styleguide/cppguide.html at 4f784372d815b7b7f532224882b5f5564c3395e7 · google/styleguide · GitHub

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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Google C++ Style Guide</title>
<link rel="stylesheet" href="include/styleguide.css">
<script src="include/styleguide.js"></script>
<link rel="shortcut icon" href="https://www.google.com/favicon.ico">
</head>
<body onload="initStyleGuide();">
<div id="content">
<h1>Google C++ Style Guide</h1>
<div class="horizontal_toc" id="tocDiv"></div>

<h2 id="Background" class="ignoreLink">Background</h2>

<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
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>
Most 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>

<h3 id="Goals">Goals of the Style Guide</h3>

<p>Why do we have this document?</p>

<p>There are a few core goals that we believe this guide should
serve. These are the fundamental <b>why</b>s that
underlie all of the individual rules. By bringing these ideas to
the fore, we hope to ground discussions and make it clearer to our
broader community why the rules are in place and why particular
decisions have been made. If you understand what goals each rule is
serving, it should be clearer to everyone when a rule may be waived
(some can be), and what sort of argument or alternative would be
necessary to change a rule in the guide.</p>

<p>The goals of the style guide as we currently see them are as follows:</p>
<dl>
<dt>Style rules should pull their weight</dt>
<dd>The benefit of a style rule
must be large enough to justify asking all of our engineers to
remember it. The benefit is measured relative to the codebase we would
get without the rule, so a rule against a very harmful practice may
still have a small benefit if people are unlikely to do it
anyway. This principle mostly explains the rules we don’t have, rather
than the rules we do: for example, <code>goto</code> contravenes many
of the following principles, but is already vanishingly rare, so the Style
Guide doesn’t discuss it.</dd>

<dt>Optimize for the reader, not the writer</dt>
<dd>Our codebase (and most individual components submitted to it) is
expected to continue for quite some time. As a result, more time will
be spent reading most of our code than writing it. We explicitly
choose to optimize for the experience of our average software engineer
reading, maintaining, and debugging code in our codebase rather than
ease when writing said code.  "Leave a trace for the reader" is a
particularly common sub-point of this principle: When something
surprising or unusual is happening in a snippet of code (for example,
transfer of pointer ownership), leaving textual hints for the reader
at the point of use is valuable (<code>std::unique_ptr</code>
demonstrates the ownership transfer unambiguously at the call
site). </dd>

<dt>Be consistent with existing code</dt>
<dd>Using one style consistently through our codebase lets us focus on
other (more important) issues. Consistency also allows for automation:
tools that format your code or adjust your <code>#include</code>s only
work properly when your code is consistent with the expectations of
the tooling. In many cases, rules that are attributed to "Be
Consistent" boil down to "Just pick one and stop worrying about it";
the potential value of allowing flexibility on these points is
outweighed by the cost of having people argue over them. However,
there are limits to consistency; it is a good tie breaker when there
is no clear technical argument, nor a long-term direction. It applies
more heavily locally (per file, or for a tightly-related set of
interfaces). Consistency should not generally be used as a
justification to do things in an old style without considering the
benefits of the new style, or the tendency of the codebase to converge
on newer styles over time.</dd>

<dt>Be consistent with the broader C++ community when appropriate</dt>
<dd>Consistency with the way other organizations use C++ has value for
the same reasons as consistency within our code base. If a feature in
the C++ standard solves a problem, or if some idiom is widely known
and accepted, that's an argument for using it. However, sometimes
standard features and idioms are flawed, or were just designed without
our codebase's needs in mind. In those cases (as described below) it's
appropriate to constrain or ban standard features.  In some cases we
prefer a homegrown or third-party library over a library defined in
the C++ Standard, either out of perceived superiority or insufficient
value to transition the codebase to the standard interface.</dd>

<dt>Avoid surprising or dangerous constructs</dt>
<dd>C++ has features that are more surprising or dangerous than one
might think at a glance. Some style guide restrictions are in place to
prevent falling into these pitfalls. There is a high bar for style
guide waivers on such restrictions, because waiving such rules often
directly risks compromising program correctness.
</dd>

