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<tr class="t-nv"><td colspan="5"> <a href="sizeof....html" title="cpp/language/sizeof...">sizeof...</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="noexcept.html" title="cpp/language/noexcept">noexcept</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operators.html" title="cpp/language/operators">Operator overloading</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Conversions </td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast">const_cast</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <strong class="selflink">User-defined conversion</strong></td></tr>
</table></div>
</div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/language/expressions/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Enables <a href="implicit_cast.html" title="cpp/language/implicit cast">implicit conversion</a> or <a href="explicit_cast.html" title="cpp/language/explicit cast">explicit conversion</a> from a <a href="class.html" title="cpp/language/class">class type</a> to another type.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/cast_operator&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
<p>Conversion function is declared like a <a href="member_functions.html" title="cpp/language/member functions">non-static member function</a> or member <a href="function_template.html" title="cpp/language/function template">function template</a> with no explicit return type and with the name of the form:
</p>
<table class="t-sdsc-begin">
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>operator</b></code> <span class="t-spar">conversion-type-id</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>explicit</b></code> <code><b>operator</b></code> <span class="t-spar">conversion-type-id</span>
</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> Declares a user-defined conversion function that participates in all <a href="implicit_cast.html" title="cpp/language/implicit cast">implicit</a> and <a href="explicit_cast.html" title="cpp/language/explicit cast">explicit conversions</a></div>
<div class="t-li1"><span class="t-li">2)</span> Declares a user-defined conversion function that participates in <a href="direct_initialization.html" title="cpp/language/direct initialization">direct-initialization</a> and <a href="explicit_cast.html" title="cpp/language/explicit cast">explicit conversions</a> only.</div>
<p><span class="t-spar">conversion-type-id</span> is a <a href="type-id.html#Type_naming" title="cpp/language/type">type-id</a> except that function and array operators <code><b>[]</b></code> or <code><b>()</b></code> are not allowed in its declarator (thus conversion to types such as pointer to array requires a type alias/typedef or an identity template: see below). Regardless of typedef, <span class="t-spar">conversion-type-id</span> cannot represent an array or a function type.
</p><p>Although the return type is not allowed in the declaration of a user-defined conversion function, the <span class="t-spar">decl-specifier-seq</span> of <a href="declarations.html#Specifiers" title="cpp/language/declarations">the declaration grammar</a> may be present and may include any specifier other than <span class="t-spar">type-specifier</span> or the keyword static, In particular, besides <code>explicit</code>, the specifiers <a href="inline.html" title="cpp/language/inline">inline</a>, <a href="virtual.html" title="cpp/language/virtual">virtual</a>, <a href="friend.html" title="cpp/language/friend">friend</a>, and <a href="constexpr.html" title="cpp/language/constexpr">constexpr</a> are also allowed.
</p><p>When such member function is declared in class X, it performs conversion from X to
<span class="t-spar">conversion-type-id</span>:
</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="co1">//implicit conversion</span>
operator <span class="kw4">int</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">7</span><span class="sy4">;</span> <span class="br0">}</span>
 
<span class="co1">// explicit conversion</span>
<span class="kw1">explicit</span> operator <span class="kw4">int</span><span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> nullptr<span class="sy4">;</span> <span class="br0">}</span>
 
<span class="co1">// Error: array operator not allowed in conversion-type-id</span>
<span class="co1">// operator int(*)[3]() const { return nullptr; }</span>
<span class="kw1">using</span> arr_t <span class="sy1">=</span> <span class="kw4">int</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="sy4">;</span>
operator arr_t<span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> nullptr<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// OK if done through typedef</span>
<span class="co1">// operator arr_t () const; // Error: conversion to array not allowed in any case</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>
X x<span class="sy4">;</span>
 
<span class="kw4">int</span> n <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: sets n to 7</span>
<span class="kw4">int</span> m <span class="sy1">=</span> x<span class="sy4">;</span> <span class="co1">// OK: sets m to 7</span>
 
<span class="kw4">int</span><span class="sy2">*</span> p <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: sets p to null</span>
<span class="co1">// int* q = x; // Error: no implicit conversion</span>
 
