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<p>An <i>identifier</i> is an arbitrary long sequence of digits, underscores, lowercase and uppercase Latin letters, and Unicode characters. A valid identifier must begin with a non-digit character (Latin letter, underscore, or Unicode non-digit character). Identifiers are case-sensitive (lowercase and uppercase letters are distinct), and every character is significant.
</p><p>Note: C++ grammar formally requires Unicode characters to be <a href="escape.html" title="cpp/language/escape">escaped</a> with <code>\u</code> or <code>\U</code>, but due to <a href="translation_phases.html" title="cpp/language/translation phases">translation phase 1</a>, that is exactly how raw unicode characters from the source code are presented to the compiler. Also note that support of this feature may be limited, e.g. <a rel="nofollow" class="external text" href="https://gcc.gnu.org/wiki/FAQ#What_is_the_status_of_adding_the_UTF-8_support_for_identifier_names_in_GCC.3F">gcc</a>
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="name.html#In_declarations"><span class="tocnumber">1</span> <span class="toctext">In declarations</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="name.html#In_expressions"><span class="tocnumber">2</span> <span class="toctext">In expressions</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="name.html#Names"><span class="tocnumber">3</span> <span class="toctext">Names</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="name.html#Qualified_identifiers"><span class="tocnumber">4</span> <span class="toctext">Qualified identifiers</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="name.html#See_also"><span class="tocnumber">5</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/identifiers&action=edit&section=1" title="Edit section: In declarations">edit</a>]</span> <span class="mw-headline" id="In_declarations">In declarations</span></h3>
<p>Identifiers can be used <a href="declarations.html" title="cpp/language/declarations">to name</a> objects, types, namespaces, and other entities, with the following exceptions:
</p>
<ul><li> the identifiers that are <a href="../keywords.html" title="cpp/keyword">keywords</a> cannot be used for other purposes;
</li><li> the identifiers with a double underscore anywhere are reserved;
</li><li> the identifiers that begin with an underscore followed by an uppercase letter are reserved;
</li><li> the identifiers that begin with an underscore are reserved for use at global namespace.
</li></ul>
<p>"Reserved" here means that the standard library headers #define or declare such identifiers for their internal needs, the compiler may predefine non-standard identifiers of that kind, and that name mangling algorithm may assume that some of these identifiers are not in use. If the programmer uses such identifiers, the behavior is undefined.
</p><p>In addition, it's undefined behavior to #define or #undef names identical to <a href="../keywords.html" title="cpp/keywords" class="mw-redirect">keywords</a>. If at least one standard library header is included, it's undefined behavior to #define or #undef identifiers identical to names declared in any standard library header.
</p>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br />Reason: +other contents of 17.6.4.3 [reserved.names] </td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/identifiers&action=edit&section=2" title="Edit section: In expressions">edit</a>]</span> <span class="mw-headline" id="In_expressions">In expressions</span></h3>
<p>An identifier that names an variable or a function can be used as an <a href="expressions.html" title="cpp/language/expressions">expression</a>. The expression consisting of just the identifier returns the entity named by the identifier. The <a href="value_category.html" title="cpp/language/value category">value category</a> of the expression is <i>lvalue</i> if the identifier names a function, a variable, or a data member, and it is <i>prvalue</i> otherwise (e.g. an <a href="enum.html" title="cpp/language/enum">enumerator</a> is a prvalue expression)
</p><p>Within the body of a non-static <a href="member_functions.html" title="cpp/language/member functions">member function</a>, each identifier that names a non-static member is implicitly transformed to a class member access expression <span class="t-c"><span class="mw-geshi cpp source-cpp">this<span class="sy2">-</span><span class="sy1">></span>member</span></span>.
</p><p>Besides simple names of objects and functions, the following language constructs can be used in expressions in the same role (together with identifiers they are known as <i>id-expressions</i>)
</p>
<ul><li> the name of an operator function, such as <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">+</span></span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../memory/new/operator_new.html"><span class="kw589">operator new</span></a></span></span>;
</li><li> the name of a user-defined conversion function, such as <span class="t-c"><span class="mw-geshi cpp source-cpp">operator <span class="kw4">bool</span></span></span>;
</li><li> the name of a user-defined literal operator function, such as <span class="t-c"><span class="mw-geshi cpp source-cpp">operator <span class="st0">""</span> _km</span></span>;
</li><li> the character <span class="t-c"><span class="mw-geshi cpp source-cpp">~</span></span> followed by class name, such as <span class="t-c"><span class="mw-geshi cpp source-cpp">~MyClass</span></span>;
</li><li> the character <span class="t-c"><span class="mw-geshi cpp source-cpp">~</span></span> followed by <a href="decltype.html" title="cpp/language/decltype">decltype</a> specifier, such as <span class="t-c"><span class="mw-geshi cpp source-cpp">~decltype<span class="br0">(</span>str<span class="br0">)</span></span></span>;
</li><li> a <a href="templates.html" title="cpp/language/templates">template</a> identifier, such as <span class="t-c"><span class="mw-geshi cpp source-cpp">MyTemplate<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span></span></span>;
</li><li> <i>qualified identifiers</i>, such as <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a></span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span><span class="me2">tolower</span></span></span>.