<dt>Avoid constructs that our average C++ programmer would find tricky
or hard to maintain</dt>
<dd>C++ has features that may not be generally appropriate because of
the complexity they introduce to the code. In widely used
code, it may be more acceptable to use
trickier language constructs, because any benefits of more complex
implementation are multiplied widely by usage, and the cost in understanding
the complexity does not need to be paid again when working with new
portions of the codebase. When in doubt, waivers to rules of this type
can be sought by asking
your project leads. This is specifically
important for our codebase because code ownership and team membership
changes over time: even if everyone that works with some piece of code
currently understands it, such understanding is not guaranteed to hold a
few years from now.</dd>

<dt>Be mindful of our scale</dt>
<dd>With a codebase of 100+ million lines and thousands of engineers,
some mistakes and simplifications for one engineer can become costly
for many. For instance it's particularly important to
avoid polluting the global namespace: name collisions across a
codebase of hundreds of millions of lines are difficult to work with
and hard to avoid if everyone puts things into the global
namespace.</dd>

<dt>Concede to optimization when necessary</dt>
<dd>Performance optimizations can sometimes be necessary and
appropriate, even when they conflict with the other principles of this
document.</dd>
</dl>

<p>The intent of this document is to provide maximal guidance with
reasonable restriction. As always, common sense and good taste should
prevail. By this we specifically refer to the established conventions
of the entire Google C++ community, not just your personal preferences
or those of your team. Be skeptical about and reluctant to use
clever or unusual constructs: the absence of a prohibition is not the
same as a license to proceed.  Use your judgment, and if you are
unsure, please don't hesitate to ask your project leads to get additional
input.</p>


<h2 id="C++_Version">C++ Version</h2>

<p>Currently, code should target C++20, i.e., should not use C++23
  features. The C++ version targeted by this guide will advance
  (aggressively) over time.</p>


<p>Do not use
  <a href="#Nonstandard_Extensions">non-standard extensions</a>.</p>

<div>
<p>Consider portability to other environments before using features
from C++17 and C++20 in your project.</p>
</div>

<h2 id="Header_Files">Header Files</h2>

<p>In general, every <code>.cc</code> file should have an
associated <code>.h</code> file. There are some common
exceptions, such as unit tests 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>

<a id="The_-inl.h_Files"></a>
<h3 id="Self_contained_Headers">Self-contained Headers</h3>

<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> and be used sparingly.</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>When a header declares inline functions or templates that clients of the
header will instantiate, the inline functions and templates must also have
definitions in the header, either directly or in files it includes.  Do not move
these definitions to separately included header (<code>-inl.h</code>) files;
this practice was common in the past, but is no longer allowed.  When all
instantiations of a template occur in one <code>.cc</code> file, either because
they're <a href="https://en.cppreference.com/w/cpp/language/class_template#Explicit_instantiation">
explicit</a> or because the definition is accessible to only
the <code>.cc</code> file, the template definition can be kept in that file.</p>

<p>There are rare cases where a file designed to be included is not
self-contained.  These are typically intended to be included at unusual
locations, such as the middle of another file.  They might not
use <a href="#The__define_Guard">header guards</a>, and might not include
their prerequisites.  Name such files with the <code>.inc</code>
extension.  Use sparingly, and prefer self-contained headers when
possible.</p>

<h3 id="The__define_Guard">The #define Guard</h3>

<p>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>.</p>


<div>
<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>
</div>

<pre>#ifndef FOO_BAR_BAZ_H_
#define FOO_BAR_BAZ_H_

...

#endif  // FOO_BAR_BAZ_H_
</pre>


<h3 id="Include_What_You_Use">Include What You Use</h3>

<p>If a source or header file refers to a symbol defined elsewhere,
the file should directly include a header file which properly intends
to provide a declaration or definition of that symbol. It should not
include header files for any other reason.
</p>

<p>Do not rely on transitive inclusions. This allows people to remove
no-longer-needed <code>#include</code> statements from their headers without
breaking clients. This also applies to related headers
- <code>foo.cc</code> should include <code>bar.h</code> if it uses a
symbol from it even if <code>foo.h</code>
includes <code>bar.h</code>.</p>

<h3 id="Forward_Declarations">Forward Declarations</h3>

<p>Avoid using forward declarations where possible.
  Instead, <a href="#Include_What_You_Use">include the headers you need</a>.
</p>


<p class="definition"></p>
<p>A "forward declaration" is a declaration of an entity
  without an associated definition.</p>
<pre>// In a C++ source file:
class B;
void FuncInB();
extern int variable_in_b;
ABSL_DECLARE_FLAG(flag_in_b);
</pre>

