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<tr class="t-nv"><td colspan="5"> <a href="derived_class.html" title="cpp/language/derived class">base and derived classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink">virtual member functions</strong></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override">override</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final">final</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="abstract_class.html" title="cpp/language/abstract class">pure virtual functions and abstract classes</a></td></tr>
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</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/classes/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Specifies that a non-static <a href="member_functions.html" title="cpp/language/member functions">member function</a> is <i>virtual</i> and supports dynamic binding.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="virtual.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="virtual.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="virtual.html#In_detail"><span class="tocnumber">3</span> <span class="toctext">In detail</span></a>
<ul>
<li class="toclevel-2 tocsection-4"><a href="virtual.html#Covariant_return_types"><span class="tocnumber">3.1</span> <span class="toctext">Covariant return types</span></a></li>
<li class="toclevel-2 tocsection-5"><a href="virtual.html#Virtual_destructor"><span class="tocnumber">3.2</span> <span class="toctext">Virtual destructor</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-6"><a href="virtual.html#During_construction_and_destruction"><span class="tocnumber">4</span> <span class="toctext">During construction and destruction</span></a></li>
<li class="toclevel-1 tocsection-7"><a href="virtual.html#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/virtual&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
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<td colspan="10" class="t-sdsc-sep">
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<td class="t-sdsc-nopad"> <code><b>virtual</b></code> <span class="t-spar">function_declaration</span> <code><b>;</b></code>
</td>
<td class="t-sdsc-nopad">
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<td class="t-sdsc-nopad">
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<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/virtual&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>Virtual functions are member functions whose behavior can be overridden in derived classes. As opposed to non-virtual functions, the overridden behavior is preserved even if there is no compile-time information about the actual type of the class. If a derived class is handled using pointer or reference to the base class, a call to a overridden virtual function would invoke the behavior defined in the derived class. This behavior is suppressed if the function is selected using <a href="lookup.html" title="cpp/language/lookup">qualified name lookup</a> (that is, if the function's name appears to the right of the scope resolution operator <code>::</code>)
</p>
<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 <iostream></span>
<span class="kw1">struct</span> Base <span class="br0">{</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"base<span class="es1">\n</span>"</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> Derived <span class="sy4">:</span> Base <span class="br0">{</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> override <span class="br0">{</span> <span class="co1">// 'override' is optional</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"derived<span class="es1">\n</span>"</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>
Base b<span class="sy4">;</span>
Derived d<span class="sy4">;</span>
 
<span class="co1">// virtual function call through reference</span>
Base<span class="sy3">&</span> br <span class="sy1">=</span> b<span class="sy4">;</span> <span class="co1">// the type of br is Base&</span>
Base<span class="sy3">&</span> dr <span class="sy1">=</span> d<span class="sy4">;</span> <span class="co1">// the type of dr is Base& as well</span>
br.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// prints "base"</span>
dr.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// prints "derived"</span>
 
<span class="co1">// virtual function call through pointer</span>
Base<span class="sy2">*</span> bp <span class="sy1">=</span> <span class="sy3">&</span>b<span class="sy4">;</span> <span class="co1">// the type of bp is Base*</span>
Base<span class="sy2">*</span> dp <span class="sy1">=</span> <span class="sy3">&</span>d<span class="sy4">;</span> <span class="co1">// the type of dp is Base* as well</span>
bp<span class="sy2">-</span><span class="sy1">></span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// prints "base"</span>
dp<span class="sy2">-</span><span class="sy1">></span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// prints "derived"</span>
 
