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<p>A copy constructor of class <code>T</code> is a non-template constructor whose first parameter is <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span></span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> T<span class="sy3">&</span></span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">volatile</span> T<span class="sy3">&</span></span></span>, or <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">volatile</span> T<span class="sy3">&</span></span></span>, and either there are no other parameters, or the rest of the parameters all have default values.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="copy_constructor.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="copy_constructor.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="copy_constructor.html#Implicitly-declared_copy_constructor"><span class="tocnumber">3</span> <span class="toctext">Implicitly-declared copy constructor</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="copy_constructor.html#Deleted_implicitly-declared_copy_constructor"><span class="tocnumber">4</span> <span class="toctext">Deleted implicitly-declared copy constructor</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="copy_constructor.html#Trivial_copy_constructor"><span class="tocnumber">5</span> <span class="toctext">Trivial copy constructor</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="copy_constructor.html#Implicitly-defined_copy_constructor"><span class="tocnumber">6</span> <span class="toctext">Implicitly-defined copy constructor</span></a></li>
<li class="toclevel-1 tocsection-7"><a href="copy_constructor.html#Notes"><span class="tocnumber">7</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-8"><a href="copy_constructor.html#Example"><span class="tocnumber">8</span> <span class="toctext">Example</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/copy_constructor&action=edit&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> <span class="t-spar">class_name</span> ( <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> <span class="t-spar">class_name</span> <code>&</code> )
</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> <span class="t-spar">class_name</span> ( <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> <span class="t-spar">class_name</span> <code>&</code> ) = <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">default</span><span class="sy4">;</span></span></span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">class_name</span> ( <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> <span class="t-spar">class_name</span> <code>&</code> ) = <span class="t-c"><span class="mw-geshi cpp source-cpp">delete<span class="sy4">;</span></span></span>
</td>
<td> (3)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/copy_constructor&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<ol><li> Typical declaration of a copy constructor
</li><li> Forcing a copy constructor to be generated by the compiler
</li><li> Avoiding implicit generation of the copy constructor
</li></ol>
<p>The copy constructor is called whenever an object is initialized from another object of the same type (unless <a href="overload_resolution.html" title="cpp/language/overload resolution">overload resolution</a> selects the <a href="move_constructor.html" title="cpp/language/move constructor">move constructor</a> as a better match), which includes
</p>
<ul><li> <a href="direct_initialization.html" title="cpp/language/direct initialization">direct initialization</a> <span class="t-c"><span class="mw-geshi cpp source-cpp">T a<span class="br0">(</span>b<span class="br0">)</span><span class="sy4">;</span></span></span>, where b is of type <code>T</code>
</li><li> <a href="copy_initialization.html" title="cpp/language/copy initialization">copy initialization</a>, including by-value function parameter passing, by-value returns ,exception throwing and handling, and <span class="t-c"><span class="mw-geshi cpp source-cpp">T a <span class="sy1">=</span> b<span class="sy4">;</span></span></span>, where b is of type <code>T</code> or derived from <code>T</code>
</li></ul>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/copy_constructor&action=edit&section=3" title="Edit section: Implicitly-declared copy constructor">edit</a>]</span> <span class="mw-headline" id="Implicitly-declared_copy_constructor">Implicitly-declared copy constructor</span></h3>
<p>If no user-defined copy constructors are provided for a class type (<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span></span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">class</span></span></span>, or <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">union</span></span></span>), the compiler will always declare a copy constructor as a non-<a href="explicit.html" title="cpp/language/explicit">explicit</a> <code>inline public</code> member of its class. This implicitly-declared copy constructor has the form <code>T::T(const T&)</code> if all of the following is true:
</p>
<ul><li> all direct and virtual bases of <code>T</code> have copy constructors with references to const or to const volatile as their first parameters
</li><li> all non-static members of <code>T</code> have copy constructors with references to const or to const volatile as their first parameters
</li></ul>
<p>Otherwise, the implicitly-declared copy constructor is <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy4">::</span><span class="me2">T</span><span class="br0">(</span>T<span class="sy3">&</span><span class="br0">)</span></span></span>. (Note that due to these rules, the implicitly-declared copy constructor cannot bind to a volatile lvalue argument)
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>A class can have multiple copy constructors, e.g. both <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy4">::</span><span class="me2">T</span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span><span class="br0">)</span></span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy4">::</span><span class="me2">T</span><span class="br0">(</span>T<span class="sy3">&</span><span class="br0">)</span></span></span>. If some user-defined copy constructors are present, the user may still force the generation of the implicitly declared copy constructor with the keyword <code>default</code>
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/copy_constructor&action=edit&section=4" title="Edit section: Deleted implicitly-declared copy constructor">edit</a>]</span> <span class="mw-headline" id="Deleted_implicitly-declared_copy_constructor">Deleted implicitly-declared copy constructor</span></h3>
