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<p>A default constructor is a constructor which can be called with no arguments (either defined with an empty parameter list, or with default arguments provided for every parameter). A type with a public default constructor is <a href="../concept/DefaultConstructible.html" title="cpp/concept/DefaultConstructible"><code>DefaultConstructible</code></a>.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="default_constructor.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="default_constructor.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="default_constructor.html#Implicitly-declared_default_constructor"><span class="tocnumber">3</span> <span class="toctext">Implicitly-declared default constructor</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="default_constructor.html#Implicitly-defined_default_constructor"><span class="tocnumber">4</span> <span class="toctext">Implicitly-defined default constructor</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="default_constructor.html#Deleted_implicitly-declared_default_constructor"><span class="tocnumber">5</span> <span class="toctext">Deleted implicitly-declared default constructor</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="default_constructor.html#Trivial_default_constructor"><span class="tocnumber">6</span> <span class="toctext">Trivial default constructor</span></a></li>
<li class="toclevel-1 tocsection-7"><a href="default_constructor.html#Example"><span class="tocnumber">7</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/default_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> <code><b>ClassName</b></code> <code><b>(</b></code> <code><b>)</b></code> <code><b>;</b></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> <code><b>ClassName</b></code> <code><b>::</b></code> <code><b>ClassName</b></code> <code><b>(</b></code> <code><b>)</b></code> <span class="t-spar">body</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> <code><b>ClassName()</b></code> <code><b>=</b></code> <code><b>delete</b></code> <code><b>;</b></code>
</td>
<td> (3)
</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>
<tr class="t-sdsc">
<td> <code><b>ClassName()</b></code> <code><b>=</b></code> <code><b>default</b></code> <code><b>;</b></code>
</td>
<td> (4)
</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>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/default_constructor&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<div class="t-li1"><span class="t-li">1)</span> Declaration of a default constructor.</div>
<div class="t-li1"><span class="t-li">2)</span> Definition of the constructor outside the class body.</div>
<div class="t-li1"><span class="t-li">3)</span> Inhibiting the automatic generation of a default constructor by the compiler.</div>
<div class="t-li1"><span class="t-li">4)</span> Explicitly forcing the automatic generation of a default constructor by the compiler.</div>
<p><code><b>ClassName</b></code> is the identifier of the enclosing class
</p><p>Default constructors are called during <a href="default_initialization.html" title="cpp/language/default initialization">default initializations</a> and <a href="value_initialization.html" title="cpp/language/value initialization">value initializations</a>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/default_constructor&action=edit&section=3" title="Edit section: Implicitly-declared default constructor">edit</a>]</span> <span class="mw-headline" id="Implicitly-declared_default_constructor">Implicitly-declared default constructor</span></h3>
<p>If no user-defined constructors of any kind 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 default constructor as an <code>inline public</code> member of its class.
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>If some user-defined constructors are present, the user may still force the automatic generation of a default constructor by the compiler that would be implicitly-declared otherwise with the keyword <code>default</code>.</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/default_constructor&action=edit&section=4" title="Edit section: Implicitly-defined default constructor">edit</a>]</span> <span class="mw-headline" id="Implicitly-defined_default_constructor">Implicitly-defined default constructor</span></h3>
<p>If the implicitly-declared default constructor is not defined as deleted, it is defined (that is, a function body is generated and compiled) by the compiler, and it has exactly the same effect as a user-defined constructor with empty body and empty initializer list. That is, it calls the default constructors of the bases and of the non-static members of this class.
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>If some user-defined constructors are present, the user may still force the automatic generation of a default constructor by the compiler that would be implicitly-declared otherwise with the keyword <code>default</code>.</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/default_constructor&action=edit&section=5" title="Edit section: Deleted implicitly-declared default constructor">edit</a>]</span> <span class="mw-headline" id="Deleted_implicitly-declared_default_constructor">Deleted implicitly-declared default constructor</span></h3>
<p>The implicitly-declared or defaulted default constructor for class <code>T</code> is <span class="t-rev-inl t-until-cxx11"><span> undefined </span> <span><span class="t-mark-rev t-until-cxx11">(until C++11)</span></span></span><span class="t-rev-inl t-since-cxx11"><span>defined as deleted</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span> if any of the following is true:
</p>
<ul><li> <code>T</code> has a member of reference type <span class="t-rev-inl t-since-cxx11"><span>without a default initializer.</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>
</li><li> <code>T</code> has a <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> member without user-defined default constructor <span class="t-rev-inl t-since-cxx11"><span>or a default initializer</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>.
