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virtual [C++ Reference]
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<h2><a name="virtual" id="virtual">virtual</a></h2>
<div class="level2">
<p>
Syntax:
</p>
<pre class="c code c++" style="font-family:monospace;"> virtual return<span class="sy0">-</span>type name<span class="br0">(</span> parameter<span class="sy0">-</span>list <span class="br0">)</span><span class="sy0">;</span>
virtual return<span class="sy0">-</span>type name<span class="br0">(</span> parameter<span class="sy0">-</span>list <span class="br0">)</span> <span class="sy0">=</span> <span class="nu0">0</span><span class="sy0">;</span></pre>
<p>
The virtual keyword can be used to create virtual functions, which can be
overridden by derived classes.
</p>
<ul>
<li class="level1"><div class="li"> A virtual function indicates that a function can be overridden in a subclass, and that the overridden function will actually be used.</div>
</li>
<li class="level1"><div class="li"> When a base object pointer points to a derived object that contains a virtual function, the decision about which version of that function to call is based on the type of object pointed to by the pointer, and this process happens at runtime.</div>
</li>
<li class="level1"><div class="li"> A base object can point to different derived objects and have different versions of the virtual function run.</div>
</li>
</ul>
<p>
If the function is specified as a pure virtual function (denoted by the = 0), it must be overridden by a derived class.
</p>
<p>
For example, the following code snippet shows how a child class can override a
virtual method of its parent, and how a non-virtual method in the parent cannot
be overridden:
</p>
<pre class="c code c++" style="font-family:monospace;"> class Base <span class="br0">{</span>
public<span class="sy0">:</span>
<span class="kw4">void</span> nonVirtualFunc<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="st0">"Base: non-virtual function"</span> <span class="sy0"><<</span> endl<span class="sy0">;</span>
<span class="br0">}</span>
virtual <span class="kw4">void</span> virtualFunc<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="st0">"Base: virtual function"</span> <span class="sy0"><<</span> endl<span class="sy0">;</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy0">;</span>
class Child <span class="sy0">:</span> public Base <span class="br0">{</span>
public<span class="sy0">:</span>
<span class="kw4">void</span> nonVirtualFunc<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="st0">"Child: non-virtual function"</span> <span class="sy0"><<</span> endl<span class="sy0">;</span>
<span class="br0">}</span>
<span class="kw4">void</span> virtualFunc<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="st0">"Child: virtual function"</span> <span class="sy0"><<</span> endl<span class="sy0">;</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy0">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
Base<span class="sy0">*</span> basePointer <span class="sy0">=</span> new Child<span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span>
basePointer<span class="sy0">-></span>nonVirtualFunc<span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span>
basePointer<span class="sy0">-></span>virtualFunc<span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span>
<span class="kw1">return</span> <span class="nu0">0</span><span class="sy0">;</span>
<span class="br0">}</span></pre>
<p>
When run, the following code displays:
</p>
<pre class="code">
Base: non-virtual function
Child: virtual function</pre>
<p>
Related Topics: <a href="../keywords/class.html" class="wikilink1" title="keywords:class">class</a>
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