<p class="pros"></p>
<ul>
  <li>Forward declarations can save compile time, as
  <code>#include</code>s force the compiler to open
  more files and process more input.</li>

  <li>Forward declarations can save on unnecessary
  recompilation. <code>#include</code>s can force
  your code to be recompiled more often, due to unrelated
  changes in the header.</li>
</ul>

<p class="cons"></p>
<ul>
  <li>Forward declarations can hide a dependency, allowing
  user code to skip necessary recompilation when headers
  change.</li>

  <li>A forward declaration as opposed to an <code>#include</code> statement
    makes it difficult for automatic tooling to discover the module
    defining the symbol.</li>

  <li>A forward declaration may be broken by subsequent
  changes to the library. Forward declarations of functions
  and templates can prevent the header owners from making
  otherwise-compatible changes to their APIs, such as
  widening a parameter type, adding a template parameter
  with a default value, or migrating to a new namespace.</li>

  <li>Forward declaring symbols from namespace
  <code>std::</code> yields undefined behavior.</li>

  <li>It can be difficult to determine whether a forward
  declaration or a full <code>#include</code> is needed.
  Replacing an <code>#include</code> with a forward
  declaration can silently change the meaning of
  code:
<pre>// b.h:
struct B {};
struct D : B {};

// good_user.cc:
#include "b.h"
void f(B*);
void f(void*);
void test(D* x) { f(x); }  // Calls f(B*)
</pre>
  If the <code>#include</code> was replaced with forward
  decls for <code>B</code> and <code>D</code>,
  <code>test()</code> would call <code>f(void*)</code>.
  </li>

  <li>Forward declaring multiple symbols from a header
  can be more verbose than simply
  <code>#include</code>ing the header.</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>


</ul>

<p class="decision"></p>
<p>Try to avoid forward declarations of entities
defined in another project.</p>

<h3 id="Inline_Functions">Inline Functions</h3>

<p>Define functions inline only when they are small, say, 10
lines or fewer.</p>

<p class="definition"></p>
<p>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.</p>

<p class="pros"></p>
<p>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.</p>

<p class="cons"></p>
<p>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.</p>

<p class="decision"></p>
<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>

<h3 id="Names_and_Order_of_Includes">Names and Order of Includes</h3>

<p>Include headers in the following order: Related header, C system headers,
C++ standard library headers,
other libraries' headers, your project's
headers.</p>

<p>
All of a project's header files should be
listed as descendants of the project's source
directory without use of UNIX directory aliases
<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>

<pre>#include "base/logging.h"
</pre>

<p>Headers should only be included using an angle-bracketed path if the library
requires you to do so. In particular, the following headers require angle
brackets:</p>

<ul>
<li>C and C++ standard library headers (e.g. <code>&lt;stdlib.h&gt;</code>
  and <code>&lt;string&gt;</code>).</li>
<li>POSIX, Linux, and Windows system headers (e.g. <code>&lt;unistd.h&gt;</code>
  and <code>&lt;windows.h&gt;</code>).</li>
<li>In rare cases, third_party libraries (e.g. <code>&lt;Python.h&gt;</code>).</li>
</ul>

<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>.</li>

  <li>A blank line</li>

  <li>C system headers, and any other headers in angle brackets with the
    <code>.h</code> extension, e.g., <code>&lt;unistd.h&gt;</code>,
    <code>&lt;stdlib.h&gt;</code>, <code>&lt;Python.h&gt;</code>.</li>

  <li>A blank line</li>

  <li>C++ standard library headers (without file extension), e.g.,
    <code>&lt;algorithm&gt;</code>, <code>&lt;cstddef&gt;</code>.</li>

  <li>A blank line</li>

  <div>
  <li>Other libraries' <code>.h</code> files.</li>

  <li>A blank line</li>
  </div>

  <li>
  Your project's <code>.h</code>
  files.</li>
</ol>

<p>Separate each non-empty group with one blank line.</p>

<p>With the preferred ordering, if the related header
<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 usually in the same
directory (e.g., <code>base/basictypes_test.cc</code> and
<code>base/basictypes.h</code>), but may sometimes be in different
directories too.</p>


<p>Note that the C headers such as <code>stddef.h</code>
are essentially interchangeable with their C++ counterparts
(<code>cstddef</code>).
Either style is acceptable, but prefer consistency with existing code.</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>

