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<title>std::basic_string&lt;CharT,Traits,Allocator&gt;::at</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::basic_string&lt;CharT,Traits,Allocator&gt;::</span>at</h1>
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<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-string.html">string</a>&lrm; | <a href="cpp-string-basic_string.html">basic string</a></span></div>
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<div><span class="mw-geshi cpp source-cpp">reference &nbsp; &nbsp; &nbsp; at<span class="br0">(</span> size_type pos <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 &#21069;)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> reference &nbsp; &nbsp; &nbsp; at<span class="br0">(</span> size_type pos <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span></td>
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<div><span class="mw-geshi cpp source-cpp">const_reference at<span class="br0">(</span> size_type pos <span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 &#21069;)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> const_reference at<span class="br0">(</span> size_type pos <span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span></td>
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<p>&#36820;&#22238;&#21040;&#20301;&#20110;&#25351;&#23450;&#20301;&#32622; <code>pos</code> &#30340;&#23383;&#31526;&#30340;&#24341;&#29992;&#12290;&#36827;&#34892;&#36793;&#30028;&#26816;&#26597;&#65292;&#38750;&#27861;&#35775;&#38382;&#26102;&#25243;&#20986; <span class="t-lc"><a href="cpp-error-out_of_range.html">std::out_of_range</a></span> &#31867;&#22411;&#30340;&#24322;&#24120;&#12290;</p>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">&#21442;&#25968;</span></h3>
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<td>pos</td>
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<td>&#35201;&#36820;&#22238;&#30340;&#23383;&#31526;&#20301;&#32622;</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">&#36820;&#22238;&#20540;</span></h3>
<p>&#21040;&#35831;&#27714;&#30340;&#23383;&#31526;&#30340;&#24341;&#29992;&#12290;</p>
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">&#24322;&#24120;</span></h3>
<p>&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp">pos <span class="sy1">&gt;=</span> size<span class="br0">(</span><span class="br0">)</span></span></span> &#21017;&#25243;&#20986; <span class="t-lc"><a href="cpp-error-out_of_range.html">std::out_of_range</a></span> &#12290;</p>
<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">&#22797;&#26434;&#24230;</span></h3>
<p>&#24120;&#25968;&#12290;</p>
<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;stdexcept&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&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="kw1230">std::<span class="me2">string</span></span></a> s<span class="br0">(</span><span class="st0">&quot;message&quot;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// &#20026;&#23481;&#37327;</span>
s <span class="sy1">=</span> <span class="st0">&quot;abc&quot;</span><span class="sy4">;</span>
s.<span class="me1">at</span><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span> <span class="sy1">=</span> <span class="st0">&#39;x&#39;</span><span class="sy4">;</span> <span class="co1">// ok</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;string size = &quot;</span> <span class="sy1">&lt;&lt;</span> s.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;string capacity = &quot;</span> <span class="sy1">&lt;&lt;</span> s.<span class="me1">capacity</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="kw1">try</span> <span class="br0">{</span>
<span class="co1">// &#25243;&#20986;&#65292;&#21363;&#20351;&#23481;&#37327;&#20801;&#35768;&#35775;&#38382;&#20803;&#32032;</span>
s.<span class="me1">at</span><span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span> <span class="sy1">=</span> <span class="st0">&#39;x&#39;</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">catch</span> <span class="br0">(</span><a href="cpp-error-out_of_range.html"><span class="kw768">std::<span class="me2">out_of_range</span></span></a> <span class="kw4">const</span><span class="sy3">&amp;</span> exc<span class="br0">)</span> <span class="br0">{</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> exc.<span class="me1">what</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span></pre></div>
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<p>&#21487;&#33021;&#30340;&#36755;&#20986;&#65306;</p>
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<pre class="de1">abx
string size = 3
string capacity = 7
basic_string::at</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-string-basic_string-operator_at.html"><span class="t-lines"><span>operator[]</span></span></a></div>
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<td>&#35775;&#38382;&#25351;&#23450;&#23383;&#31526;<br />
<span class="t-mark">(&#20844;&#24320;&#25104;&#21592;&#20989;&#25968;)</span></td>
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