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<p>Template declarations (<a href="class_template.html" title="cpp/language/class template">class</a>, <a href="function_template.html" title="cpp/language/function template">function</a><span class="t-rev-inl t-since-cxx14"><span>, and <a href="variable_template.html" title="cpp/language/variable template">variables</a></span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span>) can appear inside a <a href="class.html" title="cpp/language/class">member specification</a> of any class, struct, or union that aren't <a href="class.html#Local_classes" title="cpp/language/class">local classes</a>.
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<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include <iostream></span>
<span class="co2">#include <vector></span>
<span class="co2">#include <algorithm></span>
 
<span class="kw1">struct</span> Printer <span class="br0">{</span> <span class="co1">// generic functor</span>
<a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os<span class="sy4">;</span>
Printer<span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os<span class="br0">)</span> <span class="sy4">:</span> os<span class="br0">(</span>os<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
<span class="kw4">void</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> obj<span class="br0">)</span> <span class="br0">{</span> os <span class="sy1"><<</span> obj <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// member template</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../container/vector.html"><span class="kw1094">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>,<span class="nu0">2</span>,<span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span>
<a href="../algorithm/for_each.html"><span class="kw1302">std::<span class="me2">for_each</span></span></a><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, Printer<span class="br0">(</span><a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> s <span class="sy1">=</span> <span class="st0">"abc"</span><span class="sy4">;</span>
<a href="../algorithm/for_each.html"><span class="kw1302">std::<span class="me2">for_each</span></span></a><span class="br0">(</span>s.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, s.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, Printer<span class="br0">(</span><a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">1 2 3 a b c</pre></div></div>
</div>
<p>If the enclosing class declaration is, in turn, a class template, when a member template is defined outside of the class body, it takes two sets of template parameters: one for the enclosing class, and another one for itself:
</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">typename</span> T1<span class="sy1">></span>
<span class="kw1">struct</span> string <span class="br0">{</span>
<span class="co1">// member template function</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T2<span class="sy1">></span>
<span class="kw4">int</span> compare<span class="br0">(</span><span class="kw4">const</span> T2<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// constructors can be templates too</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T2<span class="sy1">></span>
string<span class="br0">(</span><span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1070">std::<span class="me2">basic_string</span></span></a><span class="sy1"><</span>T2<span class="sy1">></span><span class="sy3">&</span> s<span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/*...*/</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// out of class definition of string<T1>::compare<T2> </span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T1<span class="sy1">></span> <span class="co1">// for the enclosing class template</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T2<span class="sy1">></span> <span class="co1">// for the member template</span>
<span class="kw4">int</span> string<span class="sy1"><</span>T1<span class="sy1">></span><span class="sy4">::</span><span class="me2">compare</span><span class="br0">(</span><span class="kw4">const</span> T2<span class="sy3">&</span> s<span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/* ... */</span> <span class="br0">}</span></pre></div></div>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="member_template.html#Member_function_templates"><span class="tocnumber">1</span> <span class="toctext">Member function templates</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="member_template.html#Conversion_function_templates"><span class="tocnumber">2</span> <span class="toctext">Conversion function templates</span></a></li>
<li class="toclevel-1"><a href="member_template.html#Member_variable_templates"><span class="tocnumber">3</span> <span class="toctext">Member variable templates</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="member_template.html#Defect_reports"><span class="tocnumber">4</span> <span class="toctext">Defect reports</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/member_template&action=edit&section=1" title="Edit section: Member function templates">edit</a>]</span> <span class="mw-headline" id="Member_function_templates">Member function templates</span></h3>
<p>Destructors and copy constructors cannot be templates, but other special member functions can be. If a template constructor is declared which could be instantiated with the type signature of a <a href="copy_constructor.html" title="cpp/language/copy constructor">copy constructor</a>, the implicitly-declared copy constructor is used instead.
</p><p>A member function template cannot be virtual, and a member function template in a derived class cannot override a virtual member function from the base class.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> Base <span class="br0">{</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> Derived <span class="sy4">:</span> Base <span class="br0">{</span>
<span class="co1">// this member template does not override B::f</span>
<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>
 
