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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>
<div class="main_body">
<h2 class="ignoreLink" id="Background">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>
<div class="stylebody">
<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. </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>
</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 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>
<a id="The_-inl.h_Files"></a>
<h3 id="Self_contained_Headers">Self-contained Headers</h3>
<div class="summary">
<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>
</div>
<div class="stylebody">
<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>Prefer placing the definitions for template and inline functions in
the same file as their declarations. 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. If
declarations and definitions are in different files, including the
former should transitively include the latter. 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>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>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>
</div>
<h3 id="The__define_Guard">The #define Guard</h3>
<div class="summary">
<p>All header files should have <code>#define</code> guards to
prevent multiple inclusion. The format of the symbol name
should be
<code><i><PROJECT></i>_<i><PATH></i>_<i><FILE></i>_H_</code>.</p>
</div>
<div class="stylebody">
<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>
<pre>#ifndef FOO_BAR_BAZ_H_
#define FOO_BAR_BAZ_H_
...
#endif // FOO_BAR_BAZ_H_
</pre>
</div>
<h3 id="Forward_Declarations">Forward Declarations</h3>
<div class="summary">
<p>Avoid using forward declarations where possible.
Just <code>#include</code> the headers you need.</p>
</div>
<div class="stylebody">
<div class="definition">
<p>A "forward declaration" is a declaration of a class,
function, or template without an associated definition.</p>
</div>
<div class="pros">
<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>
</div>
<div class="cons">
<ul>
<li>Forward declarations can hide a dependency, allowing
user code to skip necessary recompilation when headers
change.</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>
</div>
<div class="decision">
<ul>
<li>Try to avoid forward declarations of entities
defined in another project.</li>
<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>
</ul>
<p>Please see <a href="#Names_and_Order_of_Includes">Names and Order
of Includes</a> for rules about when to #include a header.</p>
</div>
</div>
<h3 id="Inline_Functions">Inline Functions</h3>
<div class="summary">
<p>Define functions inline only when they are small, say, 10
lines or fewer.</p>
</div>
<div class="stylebody">
<div class="definition">
<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>
</div>
<div class="pros">
<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>
</div>
<div class="cons">
<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>
</div>
<div class="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>
</div>
</div>
<h3 id="Names_and_Order_of_Includes">Names and Order of Includes</h3>
<div class="summary">
<p>Use standard order for readability and to avoid hidden
dependencies: Related header, C library, C++ library, other libraries'
<code>.h</code>, your project's <code>.h</code>.</p>
</div>
<div class="stylebody">
<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>
<pre>#include "base/logging.h"
</pre>
<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 files.</li>
<li>C++ system files.</li>
<li>A blank line</li>
<li>Other libraries' <code>.h</code>
files.</li>
<li>
Your project's <code>.h</code>
files.</li>
</ol>
<p>Note that any adjacent blank lines should be collapsed.</p>
<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 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 compatibility 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>You should include all the headers that define the symbols you rely
upon, except in the unusual case of <a href="#Forward_Declarations">forward
declaration</a>. If you rely on symbols from <code>bar.h</code>,
don't count on the fact that you included <code>foo.h</code> which
(currently) includes <code>bar.h</code>: include <code>bar.h</code>
yourself, unless <code>foo.h</code> explicitly demonstrates its intent
to provide you the symbols of <code>bar.h</code>. However, any
includes present in the related header do not need to be included
again in the related <code>cc</code> (i.e., <code>foo.cc</code> can
rely on <code>foo.h</code>'s includes).</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 <sys/types.h>
#include <unistd.h>
#include <vector>
#include "base/basictypes.h"
#include "base/commandlineflags.h"
#include "foo/server/bar.h"
</pre>
<p class="exception">Sometimes, system-specific code needs
conditional includes. Such code can put conditional
includes after other includes. Of course, keep your
system-specific code small and localized. Example:</p>
<pre>#include "foo/public/fooserver.h"
#include "base/port.h" // For LANG_CXX11.
