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dynamic_cast [C++ Reference]
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<h2><a name="dynamic_cast" id="dynamic_cast">dynamic_cast</a></h2>
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<p>
Syntax:
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<pre class="cpp code cpp" style="font-family:monospace;"> TYPE<span class="sy3">&</span> <span class="kw2">dynamic_cast</span><span class="sy1"><</span>TYPE<span class="sy3">&</span><span class="sy1">></span> <span class="br0">(</span>object<span class="br0">)</span><span class="sy4">;</span>
TYPE<span class="sy2">*</span> <span class="kw2">dynamic_cast</span><span class="sy1"><</span>TYPE<span class="sy2">*</span><span class="sy1">></span> <span class="br0">(</span>object<span class="br0">)</span><span class="sy4">;</span></pre>
<p>
The dynamic_cast keyword casts a datum from one pointer or reference type to another, performing a runtime check to ensure the validity of the cast.
</p>
<p>
If you attempt to cast to a pointer type, and that type is not an actual type of the argument object, then the result of the cast will be <strong>NULL</strong>.
</p>
<p>
If you attempt to cast to a reference type, and that type is not an actual type of the argument object, then the cast will throw a <a href="../typeinfo/start#bad_cast.html" class="wikilink1" title="typeinfo:start">std::bad_cast</a> exception.
</p>
<pre class="cpp code cpp" style="font-family:monospace;"> <span class="kw4">struct</span> A <span class="br0">{</span>
<span class="kw2">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">struct</span> B <span class="sy4">:</span> <span class="kw2">public</span> A <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">struct</span> C <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">void</span> f <span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
A a<span class="sy4">;</span>
B b<span class="sy4">;</span>
A<span class="sy2">*</span> ap <span class="sy1">=</span> <span class="sy3">&</span>b<span class="sy4">;</span>
B<span class="sy2">*</span> b1 <span class="sy1">=</span> <span class="kw2">dynamic_cast</span><span class="sy1"><</span>B<span class="sy2">*</span><span class="sy1">></span> <span class="br0">(</span><span class="sy3">&</span>a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// NULL, because 'a' is not a 'B'</span>
B<span class="sy2">*</span> b2 <span class="sy1">=</span> <span class="kw2">dynamic_cast</span><span class="sy1"><</span>B<span class="sy2">*</span><span class="sy1">></span> <span class="br0">(</span>ap<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 'b'</span>
C<span class="sy2">*</span> c <span class="sy1">=</span> <span class="kw2">dynamic_cast</span><span class="sy1"><</span>C<span class="sy2">*</span><span class="sy1">></span> <span class="br0">(</span>ap<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// NULL.</span>
A<span class="sy3">&</span> ar <span class="sy1">=</span> <span class="kw2">dynamic_cast</span><span class="sy1"><</span>A<span class="sy3">&</span><span class="sy1">></span> <span class="br0">(</span><span class="sy2">*</span>ap<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Ok.</span>
B<span class="sy3">&</span> br <span class="sy1">=</span> <span class="kw2">dynamic_cast</span><span class="sy1"><</span>B<span class="sy3">&</span><span class="sy1">></span> <span class="br0">(</span><span class="sy2">*</span>ap<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Ok.</span>
C<span class="sy3">&</span> cr <span class="sy1">=</span> <span class="kw2">dynamic_cast</span><span class="sy1"><</span>C<span class="sy3">&</span><span class="sy1">></span> <span class="br0">(</span><span class="sy2">*</span>ap<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// std::bad_cast</span>
<span class="br0">}</span></pre>
<p>
Related Topics: <a href="../keywords/const_cast.html" class="wikilink1" title="keywords:const_cast">const_cast</a>, <a href="../keywords/reinterpret_cast.html" class="wikilink1" title="keywords:reinterpret_cast">reinterpret_cast</a>, <a href="../keywords/static_cast.html" class="wikilink1" title="keywords:static_cast">static_cast</a>, <a href="../keywords/casting_comparison.html" class="wikilink1" title="keywords:casting_comparison">A comparison of the C++ casting operators</a>
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