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<p>A class template defines a family of classes.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="class_template.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="class_template.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="class_template.html#Class_template_instantiation"><span class="tocnumber">3</span> <span class="toctext">Class template instantiation</span></a>
<ul>
<li class="toclevel-2 tocsection-4"><a href="class_template.html#Explicit_instantiation"><span class="tocnumber">3.1</span> <span class="toctext">Explicit instantiation</span></a></li>
<li class="toclevel-2 tocsection-5"><a href="class_template.html#Implicit_instantiation"><span class="tocnumber">3.2</span> <span class="toctext">Implicit instantiation</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-6"><a href="class_template.html#See_also"><span class="tocnumber">4</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/class_template&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
<table class="t-sdsc-begin">
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>template</b></code> <code><b><</b></code> <span class="t-spar">parameter-list</span> <code><b>></b></code> <span class="t-spar">class-declaration</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>export</b></code> <code><b>template</b></code> <code><b><</b></code> <span class="t-spar">parameter-list</span> <code><b>></b></code> <span class="t-spar">class-declaration</span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-until-cxx11">(until C++11)</span>
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/class_template&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<table class="t-par-begin">
<tr class="t-par">
<td> <span class="t-spar">class-declaration</span>
</td>
<td> -
</td>
<td> a <a href="class.html" title="cpp/language/class">class declaration</a>. The class name declared become a template name.
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">parameter-list</span>
</td>
<td> -
</td>
<td> a non-empty comma-separated list of the <a href="template_parameters.html" title="cpp/language/template parameters">template parameters</a>, each of which is either <a href="template_parameters.html#Non-type_template_parameter" title="cpp/language/template parameters">non-type parameter</a>, a <a href="template_parameters.html#Type_template_parameter" title="cpp/language/template parameters">type parameter</a>, a <a href="template_parameters.html#Template_template_parameter" title="cpp/language/template parameters">template parameter</a>, or a <a href="parameter_pack.html" title="cpp/language/parameter pack">parameter pack</a> of any of those.
</td></tr></table>
<table class="t-rev-begin">
<tr class="t-rev t-until-cxx11"><td> <code>export</code> was an optional modifier which declared the template as <i>exported</i> (when used with a class template, it declared all of its members exported as well). Files that instantiated exported templates did not need to include their definitions: the declaration was sufficient. Implementations of <code>export</code> were rare and disagreed with each other on details.
</td>
<td><span class="t-mark-rev t-until-cxx11">(until C++11)</span></td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/class_template&action=edit&section=3" title="Edit section: Class template instantiation">edit</a>]</span> <span class="mw-headline" id="Class_template_instantiation">Class template instantiation</span></h3>
<p>A class template by itself is not a type, or an object, or any other entity. No code is generated from a source file that contains only template definitions. In order for any code to appear, a template must be instantiated: the template arguments must be provided so that the compiler can generate an actual class (or function, from a function template).
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/class_template&action=edit&section=4" title="Edit section: Explicit instantiation">edit</a>]</span> <span class="mw-headline" id="Explicit_instantiation">Explicit instantiation</span></h4>
<table class="t-sdsc-begin">
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>template</b></code> <span class="t-spar">class</span> <span class="t-spar">name</span> <code><b><</b></code> <span class="t-spar">argument-list</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>
<tr class="t-sdsc">
<td> <code><b>extern</b></code> <code><b>template</b></code> <span class="t-spar">class</span> <span class="t-spar">name</span> <code><b><</b></code> <span class="t-spar">argument-list</span> <code><b>> ;</b></code>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> Explicit instantiation definition</div>
<div class="t-li1"><span class="t-li">2)</span> Explicit instantiation declaration</div>
<p>An explicit instantiation definition forces instantiation of the class, struct, or union they refer to. It may appear in the program anywhere after the template definition, and for a given argument-list, is only allowed to appear once in the entire program.
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
An explicit instantiation declaration (an extern template) prevents implicit instantiations: the code that would otherwise cause an implicit instantiation has to use the explicit instantiation definition provided somewhere else in the program (typically, in another file: this can be used to reduce compilation times)</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<p>Classes, functions, variables, and member template specializations can be explicitly instantiated from their templates. Member functions, member classes, and static data members of class templates can be explicitly instantiated from their member definitions.