<span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>pa<span class="br0">)</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="sy1">=</span> x<span class="sy4">;</span> <span class="co1">// OK</span>
<span class="br0">}</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/cast_operator&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>User-defined conversion function is invoked on the second stage of the <a href="implicit_cast.html" title="cpp/language/implicit cast">implicit conversion</a>, which consists of zero or one <a href="converting_constructor.html" title="cpp/language/converting constructor">converting constructor</a> or zero or one user-defined conversion function.
</p><p>If both conversion functions and converting constructors can be used to perform some user-defined conversion, the conversion functions and constructors are both considered by <a href="overload_resolution.html" title="cpp/language/overload resolution">overload resolution</a> in <a href="copy_initialization.html" title="cpp/language/copy initialization">copy-initialization</a> and <a href="reference_initialization.html" title="cpp/language/reference initialization">reference-initialization</a> contexts, but only the constructors are considered in <a href="direct_initialization.html" title="cpp/language/direct initialization">direct-initialization</a> contexts.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> To <span class="br0">{</span>
To<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
To<span class="br0">(</span><span class="kw4">const</span> <span class="kw1">struct</span> From<span class="sy3">&</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// converting constructor</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> From <span class="br0">{</span>
operator To<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span><span class="kw1">return</span> To<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span><span class="br0">}</span> <span class="co1">// conversion function</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>
From f<span class="sy4">;</span>
To t1<span class="br0">(</span>f<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// direct-initialization: calls the constructor</span>
<span class="co1">// (note, if converting constructor is not available, implicit copy constructor</span>
<span class="co1">// will be selected, and conversion function will be called to prepare its argument)</span>
To t2 <span class="sy1">=</span> f<span class="sy4">;</span> <span class="co1">// copy-initialization: ambiguous</span>
<span class="co1">// (note, if conversion function is from a non-const type, e.g.</span>
<span class="co1">// From::operator To();, it will be selected instead of the ctor in this case)</span>
To t3 <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1"><</span>To<span class="sy1">></span><span class="br0">(</span>f<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// direct-initialization: calls the constructor</span>
<span class="kw4">const</span> To<span class="sy3">&</span> r <span class="sy1">=</span> f<span class="sy4">;</span> <span class="co1">// reference-initialization: ambiguous</span>
<span class="br0">}</span></pre></div></div>
<p>Conversion function to its own (possibly cv-qualified) class (or to a reference to it), to the base of its own class (or to a reference to it), and to the type <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> can be defined, but can not be executed as part of the conversion sequence, except, in some cases, through <a href="virtual.html" title="cpp/language/virtual">virtual</a> dispatch:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> D<span class="sy4">;</span>
<span class="kw1">struct</span> B <span class="br0">{</span>
<span class="kw1">virtual</span> operator D<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> B
<span class="br0">{</span>
operator D<span class="br0">(</span><span class="br0">)</span> override <span class="br0">{</span> <span class="kw1">return</span> D<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</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>
D obj<span class="sy4">;</span>
D obj2 <span class="sy1">=</span> obj<span class="sy4">;</span> <span class="co1">// does not call D::operator D()</span>
B<span class="sy3">&</span> br <span class="sy1">=</span> obj<span class="sy4">;</span>
D obj3 <span class="sy1">=</span> br<span class="sy4">;</span> <span class="co1">// calls D::operator D() through virtual dispatch</span>
<span class="br0">}</span></pre></div></div>
<p>It can also be called using member function call syntax:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> X <span class="sy4">:</span> B <span class="br0">{</span>
operator B<span class="sy3">&</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</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>
X x<span class="sy4">;</span>
B<span class="sy3">&</span> b1 <span class="sy1">=</span> x<span class="sy4">;</span> <span class="co1">// does not call X::operatorB&()</span>
B<span class="sy3">&</span> b2 <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1"><</span>B<span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// does not call X::operatorB&</span>
B<span class="sy3">&</span> b3 <span class="sy1">=</span> x.<span class="me1">operator</span> B<span class="sy3">&</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls X::operator&</span>
<span class="br0">}</span></pre></div></div>
<p>When making an explicit call to the conversion function, the type-id is greedy: it is the longest possible sequence of tokens that is a valid type id:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="sy3">&</span> x.<span class="me1">operator</span> <span class="kw4">int</span> <span class="sy2">*</span> a<span class="sy4">;</span> <span class="co1">// parsed as & (x.operator int*) a</span>
<span class="co1">// not as & (x.operator int) * a</span></pre></div></div>
<p>Conversion functions can be inherited and can be <a href="virtual.html" title="cpp/language/virtual">virtual</a>, but cannot be <a href="static.html" title="cpp/language/static">static</a>. A conversion function in the derived class does not hide a conversion function in the base class unless they are converting to the same type.
</p><p>Conversion function can be a template member function, for example, <a href="../memory/auto_ptr/operator_auto_ptr.html" title="cpp/memory/auto ptr/operator auto ptr">std::auto_ptr<T>::operator auto_ptr<Y></a>. See <a href="member_template.html#Conversion_function_templates" title="cpp/language/member template">member template</a> and <a href="template_argument_deduction.html#conversion_function_template" title="cpp/language/template argument deduction">template argument deduction</a> for applicable special rules.
</p>
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