#ifdef LANG_CXX11
#include <initializer_list>
#endif // LANG_CXX11
</pre>
</div>
<h2 id="Scoping">Scoping</h2>
<h3 id="Namespaces">Namespaces</h3>
<div class="summary">
<p>With few exceptions, place code in a namespace. Namespaces
should have unique names based on the project name, and possibly
its path. Do not use <i>using-directives</i> (e.g.
<code>using namespace foo</code>). Do not use
inline namespaces. For unnamed namespaces, see
<a href="#Unnamed_Namespaces_and_Static_Variables">Unnamed Namespaces and
Static Variables</a>.
</p></div>
<div class="stylebody">
<div class="definition">
<p>Namespaces subdivide the global scope
into distinct, named scopes, and so are useful for preventing
name collisions in the global scope.</p>
</div>
<div class="pros">
<p>Namespaces provide a method for preventing name conflicts
in large programs while allowing most code to use reasonably
short names.</p>
<p>For example, if two different projects have a class
<code>Foo</code> in the global scope, these symbols may
collide at compile time or at runtime. If each project
places their code in a namespace, <code>project1::Foo</code>
and <code>project2::Foo</code> are now distinct symbols that
do not collide, and code within each project's namespace
can continue to refer to <code>Foo</code> without the prefix.</p>
<p>Inline namespaces automatically place their names in
the enclosing scope. Consider the following snippet, for
example:</p>
<pre>namespace 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>
</div>
<div class="cons">
<p>Namespaces can be confusing, because they complicate
the mechanics of figuring out what definition a name refers
to.</p>
<p>Inline namespaces, in particular, can be confusing
because names aren't actually restricted to the namespace
where they are declared. They are only useful as part of
some larger versioning policy.</p>
<p>In some contexts, it's necessary to repeatedly refer to
symbols by their fully-qualified names. For deeply-nested
namespaces, this can add a lot of clutter.</p>
</div>
<div class="decision">
<p>Namespaces should be used as follows:</p>
<ul>
<li>Follow the rules on <a href="#Namespace_Names">Namespace Names</a>.
</li><li>Terminate namespaces with comments as shown in the given examples.
</li><li>
<p>Namespaces wrap the entire source file after
includes,
<a href="https://gflags.github.io/gflags/">
gflags</a> definitions/declarations
and forward declarations of classes from other namespaces.</p>
<pre>// In the .h file
namespace mynamespace {
// All declarations are within the namespace scope.
// Notice the lack of indentation.
class MyClass {
public:
...
void Foo();
};
} // namespace mynamespace
</pre>
<pre>// In the .cc file
namespace mynamespace {
// Definition of functions is within scope of the namespace.
void MyClass::Foo() {
...
}
} // namespace mynamespace
</pre>
<p>More complex <code>.cc</code> files might have additional details,
like flags or using-declarations.</p>
<pre>#include "a.h"
DEFINE_FLAG(bool, someflag, false, "dummy flag");
namespace 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 impl { // Internal, not part of the API.
namespace sidetable = ::pipeline_diagnostics::sidetable;
} // namespace impl
inline void my_inline_function() {
// namespace alias local to a function (or method).
namespace baz = ::foo::bar::baz;
...
}
} // namespace librarian
</pre>
</li><li>Do not use inline namespaces.</li>
</ul>
</div>
</div>
<h3 id="Unnamed_Namespaces_and_Static_Variables">Unnamed Namespaces and Static
Variables</h3>
<div class="summary">
<p>When definitions in a <code>.cc</code> file do not need to be
referenced outside that file, place them in an unnamed
namespace or declare them <code>static</code>. Do not use either
of these constructs in <code>.h</code> files.
</p></div>
<div class="stylebody">
<div class="definition">
<p>All declarations can be given internal linkage by placing them in unnamed
namespaces. 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>
</div>
<div class="decision">
<p>Use of internal linkage in <code>.cc</code> files is encouraged
for all code that does not need to be referenced elsewhere.
Do not use internal linkage in <code>.h</code> files.</p>
<p>Format unnamed namespaces like named namespaces. In the
terminating comment, leave the namespace name empty:</p>
<pre>namespace {
...