<pre>#include "foo/server/fooserver.h"

#include &lt;sys/types.h&gt;
#include &lt;unistd.h&gt;

#include &lt;string&gt;
#include &lt;vector&gt;

#include "base/basictypes.h"
#include "foo/server/bar.h"
#include "third_party/absl/flags/flag.h"
</pre>

<p><b>Exception:</b></p>

<p>Sometimes, system-specific code needs
conditional includes. Such code can put conditional
includes after other includes. Of course, keep your
system-specific code small and localized. Example:</p>

<pre>#include "foo/public/fooserver.h"

#include "base/port.h"  // For LANG_CXX11.

#ifdef LANG_CXX11
#include &lt;initializer_list&gt;
#endif  // LANG_CXX11
</pre>

<h2 id="Scoping">Scoping</h2>

<h3 id="Namespaces">Namespaces</h3>

<p>With few exceptions, place code in a namespace. Namespaces
should have unique names based on the project name, and possibly
its path. Do not use <i>using-directives</i> (e.g.,
<code>using namespace foo</code>). Do not use
inline namespaces. For unnamed namespaces, see
<a href="#Internal_Linkage">Internal Linkage</a>.

</p><p class="definition"></p>
<p>Namespaces subdivide the global scope
into distinct, named scopes, and so are useful for preventing
name collisions in the global scope.</p>

<p class="pros"></p>

<p>Namespaces provide a method for preventing name conflicts
in large programs while allowing most code to use reasonably
short names.</p>

<p>For example, if two different projects have a class
<code>Foo</code> in the global scope, these symbols may
collide at compile time or at runtime. If each project
places their code in a namespace, <code>project1::Foo</code>
and <code>project2::Foo</code> are now distinct symbols that
do not collide, and code within each project's namespace
can continue to refer to <code>Foo</code> without the prefix.</p>

<p>Inline namespaces automatically place their names in
the enclosing scope. Consider the following snippet, for
example:</p>

<pre class="neutralcode">namespace outer {
inline namespace inner {
  void foo();
}  // namespace inner
}  // namespace outer
</pre>

<p>The expressions <code>outer::inner::foo()</code> and
<code>outer::foo()</code> are interchangeable. Inline
namespaces are primarily intended for ABI compatibility
across versions.</p>

<p class="cons"></p>

<p>Namespaces can be confusing, because they complicate
the mechanics of figuring out what definition a name refers
to.</p>

<p>Inline namespaces, in particular, can be confusing
because names aren't actually restricted to the namespace
where they are declared. They are only useful as part of
some larger versioning policy.</p>

<p>In some contexts, it's necessary to repeatedly refer to
symbols by their fully-qualified names. For deeply-nested
namespaces, this can add a lot of clutter.</p>

<p class="decision"></p>

<p>Namespaces should be used as follows:</p>

<ul>
  <li>Follow the rules on <a href="#Namespace_Names">Namespace Names</a>.
  </li><li>Terminate multi-line namespaces with comments as shown in the given examples.
  </li><li>

  <p>Namespaces wrap the entire source file after
  includes,
  <a href="https://gflags.github.io/gflags/">
  gflags</a> definitions/declarations
  and forward declarations of classes from other namespaces.</p>

<pre>// In the .h file
namespace mynamespace {

// All declarations are within the namespace scope.
// Notice the lack of indentation.
class MyClass {
 public:
  ...
  void Foo();
};

}  // namespace mynamespace
</pre>

<pre>// In the .cc file
namespace mynamespace {

// Definition of functions is within scope of the namespace.
void MyClass::Foo() {
  ...
}

}  // namespace mynamespace
</pre>

  <p>More complex <code>.cc</code> files might have additional details,
  like flags or using-declarations.</p>

<pre>#include "a.h"

ABSL_FLAG(bool, someflag, false, "a flag");

namespace mynamespace {

using ::foo::Bar;

...code for mynamespace...    // Code goes against the left margin.

