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                <h1 id="firstHeading" class="firstHeading">explicit (full) template specialization</h1>
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<p>Allows customizing the template code for a given set of template arguments.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="template_specialization.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="template_specialization.html#In_detail"><span class="tocnumber">2</span> <span class="toctext">In detail</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="template_specialization.html#Explicit_specializations_of_function_templates"><span class="tocnumber">3</span> <span class="toctext">Explicit specializations of function templates</span></a>
<ul>
<li class="toclevel-2 tocsection-4"><a href="template_specialization.html#Members_of_specializations"><span class="tocnumber">3.1</span> <span class="toctext">Members of specializations</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-5"><a href="template_specialization.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/template_specialization&amp;action=edit&amp;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 class="t-sdsc-nopad"> <code><b>template</b></code> <code><b>&lt;&gt;</b></code> <span class="t-spar">declaration</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
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<p>Any of the following can be fully specialized:
</p>
<ol><li> <a href="function_template.html" title="cpp/language/function template">function template</a>
</li><li> <a href="class_template.html" title="cpp/language/class template">class template</a>
</li><li> <span class="t-mark-rev t-since-cxx14">(since C++14)</span><a href="variable_template.html" title="cpp/language/variable template">variable template</a>
</li><li> <a href="member_functions.html" title="cpp/language/member functions">member function</a> of a class template
</li><li> <a href="static.html" title="cpp/language/static">static data member</a> of a class template
</li><li> <a href="nested_classes.html" title="cpp/language/nested types">member class</a> of a class template
</li><li> member <a href="enum.html" title="cpp/language/enum">enumeration</a> of a class template
</li><li> <a href="member_template.html" title="cpp/language/member template">member class template</a> of a class or class template
</li><li> <a href="member_template.html#Member_function_templates" title="cpp/language/member template">member function template</a> of a class or class template
</li></ol>
<p>For example, 
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>   <span class="co1">// primary template</span>
<span class="kw1">struct</span> is_void <span class="sy4">:</span> <a href="../types/integral_constant.html"><span class="kw529">std::<span class="me2">false_type</span></span></a>
<span class="br0">&#123;</span>
<span class="br0">&#125;</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>  <span class="co1">// explicit specialization for T = void</span>
<span class="kw1">struct</span> is_void<span class="sy1">&lt;</span><span class="kw4">void</span><span class="sy1">&gt;</span> <span class="sy4">:</span> <a href="../types/integral_constant.html"><span class="kw528">std::<span class="me2">true_type</span></span></a>
<span class="br0">&#123;</span>
<span class="br0">&#125;</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">&#40;</span><span class="br0">&#41;</span>
<span class="br0">&#123;</span>
    <span class="co1">// for any type T other than void, the </span>
    <span class="co1">// class is derived from false_type</span>
    <a href="../io/cout.html"><span class="kw1446">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> is_void<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> 
    <span class="co1">// but when T is void, the class is derived</span>
    <span class="co1">// from true_type</span>
    <a href="../io/cout.html"><span class="kw1446">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> is_void<span class="sy1">&lt;</span><span class="kw4">void</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">&#125;</span></pre></div></div>
<p><br /> 
</p>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/template_specialization&amp;action=edit&amp;section=2" title="Edit section: In detail">edit</a>]</span> <span class="mw-headline" id="In_detail">In detail</span></h3>
<p>Explicit specialization can only appear in the same <a href="namespace.html" title="cpp/language/namespace">namespace</a> as the primary template, and has to appear after the non-specialized template declaration. It is always in the scope of that namespace:
</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">&#123;</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> X <span class="br0">&#123;</span> <span class="coMULTI">/*...*/</span> <span class="br0">&#125;</span><span class="sy4">;</span> <span class="co1">// primary template</span>
    <span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">class</span> X<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> <span class="br0">&#123;</span> <span class="coMULTI">/*...*/</span> <span class="br0">&#125;</span><span class="sy4">;</span> <span class="co1">// specialization in same namespace</span>
&#160;
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> Y <span class="br0">&#123;</span> <span class="coMULTI">/*...*/</span> <span class="br0">&#125;</span><span class="sy4">;</span> <span class="co1">// primary template</span>
