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<title>std::basic_string<CharT,Traits,Allocator>::operator[]</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::basic_string<CharT,Traits,Allocator>::</span>operator[]</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-string.html">string</a>‎ | <a href="cpp-string-basic_string.html">basic string</a></span></div>
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<td rowspan="142">(1)</td>
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<div><span class="mw-geshi cpp source-cpp">reference operator<span class="br0">[</span><span class="br0">]</span><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 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> reference operator<span class="br0">[</span><span class="br0">]</span><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 起)</span></td>
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<td rowspan="142">(2)</td>
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<div><span class="mw-geshi cpp source-cpp">const_reference operator<span class="br0">[</span><span class="br0">]</span><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 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> const_reference operator<span class="br0">[</span><span class="br0">]</span><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 起)</span></td>
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<p>返回到位于指定位置 <code>pos</code> 的字符的引用。不进行边界检查。若 <span class="t-c"><span class="mw-geshi cpp source-cpp">pos <span class="sy1">></span> size<span class="br0">(</span><span class="br0">)</span></span></span> ，则行为未定义。</p>
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<div class="t-li1"><span class="t-li">1)</span> 若 <span class="t-c"><span class="mw-geshi cpp source-cpp">pos <span class="sy1">==</span> size<span class="br0">(</span><span class="br0">)</span></span></span> ，则行为未定义。</div>
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<div class="t-li1"><span class="t-li">2)</span> 若 <span class="t-c"><span class="mw-geshi cpp source-cpp">pos <span class="sy1">==</span> size<span class="br0">(</span><span class="br0">)</span></span></span> ，则返回到拥有值 <span class="t-c"><span class="mw-geshi cpp source-cpp">CharT<span class="br0">(</span><span class="br0">)</span></span></span> 的字符（空字符）的引用。</div>
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<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
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<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">pos <span class="sy1">==</span> size<span class="br0">(</span><span class="br0">)</span></span></span> ，则返回到拥有值 <span class="t-c"><span class="mw-geshi cpp source-cpp">CharT<span class="br0">(</span><span class="br0">)</span></span></span> 的字符（空字符）的引用。</p>
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<p>对于第一（非 const 版本），若修改此字符为任何异于 <span class="t-c"><span class="mw-geshi cpp source-cpp">CharT<span class="br0">(</span><span class="br0">)</span></span></span> 的值，则行为未定义。</p>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<td>pos</td>
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<td>-</td>
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<td>要返回的字符位置</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>到请求字符的引用。</p>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<p>常数</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <string></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> <span class="kw4">const</span> e<span class="br0">(</span><span class="st0">"Exemplar"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">unsigned</span> i <span class="sy1">=</span> e.<span class="me1">length</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span> <span class="nu0">1</span><span class="sy4">;</span> i <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy2">/</span><span class="sy1">=</span> <span class="nu0">2</span><span class="br0">)</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> e<span class="br0">[</span>i<span class="br0">]</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> c <span class="sy1">=</span> <span class="sy3">&</span>e<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> c <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// 作为 C 字符串打印</span>
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<span class="co1">// 更改 s 的最后字符为 'y'</span>
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<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">"Exemplar "</span><span class="br0">)</span><span class="sy4">;</span>
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s<span class="br0">[</span>s.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">'y'</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> s <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">rmx
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Exemplar
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Exemplary</pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<div class="t-dsc-member-div">
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<div><a href="cpp-string-basic_string-at.html"><span class="t-lines"><span>at</span></span></a></div>
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</div>
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</td>
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<td>访问指定字符，有边界检查<br />
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<span class="t-mark">(公开成员函数)</span></td>
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