}  // namespace mynamespace
</pre>
  </li>

  <li>To place generated protocol
  message code in a namespace, use the
  <code>package</code> specifier in the
  <code>.proto</code> file. See


  <a href="https://developers.google.com/protocol-buffers/docs/reference/cpp-generated#package">
  Protocol Buffer Packages</a>
  for details.</li>

  <li>Do not declare anything in namespace
  <code>std</code>, including forward declarations of
  standard library classes. Declaring entities in
  namespace <code>std</code> is undefined behavior, i.e.,
  not portable. To declare entities from the standard
  library, include the appropriate header file.</li>

  <li><p>You may not use a <i>using-directive</i>
  to make all names from a namespace available.</p>

<pre class="badcode">// Forbidden -- This pollutes the namespace.
using namespace foo;
</pre>
  </li>

  <li><p>Do not use <i>Namespace aliases</i> at namespace scope
  in header files except in explicitly marked
  internal-only namespaces, because anything imported into a namespace
  in a header file becomes part of the public
  API exported by that file.</p>

<pre>// Shorten access to some commonly used names in .cc files.
namespace baz = ::foo::bar::baz;
</pre>

<pre>// Shorten access to some commonly used names (in a .h file).
namespace librarian {
namespace internal {  // Internal, not part of the API.
namespace sidetable = ::pipeline_diagnostics::sidetable;
}  // namespace internal

inline void my_inline_function() {
  // namespace alias local to a function (or method).
  namespace baz = ::foo::bar::baz;
  ...
}
}  // namespace librarian
</pre>

  </li><li>Do not use inline namespaces.</li>

  <li><p>Use namespaces with "internal" in the name to document parts of an API that
    should not be mentioned by users of the API.
    </p>

<pre class="badcode">// We shouldn't use this internal name in non-absl code.
using ::absl::container_internal::ImplementationDetail;
</pre>
  </li>

  <li><p>Single-line nested namespace declarations

    are preferred in new code, but are not required.</p>
  </li>
</ul>

<a id="Unnamed_Namespaces_and_Static_Variables"></a>
<h3 id="Internal_Linkage">Internal Linkage</h3>

<p>When definitions in a <code>.cc</code> file do not need to be
referenced outside that file, give them internal linkage by placing
them in an unnamed namespace or declaring them <code>static</code>.
Do not use either of these constructs in <code>.h</code> files.

</p><p class="definition"></p>
<p>All declarations can be given internal linkage by placing them in unnamed
namespaces. Functions and variables can also be given internal linkage by
declaring them <code>static</code>. This means that anything you're declaring
can't be accessed from another file. If a different file declares something with
the same name, then the two entities are completely independent.</p>

<p class="decision"></p>

<p>Use of internal linkage in <code>.cc</code> files is encouraged
for all code that does not need to be referenced elsewhere.
Do not use internal linkage in <code>.h</code> files.</p>

<p>Format unnamed namespaces like named namespaces. In the
  terminating comment, leave the namespace name empty:</p>

<pre>namespace {
...
}  // namespace
</pre>

<h3 id="Nonmember,_Static_Member,_and_Global_Functions">Nonmember, Static Member, and Global Functions</h3>

<p>Prefer placing nonmember functions in a namespace; use completely global
functions rarely. Do not use a class simply to group static members. Static
methods of a class should generally be closely related to instances of the
class or the class's static data.</p>


<p class="pros"></p>
<p>Nonmember and static member functions can be useful in
some situations. Putting nonmember functions in a
namespace avoids polluting the global namespace.</p>

<p class="cons"></p>
<p>Nonmember and static member functions may make more sense
as members of a new class, especially if they access
external resources or have significant dependencies.</p>

<p class="decision"></p>
<p>Sometimes it is useful to define a
function not bound to a class instance. Such a function
can be either a static member or a nonmember function.
Nonmember functions should not depend on external
variables, and should nearly always exist in a namespace.
Do not create classes only to group static members;
this is no different than just giving the names a
common prefix, and such grouping is usually unnecessary anyway.</p>

<p>If you define a nonmember function and it is only
needed in its <code>.cc</code> file, use
<a href="#Internal_Linkage">internal linkage</a> to limit
its scope.</p>

<h3 id="Local_Variables">Local Variables</h3>

<p>Place a function's variables in the narrowest scope
possible, and initialize variables in the declaration.</p>

<p>C++ allows you to declare variables anywhere in a function.
We encourage you to declare them in a scope as local as
possible, and as close to the first use as possible.
This makes it easier for the reader to find the
declaration and see what type the variable is and what it
was initialized to. In particular, initialization should
be used instead of declaration and assignment, e.g.,:</p>