</li></ul>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/identifiers&action=edit&section=3" title="Edit section: Names">edit</a>]</span> <span class="mw-headline" id="Names">Names</span></h3>
<p>A <i>name</i> is the use of one of the following to refer to an entity or to a label:
</p>
<ul><li> an identifier;
</li><li> operator in function notation (<span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">+</span></span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../memory/new/operator_new.html"><span class="kw589">operator new</span></a></span></span>);
</li><li> <a href="cast_operator.html" title="cpp/language/cast operator">user-defined conversion function</a> (<span class="t-c"><span class="mw-geshi cpp source-cpp">operator <span class="kw4">bool</span></span></span>);
</li><li> user-defined <a href="user_literal.html" title="cpp/language/user literal">literal operator</a> (<span class="t-c"><span class="mw-geshi cpp source-cpp">operator <span class="st0">""</span> _km</span></span>);
</li><li> template id (<code>name<arg, arg></code>).
</li></ul>
<p>Every name that denotes an entity is introduced into the program by a <a href="declarations.html" title="cpp/language/declarations">declaration</a>. Every name that denotes a label is introduced into the program either by a <a href="goto.html" title="cpp/language/goto">goto</a> statement or by a <a href="statements.html#Labels" title="cpp/language/statements">labeled statement</a>. A name used in more than one translation unit may refer to the same or different entities, depending on <a href="storage_duration.html#Linkage" title="cpp/language/storage duration">linkage</a>.
</p><p>When the compiler encounters an unknown name in a program, it associates it with the declaration that introduced the name by means of <a href="lookup.html" title="cpp/language/lookup">name lookup</a>, except for the <a href="dependent_name.html" title="cpp/language/dependent name">dependent names</a> in template declarations and definitions (for those names, the compiler determines whether they name a type, a template, or some other entity, which may require <a href="dependent_name.html" title="cpp/language/dependent name">explicit disambiguation</a>)
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/identifiers&action=edit&section=4" title="Edit section: Qualified identifiers">edit</a>]</span> <span class="mw-headline" id="Qualified_identifiers">Qualified identifiers</span></h3>
<p>A <i>qualified identifier</i> is an identifier, operator function name, literal operator name, or a template identifier that is prepended by a scope resolution operator <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span></span></span>, and optionally, a sequence of <span class="t-rev-inl t-since-cxx11"><span>enumeration, </span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>class or namespace names <span class="t-rev-inl t-since-cxx11"><span>or <span class="t-lc">decltype</span> expressions</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span> separated by scope resolution operators:
</p><p>For example, the expression <span class="t-lc"><a href="../string/basic_string/npos.html" title="cpp/string/basic string/npos">std::string::npos</a></span> is an id-expression that names the static member <span class="t-c"><span class="mw-geshi cpp source-cpp">npos</span></span> in the class <span class="t-c"><span class="mw-geshi cpp source-cpp">string</span></span> in namespace <span class="t-c"><span class="mw-geshi cpp source-cpp">std</span></span>. The expression <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span><span class="me2">tolower</span></span></span> names the function <span class="t-c"><span class="mw-geshi cpp source-cpp">tolower</span></span> in the global namespace. The expression <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span><a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a></span></span> names the global variable <span class="t-c"><span class="mw-geshi cpp source-cpp">cout</span></span> in namespace <span class="t-c"><span class="mw-geshi cpp source-cpp">std</span></span>, which is a top-level namespace. The qualified identifier <span class="t-c"><span class="mw-geshi cpp source-cpp">boost<span class="sy4">::</span><span class="me2">signals2</span><span class="sy4">::</span><span class="me2">connection</span></span></span> names the type <span class="t-c"><span class="mw-geshi cpp source-cpp">connection</span></span> declared in namespace <span class="t-c"><span class="mw-geshi cpp source-cpp">signals2</span></span>, which is declared in namespace <span class="t-c"><span class="mw-geshi cpp source-cpp">boost</span></span>.
</p><p>The keyword <a href="../keyword/template.html" title="cpp/keyword/template"><tt>template</tt></a> may appear in qualified identifiers as necessary to disambiguate <a href="dependent_name.html" title="cpp/language/dependent name">dependent template names</a>.
</p><p>See <a href="lookup.html#Qualified_lookup" title="cpp/language/lookup">qualified lookup</a> for the details of the name lookup for qualified identifiers.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/identifiers&action=edit&section=5" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td colspan="2"> <div class="t-dsc-see"><span><a href="../../c/language/identifier.html" title="c/language/identifier">C documentation</a></span> for <span class="t-dsc-see-tt"><span>identifier</span></span></div>
</td></tr>
</table>
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