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<p>Converts between types with different cv-qualification.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="const_cast.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="const_cast.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="const_cast.html#Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="const_cast.html#Keywords"><span class="tocnumber">4</span> <span class="toctext">Keywords</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="const_cast.html#Example"><span class="tocnumber">5</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="const_cast.html#See_also"><span class="tocnumber">6</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/const_cast&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 class="t-sdsc-nopad"> <code><b>const_cast <</b></code> <span class="t-spar">new_type</span> <code><b>> (</b></code> <span class="t-spar">expression</span> <code><b>)</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
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<p>Returns a value of type <code>new_type</code>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/const_cast&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>Only the following conversions can be done with <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">const_cast</span></span></span>. In particular, only <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">const_cast</span></span></span> may be used to cast away (remove) constness or volatility.
</p>
<div class="t-li1"><span class="t-li">1)</span> Two possibly multilevel pointers to the same type may be converted between each other, regardless of cv-qualifiers at each level.</div>
<div class="t-li1"><span class="t-li">2)</span> lvalue of any type <code>T</code> may be converted to a lvalue or rvalue reference to the same type <code>T</code>, more or less cv-qualified. Likewise, an rvalue may be converted to a more or less cv-qualified rvalue reference.</div>
<div class="t-li1"><span class="t-li">3)</span> Same rules apply to possibly multilevel pointers to data members <span class="t-rev-inl t-since-cxx17"><span>and possibly multilevel pointers to arrays of known and unknown bound (arrays to cv-qualified elements are considered to be cv-qualified themselves)</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span></div>
<div class="t-li1"><span class="t-li">4)</span> null pointer value may be converted to the null pointer value of <span class="t-spar">new_type</span></div>
<p>As with all cast expressions, the result is:
</p>
<ul><li> an lvalue if <span class="t-spar">new_type</span> is an lvalue reference type or an rvalue reference to function type;
</li><li> an xvalue if <span class="t-spar">new_type</span> is an rvalue reference to object type;
</li><li> a prvalue otherwise.
</li></ul>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/const_cast&action=edit&section=3" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>Pointers to functions and pointers to member functions are not subject to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">const_cast</span></span></span>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/const_cast&action=edit&section=4" title="Edit section: Keywords">edit</a>]</span> <span class="mw-headline" id="Keywords">Keywords</span></h3>
<p><a href="../keyword/const_cast.html" title="cpp/keyword/const cast"><tt>const_cast</tt></a>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/const_cast&action=edit&section=5" 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="co2">#include <iostream></span>
 
<span class="kw1">struct</span> type <span class="br0">{</span>
type<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span>i<span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="kw4">void</span> m1<span class="br0">(</span><span class="kw4">int</span> v<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
<span class="co1">// this->i = v; // compile error: this is a pointer to const</span>
<span class="kw1">const_cast</span><span class="sy1"><</span>type<span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>this<span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>i <span class="sy1">=</span> v<span class="sy4">;</span> <span class="co1">// OK as long as the type object isn't const</span>
<span class="br0">}</span>
<span class="kw4">int</span> i<span class="sy4">;</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>
<span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span> <span class="co1">// i is not declared const</span>
<span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&</span> cref_i <span class="sy1">=</span> i<span class="sy4">;</span>
<span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span>cref_i<span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">4</span><span class="sy4">;</span> <span class="co1">// OK: modifies i</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"i = "</span> <span class="sy1"><<</span> i <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
type t<span class="sy4">;</span> <span class="co1">// note, if this is const type t;, then t.m1(4); is UB</span>
t.<span class="me1">m1</span><span class="br0">(</span><span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"type::i = "</span> <span class="sy1"><<</span> t.<span class="me1">i</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
<span class="kw4">const</span> <span class="kw4">int</span> j <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span> <span class="co1">// j is declared const</span>
<span class="kw4">int</span><span class="sy2">*</span> pj <span class="sy1">=</span> <span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span><span class="sy3">&</span>j<span class="br0">)</span><span class="sy4">;</span>
<span class="sy2">*</span>pj <span class="sy1">=</span> <span class="nu0">4</span><span class="sy4">;</span> <span class="co1">// undefined behavior!</span>
 
<span class="kw4">void</span> <span class="br0">(</span>type<span class="sy4">::</span><span class="sy2">*</span>mfp<span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="kw4">const</span> <span class="sy1">=</span> <span class="sy3">&</span>type<span class="sy4">::</span><span class="me2">m1</span><span class="sy4">;</span> <span class="co1">// pointer to member function</span>
<span class="co1">// const_cast<void(type::*)(int)>(mfp); // compiler error: const_cast does not</span>
<span class="co1">// work on function pointers</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">i = 4
type::i = 4</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/const_cast&action=edit&section=6" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<ul><li> <a href="static_cast.html" title="cpp/language/static cast">static_cast</a>
</li><li> <a href="dynamic_cast.html" title="cpp/language/dynamic cast">dynamic_cast</a>
</li><li> <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast">reinterpret_cast</a>
</li><li> <a href="explicit_cast.html" title="cpp/language/explicit cast">explicit cast</a>
</li><li> <a href="implicit_cast.html" title="cpp/language/implicit cast">implicit conversions</a>
</li></ul>
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