</p>
<h3 id="General_Naming_Rules">General Naming Rules</h3>
<div class="summary">
<p>Names should be descriptive; eschew abbreviation.</p>
<p>Names should be descriptive; avoid abbreviation.</p>
</div>
<div class="stylebody">
@ -4299,14 +4279,6 @@ the trailing underscores that data members in classes have.</p>
Classes</a> for a discussion of when to use a struct
versus a class.</p>
<h4 class="stylepoint_subsection">Global Variables</h4>
<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>
</div>
<h3 id="Constant_Names">Constant Names</h3>
@ -4375,9 +4347,10 @@ of the class's member variables.</p>
<h3 id="Namespace_Names">Namespace Names</h3>
<div class="summary">
Namespace names are all lower-case. Top-level
namespace names are based on the project name
.
Namespace names are all lower-case. Top-level namespace names are
based on the project name
. Avoid collisions
between nested namespaces and well-known top-level namespaces.
</div>
<div class="stylebody">
@ -4397,21 +4370,28 @@ as variable names. Code inside the namespace seldom needs to
mention the namespace name, so there's usually no particular need
for abbreviation anyway.</p>
<p>Naming conventions for nested namespaces are at the discretion
of the team owning the enclosing namespace. If a nested namespace
is used to group related functions in a single header file, consider basing
its name on the header name, e.g. namespace <code>foo::bar</code>
for functions defined in the header file <code>bar.h</code>.
If the nested namespace is being used to distinguish implementation
details from user-facing declarations, one common choice is
<code>internal</code>.</p>
<p>Avoid nested namespaces that match well-known top-level
namespaces. Collisions between namespace names can lead to surprising
build breaks because of name lookup rules. In particular, do not
create any nested <code>std</code> namespaces. Prefer unique project
identifiers
(<code>websearch::index</code>, <code>websearch::index_util</code>)
over collision-prone names like <code>websearch::util</code>.</p>
<p>For <code>internal</code> namespaces, be wary of other code being
added to the same <code>internal</code> namespace causing a collision
(internal helpers within a team tend to be related and may lead to
collisions). In such a situation, using the filename to make a unique
internal name is helpful
(<code>websearch::index::frobber_internal</code> for use
in <code>frobber.h</code>)</p>
</div>
<h3 id="Enumerator_Names">Enumerator Names</h3>
<div class="summary">
<p>Enumerators should be named <i>either</i> like
<p>Enumerators (for both scoped and unscoped enums) should be named <i>either</i> like
<a href="#Constant_Names">constants</a> or like
<a href="#Macro_Names">macros</a>: either <code>kEnumName</code> or
<code>ENUM_NAME</code>.</p>
@ -4537,9 +4517,13 @@ comment and what style you use where.</p>
<h3 id="File_Comments">File Comments</h3>
<div class="summary">
<p> Start each file with license
boilerplate, followed by a description of its
contents.</p>
<p>Start each file with license boilerplate.</p>
<p>File comments describe the contents of a file. If a file declares,
implements, or tests exactly one abstraction that is documented by a comment
at the point of declaration, file comments are not required. All other files
must have file comments.</p>
</div>
<div class="stylebody">
@ -4559,30 +4543,22 @@ author line, consider deleting the author line.</p>
<h4 class="stylepoint_subsection">File Contents</h4>
<p>Every file should have a comment at the top describing
its contents, unless the specific conditions described below apply.</p>
<p>A file-level comment in a <code>.h</code> should broadly describe
the contents of the file. If the file declares multiple abstractions, the
file-level comment should document how they are related. A 1 or 2 sentence
file-level comment may be sufficient. The detailed documentation about
individual abstractions belongs with those abstractions, not at the file
level.</p>
<p>If a <code>.h</code> declares multiple abstractions, the file-level comment
should broadly describe the contents of the file, and how the abstractions are
related. A 1 or 2 sentence file-level comment may be sufficient. The detailed
documentation about individual abstractions belongs with those abstractions,
not at the file level.</p>
<p>Do not duplicate comments in both the <code>.h</code> and the
<code>.cc</code>. Duplicated comments diverge.</p>
<p>A file-level comment may be omitted if the file declares, implements, or
tests exactly one abstraction that is documented by a comment at the point
of declaration.</p><p>