    <span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">class</span> Y<span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">;</span> <span class="co1">// forward declare specialization for double</span>
<span class="br0">&#125;</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
<span class="kw1">class</span> N<span class="sy4">::</span><span class="me2">Y</span><span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span> <span class="br0">&#123;</span> <span class="coMULTI">/*...*/</span> <span class="br0">&#125;</span><span class="sy4">;</span> <span class="co1">// OK: specialization in same namespace</span></pre></div></div>
<p>Specialization must be declared before the first use that would cause implicit instantiation, in every translation unit where such use occurs:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> String <span class="br0">&#123;</span><span class="br0">&#125;</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> Array <span class="br0">&#123;</span> <span class="coMULTI">/*...*/</span> <span class="br0">&#125;</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> sort<span class="br0">&#40;</span>Array<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span> v<span class="br0">&#41;</span> <span class="br0">&#123;</span> <span class="coMULTI">/*...*/</span> <span class="br0">&#125;</span> <span class="co1">// primary template</span>
&#160;
<span class="kw4">void</span> f<span class="br0">&#40;</span>Array<span class="sy1">&lt;</span>String<span class="sy1">&gt;</span><span class="sy3">&amp;</span> v<span class="br0">&#41;</span> <span class="br0">&#123;</span>
    sort<span class="br0">&#40;</span>v<span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// implicitly instantiates sort(Array&lt;String&gt;&amp;), </span>
<span class="br0">&#125;</span>            <span class="co1">// using the primary template for sort()</span>
&#160;
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>  <span class="co1">// ERROR: explicit specialization of sort(Array&lt;String&gt;)</span>
<span class="kw4">void</span> sort<span class="sy1">&lt;</span>String<span class="sy1">&gt;</span><span class="br0">&#40;</span>Array<span class="sy1">&lt;</span>String<span class="sy1">&gt;</span><span class="sy3">&amp;</span> v<span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// after implicit instantiation</span></pre></div></div>
<p>A template specialization that was declared but not defined can be used just like any other <a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">incomplete type</a> (e.g. pointers and references to it may be used)
</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">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> X<span class="sy4">;</span> <span class="co1">// primary template</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">class</span> X<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">;</span> <span class="co1">// specialization (declared, not defined)</span>
X<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy2">*</span> p<span class="sy4">;</span> <span class="co1">// OK: pointer to incomplete type</span>
X<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> x<span class="sy4">;</span> <span class="co1">// error: object of incomplete type</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/template_specialization&amp;action=edit&amp;section=3" title="Edit section: Explicit specializations of function templates">edit</a>]</span> <span class="mw-headline" id="Explicit_specializations_of_function_templates">Explicit specializations of function templates</span></h3>
<p>When specializing a function template, its template arguments can be omitted if <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">template argument deduction</a> can provide them from the function arguments:
</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">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> Array <span class="br0">&#123;</span> <span class="coMULTI">/*...*/</span> <span class="br0">&#125;</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> sort<span class="br0">&#40;</span>Array<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span> v<span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// primary template</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> sort<span class="br0">&#40;</span>Array<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// specialization for T = int</span>
<span class="co1">// no need to write</span>
<span class="co1">// template&lt;&gt; void sort&lt;int&gt;(Array&lt;int&gt;&amp;);</span></pre></div></div>
<p>A function with the same name and the same argument list as a specialization is not a specialization (see template overloading in <a href="function_template.html" title="cpp/language/function template">function_template</a>)
</p><p>An explicit specialization of a function template is inline only if it is declared with the <a href="inline.html" title="cpp/language/inline">inline specifier</a> (or defined as deleted), it doesn't matter if the primary template is inline.
</p><p><a href="default_arguments.html" title="cpp/language/default arguments">Default function arguments</a> cannot be specified in explicit specializations of function templates, member function templates, and member functions of class templates when the class is implicitly instantiated.