</p><p>Explicit instantiation can only appear in the enclosing namespace of the template, unless it uses qualified-id:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">namespace</span> N <span class="br0">{</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw1">class</span> Y <span class="br0">{</span> <span class="kw4">void</span> mf<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// template definition</span>
<span class="br0">}</span>
<span class="co1">// template class Y<int>; // error: class template Y not visible in the global namespace</span>
<span class="kw1">using</span> N<span class="sy4">::</span><span class="me2">Y</span><span class="sy4">;</span>
<span class="co1">// template class Y<int>; // error: explicit instantiation outside </span>
<span class="co1">// of the namespace of the template</span>
<span class="kw1">template</span> <span class="kw1">class</span> N<span class="sy4">::</span><span class="me2">Y</span><span class="sy1"><</span><span class="kw4">char</span><span class="sy2">*</span><span class="sy1">></span><span class="sy4">;</span> <span class="co1">// OK: explicit instantiation</span>
<span class="kw1">template</span> <span class="kw4">void</span> N<span class="sy4">::</span><span class="me2">Y</span><span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">mf</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: explicit instantiation</span></pre></div></div>
<p>Explicit instantiation has no effect if an <a href="template_specialization.html" title="cpp/language/template specialization">explicit specialization</a> appeared before for the same set of template arguments.
</p><p>Only the declaration is required to be visible when explicitly instantiating a function template, a variable template, a member function or static data member of a class template,
or a member function template. The complete definition must appear before the explicit instantiation of a class template, a member class of a class template, or a member class
template, unless an explicit specialization with the same template arguments appeared before.
</p><p>If a function template, variable template, member function template, or member function or static data member of a class template is explicitly instantiated with an explicit instantiation definition, the template definition must be present in the same translation unit.
</p><p>When an explicit instantiation names a class template specialization, it serves as an explicit instantiation of the same kind (declaration or definition) of each of its non-inherited non-template members that has not been previously explicitly specialized in the translation unit. If this explicit instantiation is a definition, it is also an explicit instantiation definition only for the members that have been defined at this point.
</p><p>Explicit instantiation definitions ignore member access specifiers: parameter types and return types may be private.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/class_template&action=edit&section=5" title="Edit section: Implicit instantiation">edit</a>]</span> <span class="mw-headline" id="Implicit_instantiation">Implicit instantiation</span></h4>
<p>When code refers to a template in context that requires a completely defined type, or when the completeness of the type affects the code, and this particular type has not been explicitly instantiated, implicit instantiation occurs. For example, when an object of this type is constructed, but not when a pointer to this type is constructed.
</p><p>This applies to the members of the class template: unless the member is used in the program, it is not instantiated, and does not require a definition.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw1">struct</span> Z <span class="br0">{</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// never defined</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// template definition</span>
<span class="kw1">template</span> <span class="kw1">struct</span> Z<span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span><span class="sy4">;</span> <span class="co1">// explicit instantiation of Z<double></span>
Z<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> a<span class="sy4">;</span> <span class="co1">// implicit instantiation of Z<int></span>
Z<span class="sy1"><</span><span class="kw4">char</span><span class="sy1">></span><span class="sy2">*</span> p<span class="sy4">;</span> <span class="co1">// nothing is instantiated here</span>
p<span class="sy2">-</span><span class="sy1">></span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// implicit instantiation of Z<char> and Z<char>::f() occurs here.</span>
<span class="co1">// Z<char>::g() is never needed and never instantiated: it does not have to be defined</span></pre></div></div>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<a href="class.html#Local_classes" title="cpp/language/class">Local classes</a> and any templates used in their members are instantiated as part of the instantiation of the entity within which the local class or enumeration is declared.</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/class_template&action=edit&section=6" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<ul><li> <a href="template_parameters.html" title="cpp/language/template parameters"> template parameters and arguments</a> allow templates to be parametrized
</li><li> <a href="function_template.html" title="cpp/language/function template"> function template declaration</a> declares a function template
</li><li> <a href="template_specialization.html" title="cpp/language/template specialization"> template specialization</a> defines an existing template for a specific type
</li><li> <a href="parameter_pack.html" title="cpp/language/parameter pack"> parameter packs</a> allows the use of lists of types in templates <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</li></ul>
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