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<p>A constructor that is not declared with the specifier <a href="explicit.html" title="cpp/language/explicit">explicit</a> <span class="t-rev-inl t-until-cxx11"><span>and which has one non-default parameter</span> <span><span class="t-mark-rev t-until-cxx11">(until C++11)</span></span></span> is called a <i>converting constructor</i>
</p><p>Unlike explicit constructors, which are only considered during <a href="direct_initialization.html" title="cpp/language/direct initialization">direct initialization</a> (which includes <a href="explicit_cast.html" title="cpp/language/explicit cast">explicit conversions</a> such as <a href="static_cast.html" title="cpp/language/static cast">static_cast</a>), converting constructors are also considered during <a href="copy_initialization.html" title="cpp/language/copy initialization">copy initialization</a>, as part of <a href="implicit_cast.html" title="cpp/language/implicit cast">user-defined conversion sequence</a>.
</p><p>It is said that a converting constructor specifies an implicit conversion from the types of its arguments to the type of its class. Note that non-explicit <a href="cast_operator.html" title="cpp/language/cast operator">user-defined conversion function</a> also specifies an implicit conversion.
</p><p>Implicitly-declared and user-defined non-explicit <a href="copy_constructor.html" title="cpp/language/copy constructor">copy constructors</a> and <a href="move_constructor.html" title="cpp/language/move constructor">move constructors</a> are converting constructors.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/converting_constructor&action=edit&section=1" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<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="kw1">struct</span> A
<span class="br0">{</span>
A<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// converting constructor</span>
A<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// converting constructor (C++11)</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> B
<span class="br0">{</span>
<span class="kw1">explicit</span> B<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="kw1">explicit</span> B<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</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 a1 <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// OK: copy-initialization selects A::A(int)</span>
A a2<span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: direct-initialization selects A::A(int)</span>
A a3 <span class="br0">{</span><span class="nu0">4</span>,<span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK: direct-list-initialization selects A::A(int, int)</span>
A a4 <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">4</span>,<span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK: copy-list-initialization selects A::A(int, int)</span>
A a5 <span class="sy1">=</span> <span class="br0">(</span>A<span class="br0">)</span><span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// OK: explicit cast performs static_cast, direct-initialization</span>
 
<span class="co1">// B b1 = 1; // Error: copy-initialization does not consider B::B(int)</span>
B b2<span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: direct-initialization selects B::B(int)</span>
B b3 <span class="br0">{</span><span class="nu0">4</span>,<span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK: direct-list-initialization selects B::B(int, int)</span>
<span class="co1">// B b4 = {4,5}; // Error: copy-list-initialization does not consider B::B(int,int)</span>
B b5 <span class="sy1">=</span> <span class="br0">(</span>B<span class="br0">)</span><span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// OK: explicit cast performs static_cast, direct-initialization</span>
<span class="br0">}</span></pre></div></div>
<p><br />
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