</p><p>An explicit specialization cannot be a <a href="friend.html" title="cpp/language/friend">friend declaration</a>.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/template_specialization&amp;action=edit&amp;section=4" title="Edit section: Members of specializations">edit</a>]</span> <span class="mw-headline" id="Members_of_specializations">Members of specializations</span></h4>
<p>When defining a member of an explicitly specialized class template outside the body of the class, the syntax <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;&gt;</span></span></span> is not used, except if it's a member of an explicitly specialized member class template, which is specialized as a class template, because otherwise, the syntax would require such definition to begin with <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span>parameters<span class="sy1">&gt;</span></span></span> required by the nested template
</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">&lt;</span> <span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> A <span class="br0">&#123;</span>
    <span class="kw1">struct</span> B <span class="br0">&#123;</span><span class="br0">&#125;</span><span class="sy4">;</span>  <span class="co1">// member class </span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> U<span class="sy1">&gt;</span> <span class="kw1">struct</span> C <span class="br0">&#123;</span> <span class="br0">&#125;</span><span class="sy4">;</span> <span class="co1">// member class template</span>
<span class="br0">&#125;</span><span class="sy4">;</span>
&#160;
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="co1">// specialization</span>
<span class="kw1">struct</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> <span class="br0">&#123;</span>
    <span class="kw4">void</span> f<span class="br0">&#40;</span><span class="kw4">int</span><span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// member function of a specialization</span>
<span class="br0">&#125;</span><span class="sy4">;</span>
<span class="co1">// template&lt;&gt; not used for a member of a specialization</span>
<span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">&#40;</span><span class="kw4">int</span><span class="br0">&#41;</span> <span class="br0">&#123;</span> <span class="coMULTI">/* ... */</span> <span class="br0">&#125;</span>
&#160;
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="co1">// specialization of a member class</span>
<span class="kw1">struct</span> A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span> <span class="br0">&#123;</span>
    <span class="kw4">void</span> f<span class="br0">&#40;</span><span class="br0">&#41;</span><span class="sy4">;</span>
<span class="br0">&#125;</span><span class="sy4">;</span>
<span class="co1">// template&lt;&gt; not used for a member of a specialized member class either</span>
<span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">&#40;</span><span class="br0">&#41;</span> <span class="br0">&#123;</span> <span class="coMULTI">/* ... */</span> <span class="br0">&#125;</span>
&#160;
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="co1">// specialization of a member class template</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> U<span class="sy1">&gt;</span> <span class="kw1">struct</span> A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">C</span> <span class="br0">&#123;</span>
    <span class="kw4">void</span> f<span class="br0">&#40;</span><span class="br0">&#41;</span><span class="sy4">;</span>
<span class="br0">&#125;</span><span class="sy4">;</span>
&#160;
<span class="co1">// template&lt;&gt; is used when defining a member of an explicitly</span>
<span class="co1">// specialized member class template specialized as a class template</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> U<span class="sy1">&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">C</span><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">&#40;</span><span class="br0">&#41;</span> <span class="br0">&#123;</span> <span class="coMULTI">/* ... */</span> <span class="br0">&#125;</span></pre></div></div>
<p><br />
An explicit specialization of a static data member of a template is a definition if the declaration includes an initializer; otherwise, it is a declaration. These definitions must use braces for default initialization:
</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">&lt;&gt;</span> X Q<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">x</span><span class="sy4">;</span> <span class="co1">// declaration of a static member</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> X Q<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">x</span> <span class="br0">&#40;</span><span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// error: function declaration</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> X Q<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">x</span> <span class="br0">&#123;</span><span class="br0">&#125;</span><span class="sy4">;</span> <span class="co1">// definition of a default-initialized static member</span></pre></div></div>
<p>A member or a member template of a class template may be explicitly specialized for a given implicit instantiation of the class template, even if the member or member template is defined in the class template 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">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> A <span class="br0">&#123;</span>
    <span class="kw4">void</span> f<span class="br0">&#40;</span>T<span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// member, declared in the primary template</span>
    <span class="kw4">void</span> h<span class="br0">&#40;</span>T<span class="br0">&#41;</span> <span class="br0">&#123;</span><span class="br0">&#125;</span> <span class="co1">// member, defined in the primary template</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X1<span class="sy1">&gt;</span> <span class="kw4">void</span> g1<span class="br0">&#40;</span>T, X1<span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// member template</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X2<span class="sy1">&gt;</span> <span class="kw4">void</span> g2<span class="br0">&#40;</span>T, X2<span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// member template</span>
<span class="br0">&#125;</span><span class="sy4">;</span>
&#160;
<span class="co1">// specialization of a member</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">&#40;</span><span class="kw4">int</span><span class="br0">&#41;</span><span class="sy4">;</span>
<span class="co1">// member specialization OK even if defined in-class</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">h</span><span class="br0">&#40;</span><span class="kw4">int</span><span class="br0">&#41;</span> <span class="br0">&#123;</span><span class="br0">&#125;</span>
&#160;
<span class="co1">// out of class member template definition</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X1<span class="sy1">&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">g1</span><span class="br0">&#40;</span>T, X1<span class="br0">&#41;</span> <span class="br0">&#123;</span> <span class="br0">&#125;</span>
&#160;
<span class="co1">// member template specialization</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X1<span class="sy1">&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">g1</span><span class="br0">&#40;</span><span class="kw4">int</span>, X1<span class="br0">&#41;</span><span class="sy4">;</span>
&#160;
<span class="co1">// member template specialization</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">g2</span><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="br0">&#40;</span><span class="kw4">int</span>, <span class="kw4">char</span><span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// for X2 = char</span>
<span class="co1">// same, using template argument deduction (X1 = char)</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> 
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">g1</span><span class="br0">&#40;</span><span class="kw4">int</span>, <span class="kw4">char</span><span class="br0">&#41;</span><span class="sy4">;</span></pre></div></div>
<p>Member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, there's a <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;&gt;</span></span></span> for every
enclosing class template that is explicitly specialized.