<span class="co1">// non-virtual function call</span>
br.<span class="me1">Base</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// prints "base"</span>
dr.<span class="me1">Base</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// prints "base"</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/virtual&action=edit&section=3" title="Edit section: In detail">edit</a>]</span> <span class="mw-headline" id="In_detail">In detail</span></h3>
<p>If some member function <code>vf</code> is declared as <code>virtual</code> in a class <code>Base</code>, and some class <code>Derived</code>, which is derived, directly or indirectly, from <code>Base</code>, has a declaration for member function with the same
</p>
<ul><li> name
</li><li> parameter type list (but not the return type)
</li><li> cv-qualifiers
</li><li> ref-qualifiers
</li></ul>
<p>Then this function in the class <code>Derived</code> is also <i>virtual</i> (whether or not the keyword <code>virtual</code> is used in its declaration) and <i>overrides</i> Base::vf (whether or not the word <code>override</code> is used in its declaration).
</p><p><code>Base::vf</code> does not need to be visible (can be declared private, or inherited using private inheritance) to be overridden.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> B <span class="br0">{</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> do_f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// private member</span>
<span class="kw1">public</span><span class="sy4">:</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> do_f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// public interface</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> <span class="kw1">public</span> B <span class="br0">{</span>
<span class="kw4">void</span> do_f<span class="br0">(</span><span class="br0">)</span> override<span class="sy4">;</span> <span class="co1">// overrides B::f</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 d<span class="sy4">;</span>
B<span class="sy2">*</span> bp <span class="sy1">=</span> <span class="sy3">&</span>d<span class="sy4">;</span>
bp<span class="sy2">-</span><span class="sy1">></span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls D::do_f();</span>
<span class="br0">}</span></pre></div></div>
<p>For every virtual function, there is the <i>final overrider</i>, which is executed when a virtual function call is made. A virtual member function <code>vf</code> of a base class <code>Base</code> is the final overrider unless the derived class declares or inherits (through multiple inheritance) another function that overrides <code>vf</code>.
</p>
<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="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// A::f is virtual</span>
<span class="kw1">struct</span> B <span class="sy4">:</span> A <span class="br0">{</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// B::f overrides A::f in B</span>
<span class="kw1">struct</span> C <span class="sy4">:</span> <span class="kw1">virtual</span> B <span class="br0">{</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// C::f overrides A::f in C</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> <span class="kw1">virtual</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// D does not introduce an overrider, B::f is final in D</span>
<span class="kw1">struct</span> E <span class="sy4">:</span> C, D <span class="br0">{</span> <span class="co1">// E does not introduce an overrider, C::f is final in E</span>
<span class="kw1">using</span> A<span class="sy4">::</span><span class="me2">f</span><span class="sy4">;</span> <span class="co1">// not a function declaration, just makes A::f visible to lookup</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>
E e<span class="sy4">;</span>
e.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// virtual call calls C::f, the final overrider in e</span>
e.<span class="me1">E</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// non-virtual call calls A::f, which is visible in E</span>
<span class="br0">}</span></pre></div></div>
<p>If a function has more than one final overrider, the program is ill-formed:
</p>
<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="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> VB1 <span class="sy4">:</span> <span class="kw1">virtual</span> A <span class="br0">{</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// overrides A::f</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> VB2 <span class="sy4">:</span> <span class="kw1">virtual</span> A <span class="br0">{</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// overrides A::f</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// struct Error : VB1, VB2 {</span>
<span class="co1">// // Error: A::f has two final overriders in Error</span>
<span class="co1">// };</span>
<span class="kw1">struct</span> Okay <span class="sy4">:</span> VB1, VB2 <span class="br0">{</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: this is the final overrider for A::f</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> VB1a <span class="sy4">:</span> <span class="kw1">virtual</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// does not declare an overrider</span>
<span class="kw1">struct</span> Da <span class="sy4">:</span> VB1a, VB2 <span class="br0">{</span>
<span class="co1">// in Da, the final overrider of A::f is VB2::f</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>A function with the same name but different parameter list does not override the base function of the same name, but <i>hides</i> it: when <a href="lookup.html" title="cpp/language/lookup">unqualified name lookup</a> examines the scope of the derived class, the lookup finds the declaration and does not examine the base class.
</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="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</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>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// D::f hides B::f (wrong parameter list)</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> D2 <span class="sy4">:</span> D <span class="br0">{</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// D2::f overrides B::f (doesn't matter that it's not visible)</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>
B b<span class="sy4">;</span> B<span class="sy3">&</span> b_as_b <span class="sy1">=</span> b<span class="sy4">;</span>
D d<span class="sy4">;</span> B<span class="sy3">&</span> d_as_b <span class="sy1">=</span> d<span class="sy4">;</span> D<span class="sy3">&</span> d_as_d <span class="sy1">=</span> d<span class="sy4">;</span>
D2 d2<span class="sy4">;</span> B<span class="sy3">&</span> d2_as_b <span class="sy1">=</span> d2<span class="sy4">;</span> D<span class="sy3">&</span> d2_as_d <span class="sy1">=</span> d2<span class="sy4">;</span>
 
b_as_b.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls B::f()</span>
d_as_b.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls B::f()</span>
d2_as_b.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls D2::f()</span>
 
