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A comparison of the C++ casting operators [C++ Reference]
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<li class="level2"><div class="li"><span class="li"><a href="#a_comparison_of_the_c_casting_operators" class="toc">A comparison of the C++ casting operators</a></span></div>
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<li class="level3"><div class="li"><span class="li"><a href="#deficiencies_of_the_old_c-style_cast" class="toc">Deficiencies of the old C-style cast</a></span></div></li>
<li class="level3"><div class="li"><span class="li"><a href="#different_operators_for_different_uses" class="toc">Different operators for different uses</a></span></div></li></ul>
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<h2><a name="a_comparison_of_the_c_casting_operators" id="a_comparison_of_the_c_casting_operators">A comparison of the C++ casting operators</a></h2>
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<p>
In addition to the C-style casting operator (provided for backwards compatibility) the C++ standard defines four additional casting operators:
</p>
<ul>
<li class="level1"><div class="li"> <a href="../keywords/static_cast.html" class="wikilink1" title="keywords:static_cast">static_cast</a></div>
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<li class="level1"><div class="li"> <a href="../keywords/const_cast.html" class="wikilink1" title="keywords:const_cast">const_cast</a></div>
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<li class="level1"><div class="li"> <a href="../keywords/dynamic_cast.html" class="wikilink1" title="keywords:dynamic_cast">dynamic_cast</a></div>
</li>
<li class="level1"><div class="li"> <a href="../keywords/reinterpret_cast.html" class="wikilink1" title="keywords:reinterpret_cast">reinterpret_cast</a></div>
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<p>
The purpose of these new casting operators is to provide better type checking. Their use is encouraged over the old C-style casting operator.
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<h3><a name="deficiencies_of_the_old_c-style_cast" id="deficiencies_of_the_old_c-style_cast">Deficiencies of the old C-style cast</a></h3>
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<p>
Two forms of the C-style cast are supported in C++:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="kw4">int</span> age <span class="sy0">=</span> <span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> sqrt<span class="br0">(</span> foo <span class="sy0">/</span> 3.25 <span class="br0">)</span><span class="sy0">;</span>
<span class="kw4">int</span> age <span class="sy0">=</span> <span class="kw4">int</span><span class="br0">(</span> sqrt<span class="br0">(</span>foo<span class="sy0">/</span>3.25 <span class="br0">)</span> <span class="br0">)</span><span class="sy0">;</span></pre>
<p>
However, using the same syntax for a variety of different casting operations can make the intent of the programmer unclear.
</p>
<p>
Furthermore, it can be difficult to find a specific type of cast in a large codebase.
</p>
<p>
Finally, the generality of the C-style cast can be overkill for situations where all that is needed is a simple conversion. The ability to select between several different casting operators of differing degrees of power can prevent programmers from inadvertently casting to an incorrect type.
</p>
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<h3><a name="different_operators_for_different_uses" id="different_operators_for_different_uses">Different operators for different uses</a></h3>
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<p>
The four casting operators in C++ can be used in different cases, where each is most appropriate:
</p>
<p>
<a href="../keywords/static_cast.html" class="wikilink1" title="keywords:static_cast">static_cast</a> is the most useful cast.
It can be used to perform any implicit cast. When an implicit conversion loses some information, some compilers will produce warnings, and static_cast will eliminate these warnings. Making implicit conversion through static_cast is also useful to resolve ambiguity or to clarify the conversion presence.
It also can be used to call a unary constructor, declared as <a href="../keywords/explicit.html" class="wikilink1" title="keywords:explicit">explicit</a>.
It also can be used to cast up and down a class hierarchy, like dynamic_cast, except that no runtime checking is performed.
</p>
<p>
<a href="../keywords/const_cast.html" class="wikilink1" title="keywords:const_cast">const_cast</a> is used to apply or remove <a href="../keywords/const.html" class="wikilink1" title="keywords:const">const</a> or <a href="../keywords/volatile.html" class="wikilink1" title="keywords:volatile">volatile</a> qualifier from a variable.
</p>
<p>
<a href="../keywords/dynamic_cast.html" class="wikilink1" title="keywords:dynamic_cast">dynamic_cast</a> is used on polymorphic pointers or references to move up or down a class hierarchy. Note that dynamic_cast performs runtime-checks: if the object's type is not the one expected, it will return NULL during a pointer-cast and throw a std::bad_cast exception during a reference-cast.
</p>
<p>
<a href="../keywords/reinterpret_cast.html" class="wikilink1" title="keywords:reinterpret_cast">reinterpret_cast</a> is used to perform conversions between unrelated types, like conversion between unrelated pointers and references or conversion between an integer and a pointer.
</p>
<p>
Old-style cast may correspond to static_cast, reinterpret_cast or const_cast, or even a combination of them. This means that none of these casting operators is as powerful as old-style cast.
</p>
<p>
Related links: <a href="http://www.acm.org/crossroads/xrds3-1/ovp3-1.html" class="urlextern" title="http://www.acm.org/crossroads/xrds3-1/ovp3-1.html" rel="nofollow">http://www.acm.org/crossroads/xrds3-1/ovp3-1.html</a>
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