} // namespace
</pre>
</div>
</div>
<h3 id="Nonmember,_Static_Member,_and_Global_Functions">Nonmember, Static Member, and Global Functions</h3>
<div class="summary">
<p>Prefer placing nonmember functions in a namespace; use completely global
functions rarely. Do not use a class simply to group static functions. Static
methods of a class should generally be closely related to instances of the
class or the class's static data.</p>
</div>
<div class="stylebody">
<div class="pros">
<p>Nonmember and static member functions can be useful in
some situations. Putting nonmember functions in a
namespace avoids polluting the global namespace.</p>
</div>
<div class="cons">
<p>Nonmember and static member functions may make more sense
as members of a new class, especially if they access
external resources or have significant dependencies.</p>
</div>
<div class="decision">
<p>Sometimes it is useful to define a
function not bound to a class instance. Such a function
can be either a static member or a nonmember function.
Nonmember functions should not depend on external
variables, and should nearly always exist in a namespace.
Do not create classes only to group static member functions;
this is no different than just giving the function 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="#Unnamed_Namespaces_and_Static_Variables">internal linkage</a> to limit
its scope.</p>
</div>
</div>
<h3 id="Local_Variables">Local Variables</h3>
<div class="summary">
<p>Place a function's variables in the narrowest scope
possible, and initialize variables in the declaration.</p>
</div>
<div class="stylebody">
<p>C++ allows you to declare variables anywhere in a
function. We encourage you to declare them in as local a
scope as possible, and as close to the first use as
possible. This makes it easier for the reader to find the
declaration and see what type the variable is and what it
was initialized to. In particular, initialization should
be used instead of declaration and assignment, e.g.:</p>
<pre class="badcode">int i;
i = f(); // Bad -- initialization separate from declaration.
</pre>
<pre>int j = g(); // Good -- declaration has initialization.
</pre>
<pre class="badcode">std::vector<int> v;
v.push_back(1); // Prefer initializing using brace initialization.
v.push_back(2);
</pre>
<pre>std::vector<int> v = {1, 2}; // Good -- v starts initialized.
</pre>
<p>Variables needed for <code>if</code>, <code>while</code>
and <code>for</code> statements should normally be declared
within those statements, so that such variables are confined
to those scopes. E.g.:</p>
<pre>while (const char* p = strchr(str, '/')) str = p + 1;
</pre>
<p>There is one caveat: if the variable is an object, its
constructor is invoked every time it enters scope and is
created, and its destructor is invoked every time it goes
out of scope.</p>
<pre class="badcode">// Inefficient implementation:
for (int i = 0; i < 1000000; ++i) {
Foo f; // My ctor and dtor get called 1000000 times each.
f.DoSomething(i);
}
</pre>
<p>It may be more efficient to declare such a variable
used in a loop outside that loop:</p>
<pre>Foo f; // My ctor and dtor get called once each.
for (int i = 0; i < 1000000; ++i) {
f.DoSomething(i);
}
</pre>
</div>
<h3 id="Static_and_Global_Variables">Static and Global Variables</h3>
<div class="summary">
<p>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>
</div>
<div class="stylebody">
<div class="definition">
<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>
</div>
<div class="pros">
<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>
</div>
<div class="cons">
<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>
</div>
<div class="decision">
<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<int, 3> kArray = {{1, 2, 3}};</pre>
<pre class="badcode">// bad: non-trivial destructor
const string kFoo = "foo";
// bad for the same reason, even though kBar is a reference (the
// rule also applies to lifetime-extended temporary objects)
const string& kBar = StrCat("a", "b", "c");
void bar() {
// bad: non-trivial destructor
static std::map<int, int> 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& 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>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 where possible the
<a href="https://github.com/abseil/abseil-cpp/blob/03c1513538584f4a04d666be5eb469e3979febba/absl/base/attributes.h#L540">
<code>ABSL_CONST_INIT</code></a>
attribute. 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="neutralcode">time_t time(time_t*); // not constexpr!
int f(); // not constexpr!
struct Bar { Bar() {} };
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>
</div>
<h4>Common patterns</h4>
<ul>
<li>Global strings: if you require a global or static string constant,
consider using a simple character array, or a char pointer to the first