<pre class="badcode">int i;
i = f();      // Bad -- initialization separate from declaration.
</pre>

<pre>int i = f();  // Good -- declaration has initialization.
</pre>


<pre class="badcode">int jobs = NumJobs();
// More code...
f(jobs);      // Bad -- declaration separate from use.
</pre>

<pre>int jobs = NumJobs();
f(jobs);      // Good -- declaration immediately (or closely) followed by use.
</pre>

<pre class="badcode">std::vector&lt;int&gt; v;
v.push_back(1);  // Prefer initializing using brace initialization.
v.push_back(2);
</pre>

<pre>std::vector&lt;int&gt; v = {1, 2};  // Good -- v starts initialized.
</pre>

<p>Variables needed for <code>if</code>, <code>while</code>
and <code>for</code> statements should normally be declared
within those statements, so that such variables are confined
to those scopes.  E.g.:</p>

<pre>while (const char* p = strchr(str, '/')) str = p + 1;
</pre>

<p>There is one caveat: if the variable is an object, its
constructor is invoked every time it enters scope and is
created, and its destructor is invoked every time it goes
out of scope.</p>

<pre class="badcode">// Inefficient implementation:
for (int i = 0; i &lt; 1000000; ++i) {
  Foo f;  // My ctor and dtor get called 1000000 times each.
  f.DoSomething(i);
}
</pre>

<p>It may be more efficient to declare such a variable
used in a loop outside that loop:</p>

<pre>Foo f;  // My ctor and dtor get called once each.
for (int i = 0; i &lt; 1000000; ++i) {
  f.DoSomething(i);
}
</pre>

<h3 id="Static_and_Global_Variables">Static and Global Variables</h3>

<p>Objects with
<a href="http://en.cppreference.com/w/cpp/language/storage_duration#Storage_duration">
static storage duration</a> are forbidden unless they are
<a href="http://en.cppreference.com/w/cpp/types/is_destructible">trivially
destructible</a>. Informally this means that the destructor does not do
anything, even taking member and base destructors into account. More formally it
means that the type has no user-defined or virtual destructor and that all bases
and non-static members are trivially destructible.
Static function-local variables may use dynamic initialization.
Use of dynamic initialization for static class member variables or variables at
namespace scope is discouraged, but allowed in limited circumstances; see below
for details.</p>

<p>As a rule of thumb: a global variable satisfies these requirements if its
declaration, considered in isolation, could be <code>constexpr</code>.</p>

<p class="definition"></p>
<p>Every object has a <dfn>storage duration</dfn>, which correlates with its
lifetime. Objects with static storage duration live from the point of their
initialization until the end of the program. Such objects appear as variables at
namespace scope ("global variables"), as static data members of classes, or as
function-local variables that are declared with the <code>static</code>
specifier. Function-local static variables are initialized when control first
passes through their declaration; all other objects with static storage duration
are initialized as part of program start-up. All objects with static storage
duration are destroyed at program exit (which happens before unjoined threads
are terminated).</p>

<p>Initialization may be <dfn>dynamic</dfn>, which means that something
non-trivial happens during initialization. (For example, consider a constructor
that allocates memory, or a variable that is initialized with the current
process ID.) The other kind of initialization is <dfn>static</dfn>
initialization. The two aren't quite opposites, though: static
initialization <em>always</em> happens to objects with static storage duration
(initializing the object either to a given constant or to a representation
consisting of all bytes set to zero), whereas dynamic initialization happens
after that, if required.</p>

<p class="pros"></p>
<p>Global and static variables are very useful for a large number of
applications: named constants, auxiliary data structures internal to some
translation unit, command-line flags, logging, registration mechanisms,
background infrastructure, etc.</p>

<p class="cons"></p>
<p>Global and static variables that use dynamic initialization or have
non-trivial destructors create complexity that can easily lead to hard-to-find
bugs. Dynamic initialization is not ordered across translation units, and
neither is destruction (except that destruction
happens in reverse order of initialization). When one initialization refers to
another variable with static storage duration, it is possible that this causes
an object to be accessed before its lifetime has begun (or after its lifetime
has ended). Moreover, when a program starts threads that are not joined at exit,
those threads may attempt to access objects after their lifetime has ended if
their destructor has already run.</p>