d_as_d.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: lookup in D finds only f(int)</span>
d2_as_d.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: lookup in D finds only f(int)</span>
<span class="br0">}</span></pre></div></div>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>If a function is declared with the specifier <code>override</code>, but does not override a virtual function, the program is ill-formed:
</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="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</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>
<span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> override<span class="sy4">;</span> <span class="co1">// OK, D::f(int) overrides B::f(int)</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">long</span><span class="br0">)</span> override<span class="sy4">;</span> <span class="co1">// Error: f(long) does not override B(int)</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>If a function is declared with the specifier <code>final</code>, and another function attempts to override it, the program is ill-formed
</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="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> final<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>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span> <span class="co1">// Error: D::f attempts to override final B::f</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<p>Non-member functions and static member functions cannot be virtual.
</p><p>Functions templates cannot be declared <code>virtual</code>. This applies only to functions that are themselves templates - a regular member function of a class template can be declared virtual.
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-concepts_ts"><td>
<p>Virtual functions cannot have any associated constraints.
</p>
<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="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> requires <span class="kw2">true</span><span class="sy4">;</span> <span class="co1">// Error: constrained virtual function</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-concepts-ts t-mark-ts">(concepts TS)</span></td></tr>
</table>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/virtual&action=edit&section=4" title="Edit section: Covariant return types">edit</a>]</span> <span class="mw-headline" id="Covariant_return_types">Covariant return types</span></h4>
<p>If the function <code>Derived::f</code> overrides a function <code>Base::f</code>, their return types must either be the same or be <i>covariant</i>. Two types are covariant if they satisfy all of the following requirements:
</p>
<ul><li> both types are pointers or references (lvalue or rvalue) to classes. Multi-level pointers or references are not allowed.
</li><li> the referenced/pointed-to class in the return type of <code>Base::f()</code> must be a unambiguous and accessible direct or indirect base class of the referenced/pointed-to class of the return type of <code>Derived::f()</code>.
</li><li> the return type of <code>Derived::f()</code> must be equally or less <a href="cv.html" title="cpp/language/cv"> cv-qualified</a> than the return type of <code>Base::f()</code>.
</li></ul>
<p>The class in the return type of <code>Derived::f</code> must be either <code>Derived</code> itself, or must be a <a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">complete type</a> at the point of declaration of <code>Derived::f</code>.
</p><p>When a virtual function call is made, the type returned by the final overrider is <a href="implicit_cast.html" title="cpp/language/implicit cast">implicitly converted</a> to the return type of the overridden function that was called:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> Base <span class="br0">{</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> vf1<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> vf2<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> vf3<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">virtual</span> B<span class="sy2">*</span> vf4<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">virtual</span> B<span class="sy2">*</span> vf5<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">class</span> D <span class="sy4">:</span> <span class="kw1">private</span> B <span class="br0">{</span>
<span class="kw1">friend</span> <span class="kw1">struct</span> Derived<span class="sy4">;</span> <span class="co1">// in Derived, B is an accessible base of D</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">class</span> A<span class="sy4">;</span> <span class="co1">// forward-declared class is an incomplete type</span>
 
<span class="kw1">struct</span> Derived <span class="sy4">:</span> <span class="kw1">public</span> Base <span class="br0">{</span>
<span class="kw4">void</span> vf1<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// virtual, overrides Base::vf1()</span>
<span class="kw4">void</span> vf2<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// non-virtual, hides Base::vf2()</span>
<span class="co1">// char vf3(); // Error: overrides Base::vf3, but has different</span>
<span class="co1">// and non-covariant return type</span>
D<span class="sy2">*</span> vf4<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// overrides Base::vf4() and has covariant return type</span>
<span class="co1">// A* vf5(); // Error: A is incomplete type</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>
Derived d<span class="sy4">;</span>
Base<span class="sy3">&</span> br <span class="sy1">=</span> d<span class="sy4">;</span>
Derived<span class="sy3">&</span> dr <span class="sy1">=</span> d<span class="sy4">;</span>
 
br.<span class="me1">vf1</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls Derived::vf1()</span>
br.<span class="me1">vf2</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls Base::vf2()</span>
<span class="co1">// dr.vf2(); // Error: vf2(int) hides vf2()</span>
 
B<span class="sy2">*</span> p <span class="sy1">=</span> br.<span class="me1">vf4</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls Derived::vf4() and converts the result to B*</span>
D<span class="sy2">*</span> q <span class="sy1">=</span> dr.<span class="me1">vf4</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls Derived::vf4() and does not convert</span>
<span class="co1">// the result to B*</span>
 