<table class="t-rev-begin">
<tr class="t-rev t-until-cxx11"><td> the implicitly-declared copy constructor for class <code>T</code> is undefined if any of the following conditions are true:
</td>
<td><span class="t-mark-rev t-until-cxx11">(until C++11)</span></td></tr>
<tr class="t-rev t-since-cxx11"><td> the implicitly-declared or defaulted copy constructor for class <code>T</code> is defined as <i>deleted</i> if any of the following conditions are true:
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<ul><li> <code>T</code> has non-static data members that cannot be copied (have deleted, inaccessible, or ambiguous copy constructors)
</li><li> <code>T</code> has direct or virtual base class that cannot be copied (has deleted, inaccessible, or ambiguous copy constructors)
</li><li> <code>T</code> has direct or virtual base class with a deleted or inaccessible destructor
</li></ul>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<ul><li> <code>T</code> has a user-defined move constructor or move assignment operator
</li><li> <code>T</code> is a union and has a variant member with non-trivial copy constructor
</li><li> <code>T</code> has a data member of rvalue reference type
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/copy_constructor&action=edit&section=5" title="Edit section: Trivial copy constructor">edit</a>]</span> <span class="mw-headline" id="Trivial_copy_constructor">Trivial copy constructor</span></h3>
<p>The copy constructor for class <code>T</code> is trivial if all of the following is true:
</p>
<ul><li> It is not user-provided (that is, it is implicitly-defined or defaulted), and if it is defaulted, its signature is the same as implicitly-defined
</li><li> <code>T</code> has no virtual member functions
</li><li> <code>T</code> has no virtual base classes
</li><li> The copy constructor selected for every direct base of <code>T</code> is trivial
</li><li> The copy constructor selected for every non-static class type (or array of class type) member of <code>T</code> is trivial
</li></ul>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<ul><li> <code>T</code> has no non-static data members of <a href="cv.html" title="cpp/language/cv">volatile</a>-qualified type
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td></tr>
</table>
<p>A trivial copy constructor is a constructor that creates a bytewise copy of the object representation of the argument, and performs no other action. Objects with trivial copy constructors can be copied by copying their object representations manually, e.g. with <span class="t-lc">std::memmove</span>. All data types compatible with the C language (POD types) are trivially copyable.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/copy_constructor&action=edit&section=6" title="Edit section: Implicitly-defined copy constructor">edit</a>]</span> <span class="mw-headline" id="Implicitly-defined_copy_constructor">Implicitly-defined copy constructor</span></h3>
<p>If the implicitly-declared copy constructor is neither deleted nor trivial, it is defined (that is, a function body is generated and compiled) by the compiler if <a href="definition.html#ODR-use" title="cpp/language/definition">odr-used</a>. For <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">union</span></span></span> types, the implicitly-defined copy constructor copies the object representation (as by <span class="t-lc">std::memmove</span>). For non-union class types (<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">class</span></span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span></span></span>), the constructor performs full member-wise copy of the object's bases and non-static members, in their initialization order, using direct initialization.
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>The generation of the implicitly-defined copy constructor is deprecated if <code>T</code> has a user-defined destructor or user-defined copy assignment operator.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/copy_constructor&action=edit&section=7" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>In many situations, copy constructors are optimized out even if they would produce observable side-effects, see <a href="copy_elision.html" title="cpp/language/copy elision">copy elision</a>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/copy_constructor&action=edit&section=8" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<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="kw4">int</span> n<span class="sy4">;</span>
A<span class="br0">(</span><span class="kw4">int</span> n<span class="sy1">=</span><span class="nu0">1</span><span class="br0">)</span> <span class="sy4">:</span> n<span class="br0">(</span>n<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
A<span class="br0">(</span><span class="kw4">const</span> A<span class="sy3">&</span> a<span class="br0">)</span> <span class="sy4">:</span> n<span class="br0">(</span>a.<span class="me1">n</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// user-defined copy ctor</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> B <span class="sy4">:</span> A <span class="br0">{</span>
<span class="co1">// implicit default ctor B::B()</span>
<span class="co1">// implicit copy ctor B::B(const B&) </span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> C <span class="sy4">:</span> B <span class="br0">{</span>
C<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> B<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="kw1">private</span><span class="sy4">:</span>
C<span class="br0">(</span><span class="kw4">const</span> C<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// non-copyable, C++98 style</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="br0">(</span><span class="nu0">7</span><span class="br0">)</span><span class="sy4">;</span>
A a2<span class="br0">(</span>a1<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls the copy ctor</span>
B b<span class="sy4">;</span>
B b2 <span class="sy1">=</span> b<span class="sy4">;</span>
A a3 <span class="sy1">=</span> b<span class="sy4">;</span> <span class="co1">// conversion to A& and copy ctor</span>
<span class="kw4">volatile</span> A va<span class="br0">(</span><span class="nu0">10</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// A a4 = va; // compile error</span>
 
C c<span class="sy4">;</span>
<span class="co1">// C c2 = c; // compile error</span>
<span class="br0">}</span></pre></div></div>
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