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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>as_const</h1>
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<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-utility.html">utility</a></span></div>
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<div>&#23450;&#20041;&#20110;&#22836;&#25991;&#20214; <code><a href="cpp-header-utility.html">&lt;utility&gt;</a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> <a href="cpp-types-add_cv.html"><span class="kw604">std::<span class="me2">add_const_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span> as_const<span class="br0">(</span>T<span class="sy3">&amp;</span> t<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span><br />
<span class="kw4">void</span> as_const<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&amp;&amp;</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> &#23558;&#24038;&#20540;&#24341;&#29992;&#32452;&#25104; <code>t</code> &#30340; const &#31867;&#22411;</div>
<div class="t-li1"><span class="t-li">2)</span> &#21024;&#38500; const &#21491;&#20540;&#24341;&#29992;&#37325;&#36733;&#65292;&#20197;&#31105;&#27490;&#21491;&#20540;&#21442;&#25968;</div>
<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">&#21487;&#33021;&#30340;&#23454;&#29616;</span></h3>
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<pre class="de1"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw4">constexpr</span> <a href="cpp-types-add_cv.html"><span class="kw604">std::<span class="me2">add_const_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span> as_const<span class="br0">(</span>T<span class="sy3">&amp;</span> t<span class="br0">)</span> <span class="kw1">noexcept</span>
<span class="br0">{</span>
<span class="kw1">return</span> t<span class="sy4">;</span>
<span class="br0">}</span></pre></div>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
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<pre class="de1"><span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;cassert&gt;</span>
<span class="co2">#include &lt;utility&gt;</span>
<span class="co2">#include &lt;type_traits&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a> mutableString <span class="sy1">=</span> <span class="st0">&quot;Hello World!&quot;</span><span class="sy4">;</span>
<span class="kw4">auto</span><span class="sy3">&amp;&amp;</span> constRef <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">as_const</span><span class="br0">(</span>mutableString<span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// mutableString.clear(); // OK</span>
<span class="co1">// constRef.clear(); // &#38169;&#35823;&#65306; &#39;constRef&#39; &#26377; &#39;const&#39; &#38480;&#23450;&#65292;&#20294; &#39;clear&#39; &#19981;&#26631;&#35760;&#20026; const</span>
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span> <span class="sy3">&amp;</span>constRef <span class="sy1">==</span> <span class="sy3">&amp;</span>mutableString <span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span> <span class="sy3">&amp;</span>std<span class="sy4">::</span><span class="me2">as_const</span><span class="br0">(</span> mutableString <span class="br0">)</span> <span class="sy1">==</span> <span class="sy3">&amp;</span>mutableString <span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">using</span> ExprType <span class="sy1">=</span> <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>decltype<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">as_const</span><span class="br0">(</span>mutableString<span class="br0">)</span><span class="br0">)</span><span class="sy1">&gt;</span><span class="sy4">;</span>
static_assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1">&lt;</span><a href="cpp-types-remove_cv.html"><span class="kw598">std::<span class="me2">remove_const_t</span></span></a><span class="sy1">&lt;</span>ExprType<span class="sy1">&gt;</span>, <a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span>,
<span class="st0">&quot;ExprType should be some kind of string.&quot;</span> <span class="br0">)</span><span class="sy4">;</span>
static_assert<span class="br0">(</span><span class="sy3">!</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1">&lt;</span>ExprType, <a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span>,
<span class="st0">&quot;ExprType shouldn&#39;t be a mutable string.&quot;</span> <span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">&#21442;&#38405;</span></h3>
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<div><a href="cpp-types-is_const.html"><span class="t-lines"><span>is_const</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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<td>&#26816;&#26597;&#31867;&#22411;&#26159;&#21542;&#20026; const &#38480;&#23450;<br />
<span class="t-mark">(&#31867;&#27169;&#26495;)</span></td>
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<div><a href="cpp-types-add_cv.html"><span class="t-lines"><span>add_cv</span><span>add_const</span><span>add_volatile</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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<td>&#28155;&#21152; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> &#25110;/&#19982; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">volatile</span></span></span> &#38480;&#23450;&#31526;&#21040;&#32473;&#23450;&#31867;&#22411;<br />
<span class="t-mark">(&#31867;&#27169;&#26495;)</span></td>
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<div><a href="cpp-types-remove_cv.html"><span class="t-lines"><span>remove_cv</span><span>remove_const</span><span>remove_volatile</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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<td>&#20174;&#32473;&#23450;&#31867;&#22411;&#31227;&#38500; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> &#25110;/&#19982; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">volatile</span></span></span> &#38480;&#23450;&#31526;<br />
<span class="t-mark">(&#31867;&#27169;&#26495;)</span></td>
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