<span class="br0">}</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/virtual&action=edit&section=5" title="Edit section: Virtual destructor">edit</a>]</span> <span class="mw-headline" id="Virtual_destructor">Virtual destructor</span></h4>
<p>Even though destructors are not inherited, if a base class declares its destructor <code>virtual</code>, the derived destructor always overrides it. This makes it possible to delete dynamically allocated objects of polymorphic type through pointers to base
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> Base <span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
<span class="kw1">virtual</span> ~Base<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/* releases Base's resources */</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">class</span> Derived <span class="sy4">:</span> <span class="kw1">public</span> Base <span class="br0">{</span>
~Derived<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/* releases Derived's resources */</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>
Base<span class="sy2">*</span> b <span class="sy1">=</span> new Derived<span class="sy4">;</span>
delete b<span class="sy4">;</span> <span class="co1">// Makes a virtual function call to Base::~Base()</span>
<span class="co1">// since it is virtual, it calls Derived::~Derived() which can</span>
<span class="co1">// release resources of the derived class, and then calls</span>
<span class="co1">// Base::~Base() following the usual order of destruction</span>
<span class="br0">}</span></pre></div></div>
<p>Moreover, if a class is <i>polymorphic</i> (declares or inherits at least one virtual function), and its destructor is not virtual, deleting it is <i>undefined behavior</i> regardless of whether there are resources that would be leaked if the derived destructor is not invoked.
</p><p>A useful guideline is that the destructor of any base class must be <a rel="nofollow" class="external text" href="http://www.gotw.ca/publications/mill18.htm">public and virtual or protected and non-virtual</a>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/virtual&action=edit&section=6" title="Edit section: During construction and destruction">edit</a>]</span> <span class="mw-headline" id="During_construction_and_destruction">During construction and destruction</span></h3>
<p>When a virtual function is called directly or indirectly from a constructor or from a destructor (including during the construction or destruction of the class’s non-static data members, e.g. in a member <a href="constructor.html" title="cpp/language/initializer list">initializer list</a>), and the object to which the call applies is the object under construction or destruction, the function called is the final overrider
in the constructor’s or destructor’s class and not one overriding it in a more-derived class.
In other words, during construction or destruction, the more-derived classes do not exist.
</p><p>When constructing a complex class with multiple branches, within a constructor that belongs to one branch, polymorphism is restricted to that class and its bases: if it obtains a pointer or reference to a base subobject outside this subhierarchy, and attempts to invoke a virtual function call (e.g. using explicit member access), the behavior is undefined:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> V <span class="br0">{</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> A <span class="sy4">:</span> <span class="kw1">virtual</span> V <span class="br0">{</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// A::f is the final overrider of V::f in A</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B <span class="sy4">:</span> <span class="kw1">virtual</span> V <span class="br0">{</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// B::g is the final overrider of V::g in B</span>
B<span class="br0">(</span>V<span class="sy2">*</span>, A<span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> A, B <span class="br0">{</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// D::f is the final overrider of V::f in D</span>
<span class="kw1">virtual</span> <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// D::g is the final overrider of V::g in D</span>
 
<span class="co1">// note: A is initialized before B</span>
D<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> B<span class="br0">(</span><span class="br0">(</span>A<span class="sy2">*</span><span class="br0">)</span>this, this<span class="br0">)</span>
<span class="br0">{</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// the constructor of B, called from the constructor of D </span>
B<span class="sy4">::</span><span class="me2">B</span><span class="br0">(</span>V<span class="sy2">*</span> v, A<span class="sy2">*</span> a<span class="br0">)</span>
<span class="br0">{</span>
f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// virtual call to V::f (although D has the final overrider, D doesn't exist)</span>
g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// virtual call to B::g, which is the final overrider in B </span>
 
v<span class="sy2">-</span><span class="sy1">></span>g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// v's type V is base of B, virtual call calls B::g as before</span>
 
a<span class="sy2">-</span><span class="sy1">></span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// a’s type A is not a base of B. it belongs to a different branch of the</span>
<span class="co1">// hierarchy. Attempting a virtual call through that branch causes</span>
<span class="co1">// undefined behavior even though A was already fully constructed in this</span>
<span class="co1">// case (it was constructed before B since it appears before B in the list</span>
<span class="co1">// of the bases of D). In practice, the virtual call to A::f will be</span>
<span class="co1">// attempted using B's virtual member function table, since that's what</span>
<span class="co1">// is active during B's construction)</span>
<span class="br0">}</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/virtual&action=edit&section=7" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<ul><li> <a href="derived_class.html" title="cpp/language/derived class"> derived classes and modes of inheritance </a>
</li><li> <a href="override.html" title="cpp/language/override"> override specifier</a> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</li><li> <a href="final.html" title="cpp/language/final"> final specifier</a> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</li></ul>
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