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<p>Initializes an object from another object
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="#Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="#Example"><span class="tocnumber">4</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="#See_also"><span class="tocnumber">5</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/copy_initialization&amp;action=edit&amp;section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
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<td> <span class="t-spar">T</span> <span class="t-spar">object</span> <code><b>= </b></code> <span class="t-spar">other</span> <code><b>;</b></code>
</td>
<td> (1)
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<td> <span class="t-spar">f</span><code><b>(</b></code><span class="t-spar">other</span><code><b>)</b></code><code><b>;</b></code>
</td>
<td> (2)
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<td> <code><b>return </b></code> <span class="t-spar">other</span><code><b>;</b></code>
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<td> (3)
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<td> <code><b>catch (</b></code> <span class="t-spar">T</span> <span class="t-spar">other</span><code><b>)&#160;;</b></code>
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<td> (4)
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<td> <span class="t-spar">T</span> <span class="t-spar">array</span> <code><b>[</b></code> <span class="t-spar">N</span> <code><b>] = { </b></code> <span class="t-spar">other</span> <code><b>};</b></code>
</td>
<td> (5)
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<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/copy_initialization&amp;action=edit&amp;section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>Copy initialization is performed in the following situations:
</p>
<div class="t-li1"><span class="t-li">1)</span> when a named variable (automatic, static, or thread-local) is declared with the initializer consisting of an equals sign followed by an expression.</div>
<div class="t-li1"><span class="t-li">2)</span> when passing an argument to a function by value</div>
<div class="t-li1"><span class="t-li">3)</span> when returning from a function that returns by value</div>
<div class="t-li1"><span class="t-li">4)</span> when catching an exception by value</div>
<div class="t-li1"><span class="t-li">5)</span> as part of <a href="aggregate_initialization.html" title="cpp/language/aggregate initialization">aggregate initialization</a>, to initialize each element for which an initializer is provided</div>
<p>The effects of copy initialization are:
</p>
<ul><li> If <code>T</code> is a class type and the type of <span class="t-spar">other</span> is cv-unqualified version of <code>T</code> or a class derived from <code>T</code>, the constructors of <code>T</code> are examined and the best match is selected by overload resolution. The constructor is then called to initialize the object.
</li></ul>
<ul><li> If <code>T</code> is a class type, and the type of <span class="t-spar">other</span> is different, or if <code>T</code> is non-class type, but the type of <span class="t-spar">other</span> is a class type, <a href="implicit_cast.html" title="cpp/language/implicit cast">user-defined conversion sequences</a> that can convert from the type of <span class="t-spar">other</span> to <code>T</code> are examined and the best one is selected through overload resolution. The result of the conversion, which is a prvalue temporary of the destination type, is then used to <a href="direct_initialization.html" title="cpp/language/direct initialization">direct-initialize</a> the object. The last step is usually <a href="copy_elision.html" title="cpp/language/copy elision">eliminated</a> and the result of the conversion function is constructed directly in the memory allocated for the target object, but the copy constructor is required to be accessible even though it's not used.
</li></ul>
<ul><li> Otherwise (if neither <code>T</code> nor the type of <span class="t-spar">other</span> are class types), <a href="implicit_cast.html" title="cpp/language/implicit cast">standard conversions</a> are used, if necessary, to convert the value of <span class="t-spar">other</span> to the cv-unqualified version of <code>T</code>.
</li></ul>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/copy_initialization&amp;action=edit&amp;section=3" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>Copy-initialization is less permissive than direct-initialization: copy-initialization only considers non-explicit constructors and user-defined conversion functions.
</p><p>If <span class="t-spar">other</span> is an rvalue expression, <a href="move_constructor.html" title="cpp/language/move constructor">move constructor</a> will be selected by overload resolution and called during copy-initialization.
</p><p><a href="implicit_cast.html" title="cpp/language/implicit cast">Implicit conversion</a> is defined in terms of copy-initialization: if an object of type <code>T</code> can be copy-initialized with expression <code>E</code>, then <code>E</code> is implicitly convertible to <code>T</code>.
</p><p>The equals sign, <code><b>=</b></code>, in copy-initialization of a named variable is not related to the assignment operator. Assignment operator overloads have no effect on copy-initialization.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/copy_initialization&amp;action=edit&amp;section=4" 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="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;utility&gt;</span>
<span class="co2">#include &lt;memory&gt;</span>
&#160;
<span class="kw4">int</span> main<span class="br0">&#40;</span><span class="br0">&#41;</span>
<span class="br0">&#123;</span>
    <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a> s <span class="sy1">=</span> <span class="st0">&quot;test&quot;</span><span class="sy4">;</span> <span class="co1">// OK: constructor is non-explicit</span>
    <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a> s2 <span class="sy1">=</span> <a href="../utility/move.html"><span class="kw914">std::<span class="me2">move</span></span></a><span class="br0">&#40;</span>s<span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// this copy-initialization performs a move</span>
&#160;
<span class="co1">//  std::unique_ptr&lt;int&gt; p = new int(1); // error: constructor is explicit</span>
    <a href="../memory/unique_ptr.html"><span class="kw581">std::<span class="me2">unique_ptr</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> p<span class="br0">&#40;</span>new <span class="kw4">int</span><span class="br0">&#40;</span><span class="nu0">1</span><span class="br0">&#41;</span><span class="br0">&#41;</span><span class="sy4">;</span> <span class="co1">// OK: direct-initialization</span>
&#160;
    <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu16">3.14</span><span class="sy4">;</span>    <span class="co1">// floating-integral conversion</span>
    <span class="kw4">const</span> <span class="kw4">int</span> b <span class="sy1">=</span> n<span class="sy4">;</span> <span class="co1">// const doesn't matter</span>
    <span class="kw4">int</span> c <span class="sy1">=</span> b<span class="sy4">;</span>       <span class="co1">// ...either way</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/copy_initialization&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="default_initialization.html" title="cpp/language/default initialization">default initialization</a>
</li><li> <a href="direct_initialization.html" title="cpp/language/direct initialization">direct initialization</a>
</li><li> <a href="aggregate_initialization.html" title="cpp/language/aggregate initialization">aggregate initialization</a>
</li><li> <a href="list_initialization.html" title="cpp/language/list initialization">list initialization</a>
</li></ul>

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