</li><li> <code>T</code> has a member <span class="t-rev-inl t-since-cxx11"><span>(without a default initializer)</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span> which has a deleted default constructor, or its default constructor is ambiguous or inaccessible from this constructor.
</li><li> <code>T</code> has a direct or virtual base which has a deleted default constructor, or it is ambiguous or inaccessible from this constructor.
</li><li> <code>T</code> has a direct or virtual base which has a deleted destructor, or a destructor that is inaccessible from this constructor.
</li></ul>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<ul><li> <code>T</code> is a <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">union</span></span></span> with at least one variant member with non-trivial default constructor.</td>
</li></ul>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<ul><li> <code>T</code> is a <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">union</span></span></span> and all of its variant members are <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span>.
</li></ul>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>If no user-defined constructors are present and the implicitly-declared default constructor is not trivial, the user may still inhibit the automatic generation of an implicitly-defined default constructor by the compiler with the keyword <code>delete</code>.</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/default_constructor&action=edit&section=6" title="Edit section: Trivial default constructor">edit</a>]</span> <span class="mw-headline" id="Trivial_default_constructor">Trivial default constructor</span></h3>
<p>The default constructor for class <code>T</code> is trivial (i.e. performs no action) if all of the following is true:
</p>
<ul><li> The constructor is not user-provided (i.e., is implicitly-defined or defaulted)
</li><li> <code>T</code> has no virtual member functions
</li><li> <code>T</code> has no virtual base classes
</li></ul>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<ul><li> <code>T</code> has no non-static members with default initializers.
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<ul><li> Every direct base of <code>T</code> has a trivial default constructor
</li><li> Every non-static member of class type has a trivial default constructor
</li></ul>
<p>A trivial default constructor is a constructor that performs no action. Objects with trivial default constructors can be created by using <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast">reinterpret_cast</a> on any suitably aligned storage, e.g. on memory allocated with <span class="t-lc">std::malloc</span>. All data types compatible with the C language (POD types) are trivially default-constructible.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/default_constructor&action=edit&section=7" 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>
<span class="kw4">int</span> x<span class="sy4">;</span>
A<span class="br0">(</span><span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">:</span> x<span class="br0">(</span>x<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// user-defined default constructor</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">// B::B() is implicitly-defined, calls A::A()</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> C
<span class="br0">{</span>
A a<span class="sy4">;</span>
<span class="co1">// C::C() is implicitly-defined, calls A::A()</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> D<span class="sy4">:</span> A
<span class="br0">{</span>
D<span class="br0">(</span><span class="kw4">int</span> y<span class="br0">)</span><span class="sy4">:</span> A<span class="br0">(</span>y<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="co1">// D::D() is not declared because another constructor exists</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> E<span class="sy4">:</span> A
<span class="br0">{</span>
E<span class="br0">(</span><span class="kw4">int</span> y<span class="br0">)</span><span class="sy4">:</span> A<span class="br0">(</span>y<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
E<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span> <span class="co1">// explicitly defaulted, calls A::A()</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> F
<span class="br0">{</span>
<span class="kw4">int</span><span class="sy3">&</span> ref<span class="sy4">;</span> <span class="co1">// reference member</span>
<span class="kw4">const</span> <span class="kw4">int</span> c<span class="sy4">;</span> <span class="co1">// const member</span>
<span class="co1">// F::F() is implicitly defined as deleted</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>
B b<span class="sy4">;</span>
C c<span class="sy4">;</span>
<span class="co1">// D d; // compile error</span>
E e<span class="sy4">;</span>
<span class="co1">// F f; // compile error</span>
<span class="br0">}</span></pre></div></div>
<p><br />
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