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<p>A class is a user-defined type.
</p><p>A class type is defined by class-specifier, which appears in <span class="t-spar">decl-specifier-seq</span> of the <a href="declarations.html" title="cpp/language/declarations">declaration</a> syntax. The class specifier has the following syntax:
</p>
<table class="t-sdsc-begin">
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">class-key</span> <span class="t-spar">attr</span> <span class="t-spar">class-head-name</span> <span class="t-spar">base-clause</span> <code><b>{ </b></code> <span class="t-spar">member-specification</span> <code><b>} </b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<table class="t-par-begin">
<tr class="t-par">
<td> <span class="t-spar">class-key</span>
</td>
<td> -
</td>
<td> one of <a href="../keyword/class.html" title="cpp/keyword/class">class</a>, <a href="../keyword/struct.html" title="cpp/keyword/struct">struct</a>, <a href="../keyword/union.html" title="cpp/keyword/union">union</a>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">attr</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td> optional sequence of any number of <a href="attributes.html" title="cpp/language/attributes">attributes</a>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">class-head-name</span>
</td>
<td> -
</td>
<td> the name of the class that's being defined. Optionally prepended by <span class="t-spar">nested-name-specifier</span> (sequence of names and scope-resolution operators, ending with scope-resolution operator), optionally followed by keyword <a href="../keyword/final.html" title="cpp/keyword/final">final</a>. The name may be omitted, in which case the class is unnamed (note that unnamed class cannot be final)
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">base-clause</span>
</td>
<td> -
</td>
<td> optional list of one or more parent classes and the model of inheritance used for each (see <a href="derived_class.html" title="cpp/language/derived class">derived class</a>)
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">member-specification</span>
</td>
<td> -
</td>
<td> list of access specifiers, member object and member function declarations and definitions (see <a href="class.html" title="cpp/language/class">class definition</a>)
</td></tr></table>
<p>The class keys <code>struct</code> and <code>class</code> are indistinguishable in C++, except that the default <a href="access.html" title="cpp/language/access">access mode</a> and default <a href="derived_class.html" title="cpp/language/derived class">inheritance mode</a> are <code>public</code> if class declaration uses the <code>struct</code> class-key and <code>private</code> if the class declaration uses the <code>class</code> class-key. Both <code>class</code> and <code>struct</code> can be used in a <a href="class.html" title="cpp/language/class">class definition</a>.
</p><p>The use of the class key <code>union</code> results in a <a href="union.html" title="cpp/language/union">union definition</a>, which defines a class that holds only one of its data members at a time.
</p><p>A class can have the following kinds of members
</p>
<div class="t-li1"><span class="t-li">1)</span> <a href="data_members.html" title="cpp/language/data members">data members</a></div>
<div class="t-li2"><span class="t-li">a)</span> non-static data members, including <a href="bit_field.html" title="cpp/language/bit field">bit fields</a>.</div>
<div class="t-li2"><span class="t-li">b)</span> <a href="static.html" title="cpp/language/static">static</a> data members</div>
<div class="t-li1"><span class="t-li">2)</span> member functions</div>
<div class="t-li2"><span class="t-li">a)</span> <a href="member_functions.html" title="cpp/language/member functions">non-static member functions</a></div>
<div class="t-li2"><span class="t-li">b)</span> <a href="static.html" title="cpp/language/static">static</a> member functions</div>
<div class="t-li1"><span class="t-li">3)</span> nested types</div>
<div class="t-li2"><span class="t-li">a)</span> <a href="nested_classes.html" title="cpp/language/nested classes" class="mw-redirect">nested classes</a> and <a href="enum.html" title="cpp/language/enum">enumerations</a> defined within the class definition</div>
<div class="t-li2"><span class="t-li">b)</span> aliases of existing types, defined with <a href="typedef.html" title="cpp/language/typedef">typedef</a> or <a href="type_alias.html" title="cpp/language/type alias">type alias</a> declarations</div>
<div class="t-li2"><span class="t-li">c)</span> the name of the class within its own definition acts as a public member type alias of itself for the purpose of <a href="unqualified_lookup.html#Injected_class_name" title="cpp/language/unqualified lookup">lookup</a> (except when used to name a <a href="constructor.html" title="cpp/language/constructor" class="mw-redirect">constructor</a>): this is known as <i>injected-class-name</i></div>
<div class="t-li1"><span class="t-li">4)</span> <a href="enum.html" title="cpp/language/enum">enumerators</a> from all unscoped enumerations defined within the class</div>
<div class="t-li1"><span class="t-li">5)</span> <a href="member_template.html" title="cpp/language/member template">member templates</a> (<span class="t-rev-inl t-since-cxx14"><span>variable templates, </span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span>class templates or function templates) may appear in the body of any non-local class/struct/union.</div>
<p>All members are defined at once in the class definition, they cannot be added to an already-defined class (unlike the members of namespaces)
</p><p>A member of a class <code>T</code> cannot use <code>T</code> as its name if the member is a static data member, a member function, a member type, a member template, an enumerator of an unscoped enumeration, a member of a member anonymous union. However, a non-static data member may use the name <code>T</code> as long as there are no user-declared constructors.
</p><p>A class with at least one declared or inherited <a href="virtual.html" title="cpp/language/virtual">virtual</a> member function is <i>polymorphic</i>. Objects of this type are <a href="objects.html#Polymorphic_objects" title="cpp/language/object">polymorphic objects</a> and have runtime type information stored as part of the object representation, which may be queried with <a href="dynamic_cast.html" title="cpp/language/dynamic cast">dynamic_cast</a> and <a href="typeid.html" title="cpp/language/typeid">typeid</a>. Virtual member functions participate in dynamic binding.
</p><p>A class with at least one declared or inherited pure virtual member function is an <a href="abstract_class.html" title="cpp/language/abstract class">abstract class</a>. Objects of this type cannot be created.
</p><p>A class with a <a href="constexpr.html" title="cpp/language/constexpr">constexpr</a> constructor is a <a href="../concept/LiteralType.html" title="cpp/concept/LiteralType"><code>LiteralType</code></a>: objects of this type can be manipulated by <a href="constexpr.html" title="cpp/language/constexpr">constexpr</a> functions at compile time.
</p><p>Some member functions are special: under certain circumstances they are defined by the compiler even if not defined by the user. They are:
</p>
<ul><li> <a href="default_constructor.html" title="cpp/language/default constructor">Default constructor</a>
</li><li> <a href="copy_constructor.html" title="cpp/language/copy constructor">Copy constructor</a>
</li><li> <a href="move_constructor.html" title="cpp/language/move constructor">Move constructor</a> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</li><li> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect">Copy assignment operator</a>
</li><li> <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect">Move assignment operator</a> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</li><li> <a href="destructor.html" title="cpp/language/destructor">Destructor</a>
</li></ul>
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