</p></div>
</div>
<h3 id="Class_Comments">Class Comments</h3>
<div class="summary">
<p>Every class definition should have an accompanying comment
that describes what it is for and how it should be used.</p>
<p>Every non-obvious class declaration should have an accompanying
comment that describes what it is for and how it should be used.</p>
</div>
<div class="stylebody">
@ -4609,28 +4585,34 @@ surrounding multithreaded use.</p>
<p>The class comment is often a good place for a small example code snippet
demonstrating a simple and focused usage of the class.</p>
<p>When sufficiently separated (e.g. <code>.h</code> and <code>.cc</code>
files), comments describing the use of the class should go together with its
interface definition; comments about the class operation and implementation
should accompany the implementation of the class's methods.</p>
</div>
<h3 id="Function_Comments">Function Comments</h3>
<div class="summary">
<p>Declaration comments describe use of the function; comments
at the definition of a function describe operation.</p>
<p>Declaration comments describe use of the function (when it is
non-obvious); comments at the definition of a function describe
operation.</p>
</div>
<div class="stylebody">
<h4 class="stylepoint_subsection">Function Declarations</h4>
<p>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>Almost every function declaration should have comments immediately
preceding it that describe what the function does and how to use
it. These comments may be omitted only if the function is simple and
obvious (e.g. simple accessors for obvious properties of the
class). These comments should be descriptive ("Opens the file")
rather than imperative ("Open the file"); the comment describes the
function, it does not tell the function what to do. In general, these
comments do not describe how the function performs its task. Instead,
that should be left to comments in the function definition.</p>
<p>Types of things to mention in comments at the function
declaration:</p>
@ -4733,25 +4715,28 @@ is used for. In certain cases, more comments are required.</p>
<h4 class="stylepoint_subsection">Class Data Members</h4>
<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>
<p>The purpose of each class data member (also called an instance
variable or member variable) must be clear. If there are any
invariants (special values, relationships between members, lifetime
requirements) not clearly expressed by the type and name, they must be
commented. However, if the type and name suffice (<code>int
num_events_;</code>), no comment is needed.</p>
<p>In particular, add comments to describe the existence and meaning
of sentinel values, such as nullptr or -1, when they are not
obvious. For example:</p>
<pre>private:
// Keeps track of the total number of entries in the table.
// Used to ensure we do not go over the limit. -1 means
// Used to bounds-check table accesses. -1 means
// that we don't yet know how many entries the table has.
int num_total_entries_;
</pre>
<h4 class="stylepoint_subsection">Global Variables</h4>
<p>As with data members, all global variables should have
a comment describing what they are and what they are used
for. For example:</p>
<p>All global variables should have a comment describing what they
are, what they are used for, and (if unclear) why it needs to be
global. For example:</p>
<pre>// The total number of tests cases that we run through in this regression test.
const int kNumTestCases = 6;
@ -5226,10 +5211,10 @@ function call.</p>
<pre>class Foo {
public:
Foo(Foo&amp;&amp;) = default;
Foo(const Foo&amp;) = default;
Foo&amp; operator=(Foo&amp;&amp;) = default;
Foo&amp; operator=(const Foo&amp;) = default;
Foo(Foo&amp;&amp;);
Foo(const Foo&amp;);
Foo&amp; operator=(Foo&amp;&amp;);
Foo&amp; operator=(const Foo&amp;);
};
</pre>
@ -5254,6 +5239,12 @@ void Circle::Rotate(double /*radians*/) {}
void Circle::Rotate(double) {}
</pre>
<p>Attributes, and macros that expand to attributes, appear at the very
beginning of the function declaration or definition, before the
return type:</p>
<pre>MUST_USE_RESULT bool IsOK();
</pre>
</div>
<h3 id="Formatting_Lambda_Expressions">Lambda Expressions</h3>
@ -5790,7 +5781,7 @@ beginning of the line.</p>
<div class="stylebody">
<p>The basic format for a class declaration (lacking the
<p>The basic format for a class definition (lacking the
comments, see <a href="#Class_Comments">Class
Comments</a> for a discussion of what comments are
needed) is:</p>
@ -6210,6 +6201,6 @@ more interesting. Have fun!</p>
<hr>
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