</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">&lt;</span><span class="kw1">class</span> T1<span class="sy1">&gt;</span> <span class="kw1">class</span> A <span class="br0">&#123;</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T2<span class="sy1">&gt;</span> <span class="kw1">class</span> B <span class="br0">&#123;</span>
        <span class="kw4">void</span> mf<span class="br0">&#40;</span><span class="br0">&#41;</span><span class="sy4">;</span>
    <span class="br0">&#125;</span><span class="sy4">;</span>
<span class="br0">&#125;</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">class</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">mf</span><span class="br0">&#40;</span><span class="br0">&#41;</span><span class="sy4">;</span></pre></div></div>
<p>In such a nested declaration, some of the levels may remain unspecialized (except that it can't specialize a class member template if its enclosing class is unspecialized). For each of those levels, the declaration needs <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span>arguments<span class="sy1">&gt;</span></span></span>, because such specializations are themselves templates:
</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">&lt;</span><span class="kw1">class</span> T1<span class="sy1">&gt;</span> <span class="kw1">class</span> A <span class="br0">&#123;</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T2<span class="sy1">&gt;</span> <span class="kw1">class</span> B <span class="br0">&#123;</span>
        <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T3<span class="sy1">&gt;</span> <span class="kw4">void</span> mf1<span class="br0">&#40;</span>T3<span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// member template</span>
        <span class="kw4">void</span> mf2<span class="br0">&#40;</span><span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// non-template member</span>
     <span class="br0">&#125;</span><span class="sy4">;</span>
<span class="br0">&#125;</span><span class="sy4">;</span>
&#160;
<span class="co1">// specialization</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="co1">// for the specialized A</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X<span class="sy1">&gt;</span> <span class="co1">// for the unspecialized B</span>
<span class="kw1">class</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span> <span class="br0">&#123;</span>
    <span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> mf1<span class="br0">&#40;</span>T<span class="br0">&#41;</span><span class="sy4">;</span>
<span class="br0">&#125;</span><span class="sy4">;</span>
&#160;
<span class="co1">// specialization</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="co1">// for the specialized A</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="co1">// for the specialized B</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="co1">// for the unspecialized mf1</span>
<span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">mf1</span><span class="br0">&#40;</span>T t<span class="br0">&#41;</span> <span class="br0">&#123;</span> <span class="br0">&#125;</span>
&#160;
<span class="co1">// ERROR: B&lt;double&gt; is specialized and is a member template, so its enclosing A</span>
<span class="co1">// must be specialized also</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> Y<span class="sy1">&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span>Y<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">mf2</span><span class="br0">&#40;</span><span class="br0">&#41;</span> <span class="br0">&#123;</span> <span class="br0">&#125;</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/template_specialization&amp;action=edit&amp;section=5" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<ul><li> <a href="templates.html" title="cpp/language/templates">templates</a>
</li><li> <a href="class_template.html" title="cpp/language/class template">class template</a>
</li><li> <a href="function_template.html" title="cpp/language/function template">function template</a>
</li><li> <a href="partial_specialization.html" title="cpp/language/partial specialization">partial specialization</a>
</li></ul>

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