<p class="decision"></p>
<h4>Decision on destruction</h4>

<p>When destructors are trivial, their execution is not subject to ordering at
all (they are effectively not "run"); otherwise we are exposed to the risk of
accessing objects after the end of their lifetime. Therefore, we only allow
objects with static storage duration if they are trivially destructible.
Fundamental types (like pointers and <code>int</code>) are trivially
destructible, as are arrays of trivially destructible types. Note that
variables marked with <code>constexpr</code> are trivially destructible.</p>
<pre>const int kNum = 10;  // Allowed

struct X { int n; };
const X kX[] = {{1}, {2}, {3}};  // Allowed

void foo() {
  static const char* const kMessages[] = {"hello", "world"};  // Allowed
}

// Allowed: constexpr guarantees trivial destructor.
constexpr std::array&lt;int, 3&gt; kArray = {1, 2, 3};</pre>
<pre class="badcode">// bad: non-trivial destructor
const std::string kFoo = "foo";

// Bad for the same reason, even though kBar is a reference (the
// rule also applies to lifetime-extended temporary objects).
const std::string&amp; kBar = StrCat("a", "b", "c");

void bar() {
  // Bad: non-trivial destructor.
  static std::map&lt;int, int&gt; kData = {{1, 0}, {2, 0}, {3, 0}};
}</pre>

<p>Note that references are not objects, and thus they are not subject to the
constraints on destructibility. The constraint on dynamic initialization still
applies, though. In particular, a function-local static reference of the form
<code>static T&amp; t = *new T;</code> is allowed.</p>

<h4>Decision on initialization</h4>

<p>Initialization is a more complex topic. This is because we must not only
consider whether class constructors execute, but we must also consider the
evaluation of the initializer:</p>
<pre class="neutralcode">int n = 5;    // Fine
int m = f();  // ? (Depends on f)
Foo x;        // ? (Depends on Foo::Foo)
Bar y = g();  // ? (Depends on g and on Bar::Bar)
</pre>

<p>All but the first statement expose us to indeterminate initialization
ordering.</p>

<p>The concept we are looking for is called <em>constant initialization</em> in
the formal language of the C++ standard. It means that the initializing
expression is a constant expression, and if the object is initialized by a
constructor call, then the constructor must be specified as
<code>constexpr</code>, too:</p>
<pre class="goodcode">struct Foo { constexpr Foo(int) {} };

int n = 5;  // Fine, 5 is a constant expression.
Foo x(2);   // Fine, 2 is a constant expression and the chosen constructor is constexpr.
Foo a[] = { Foo(1), Foo(2), Foo(3) };  // Fine</pre>

<p>Constant initialization is always allowed. Constant initialization of
static storage duration variables should be marked with <code>constexpr</code>
or <code>constinit</code>.
Any non-local static storage
duration variable that is not so marked should be presumed to have
dynamic initialization, and reviewed very carefully.</p>

<p>By contrast, the following initializations are problematic:</p>

<pre class="badcode">// Some declarations used below.
time_t time(time_t*);      // Not constexpr!
int f();                   // Not constexpr!
struct Bar { Bar() {} };

// Problematic initializations.
time_t m = time(nullptr);  // Initializing expression not a constant expression.
Foo y(f());                // Ditto
Bar b;                     // Chosen constructor Bar::Bar() not constexpr.</pre>

<p>Dynamic initialization of nonlocal variables is discouraged, and in general
it is forbidden. However, we do permit it if no aspect of the program depends
on the sequencing of this initialization with respect to all other
initializations. Under those restrictions, the ordering of the initialization
does not make an observable difference. For example:</p>
<pre>int p = getpid();  // Allowed, as long as no other static variable
                   // uses p in its own initialization.</pre>

<p>Dynamic initialization of static local variables is allowed (and common).</p>


<h4>Common patterns</h4>

<ul>
  <li>Global strings: if you require a named global or static string constant,
    consider using a <code>constexpr</code> variable of
    <code>string_view</code>, character array, or character pointer, pointing
    to a string literal. String literals have static storage duration already
    and are usually sufficient.
    See <a href="https://abseil.io/tips/140">TotW #140.</a></li>
  <li>Maps, sets, and other dynamic containers: if you require a static, fixed
    collection, such as a set to search against or a lookup table, you cannot
    use the dynamic containers from the standard library as a static variable,
    since they have non-trivial destructors. Instead, consider