<span class="co1">// non-member override can call the template:</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> override <span class="br0">{</span>
f<span class="sy1"><></span><span class="br0">(</span>i<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>A non-template member function and a template member function with the same name may be declared. In case of conflict (when some template specialization matches the non-template function signature exactly), use of that name and type refers to the non-template member unless an explicit template argument list is supplied.
</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">typename</span> T<span class="sy1">></span>
<span class="kw1">struct</span> A <span class="br0">{</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// non-template member</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T2<span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>T2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// member template</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
A<span class="sy1"><</span><span class="kw4">char</span><span class="sy1">></span> ac<span class="sy4">;</span>
ac.<span class="me1">f</span><span class="br0">(</span><span class="st0">'c'</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls template function A<char>::f<char>(int)</span>
ac.<span class="me1">f</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls non-template function A<char>::f(int)</span>
ac.<span class="me1">f</span><span class="sy1"><></span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls template function A<char>::f<int>(int)</span>
<span class="br0">}</span></pre></div></div>
<p><br />
An out-of-class definition of a member function template must be <i>equivalent</i> to the declaration inside the class (see <a href="function_template.html#Function_template_overloading" title="cpp/language/function template">function template overloading</a> for the definition of equivalency), otherwise it is considered to be an overload.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> X <span class="br0">{</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> T good<span class="br0">(</span>T n<span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> T bad<span class="br0">(</span>T n<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw1">struct</span> identity <span class="br0">{</span> <span class="kw1">using</span> type <span class="sy1">=</span> T<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// OK: equivalent declaration</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> V<span class="sy1">></span>
V X<span class="sy4">::</span><span class="me2">good</span><span class="br0">(</span>V n<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> n<span class="sy4">;</span> <span class="br0">}</span>
 
<span class="co1">// Error: not equivalent to any of the declarations inside X</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
T X<span class="sy4">::</span><span class="me2">bad</span><span class="br0">(</span><span class="kw1">typename</span> identity<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span> n<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> n<span class="sy4">;</span> <span class="br0">}</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/member_template&action=edit&section=2" title="Edit section: Conversion function templates">edit</a>]</span> <span class="mw-headline" id="Conversion_function_templates">Conversion function templates</span></h3>
<p>A user-defined <a href="cast_operator.html" title="cpp/language/cast operator">conversion function</a> can be a template.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
operator T<span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// conversion to pointer to any type</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// out-of-class definition</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
A<span class="sy4">::</span><span class="me2">operator</span> T<span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="kw1">return</span> nullptr<span class="sy4">;</span><span class="br0">}</span>
 
<span class="co1">// explicit specialization for char*</span>
<span class="kw1">template</span><span class="sy1"><></span>
A<span class="sy4">::</span><span class="me2">operator</span> <span class="kw4">char</span><span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="kw1">return</span> nullptr<span class="sy4">;</span><span class="br0">}</span>
 
<span class="co1">// explicit instantiation</span>
<span class="kw1">template</span> A<span class="sy4">::</span><span class="me2">operator</span> <span class="kw4">void</span><span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
A a<span class="sy4">;</span>
<span class="kw4">int</span><span class="sy2">*</span> ip <span class="sy1">=</span> a.<span class="me1">operator</span> <span class="kw4">int</span><span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// explicit call to A::operator int*()</span>
<span class="br0">}</span></pre></div></div>
<p>During <a href="overload_resolution.html" title="cpp/language/overload resolution">overload resolution</a>, specializations of conversion function templates are not found by <a href="lookup.html" title="cpp/language/lookup">name lookup</a>. Instead, all visible conversion function templates are considered, and every specialization produced by <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">template argument deduction</a> (which has special rules for conversion function templates) is used as if found by name lookup.
</p><p>Using-declarations in derived classes cannot refer to specializations of template conversion functions from base classes.
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<p>A user-defined conversion function template cannot have a deduced return type
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> operator <span class="kw4">auto</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">42</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// error</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td></tr>
</table>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<h3> <span class="mw-headline" id="Member_variable_templates">Member variable templates</span></h3>
<p>A variable template declaration may appear at class scope, in which case it declares a static data member template. See <a href="variable_template.html" title="cpp/language/variable template">variable templates</a> for details.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td></tr>
</table>
<p><br />
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/member_template&action=edit&section=4" title="Edit section: Defect reports">edit</a>]</span> <span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1878">CWG 1878</a>
</td>
<td> C++14
</td>
<td> operator auto was technically allowed
</td>
<td> operator auto forbidden
</td></tr></table>
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