157 lines
11 KiB
HTML
157 lines
11 KiB
HTML
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<title>std::basic_string<CharT,Traits,Allocator>::substr</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>substr</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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<div><span class="mw-geshi cpp source-cpp">basic_string substr<span class="br0">(</span> size_type pos <span class="sy1">=</span> <span class="nu0">0</span>, size_type count <span class="sy1">=</span> npos <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> basic_string substr<span class="br0">(</span> size_type pos <span class="sy1">=</span> <span class="nu0">0</span>, size_type count <span class="sy1">=</span> npos <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>返回子串 <code>[pos, pos+count)</code> 。若请求的字串越过字符串的结尾，即 <code>count</code> 大于 <span class="t-c"><span class="mw-geshi cpp source-cpp">size<span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span> pos</span></span> （例如若 <span class="t-c"><span class="mw-geshi cpp source-cpp">count <span class="sy1">==</span> npos</span></span> ），则返回的子串为 <code>[pos, <a href="cpp-string-basic_string-size.html">size()</a>)</code> 。</p>
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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>要包含的首个字符的位置</td>
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</tr>
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<tr class="t-par">
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<td>count</td>
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<td>-</td>
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<td>子串的长度</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>含子串 <code>[pos, pos+count)</code> 或 <code>[pos, <a href="cpp-string-basic_string-size.html">size()</a>)</code> 的 string 。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>若 <code>pos > <a href="cpp-string-basic_string-size.html">size()</a></code> 则为 <span class="t-lc"><a href="cpp-error-out_of_range.html">std::out_of_range</a></span> 。</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>与 <code>count</code> 成线性。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>如同以 <span class="t-c"><span class="mw-geshi cpp source-cpp">basic_string<span class="br0">(</span>data<span class="br0">(</span><span class="br0">)</span><span class="sy2">+</span>pos, count<span class="br0">)</span></span></span> 构造返回的 string ，这隐含将会默认构造返回的 string 的分配器——新分配器将<i>不</i>是 <code>this-><a href="cpp-string-basic_string-get_allocator.html">get_allocator()</a></code> 的副本。</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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <string></span>
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<span class="co2">#include <iostream></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> a <span class="sy1">=</span> <span class="st0">"0123456789abcdefghij"</span><span class="sy4">;</span>
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<span class="co1">// count 为 npos ，返回 [pos, size())</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> sub1 <span class="sy1">=</span> a.<span class="me1">substr</span><span class="br0">(</span><span class="nu0">10</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> sub1 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="co1">// pos 和 pos+count 都在边界内，返回 [pos, pos+count)</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> sub2 <span class="sy1">=</span> a.<span class="me1">substr</span><span class="br0">(</span><span class="nu0">5</span>, <span class="nu0">3</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> sub2 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="co1">// pos 在边界内， pos+count 不在，返回 [pos, size()) </span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> sub4 <span class="sy1">=</span> a.<span class="me1">substr</span><span class="br0">(</span>a.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="nu0">3</span>, <span class="nu0">50</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 这等效于</span>
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<span class="co1">// std::string sub4 = a.substr(17, 3);</span>
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<span class="co1">// 因为 a.size() == 20 ， pos == a.size()-3 == 17 而 a.size()-pos == 3</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> sub4 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw1">try</span> <span class="br0">{</span>
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<span class="co1">// pos 在边界外，抛出</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> sub5 <span class="sy1">=</span> a.<span class="me1">substr</span><span class="br0">(</span>a.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">+</span><span class="nu0">3</span>, <span class="nu0">50</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> sub5 <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> <span class="kw1">catch</span><span class="br0">(</span><span class="kw4">const</span> <a href="cpp-error-out_of_range.html"><span class="kw768">std::<span class="me2">out_of_range</span></span></a><span class="sy3">&</span> e<span class="br0">)</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> <span class="st0">"pos exceeds string size<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</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">abcdefghij
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567
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hij
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pos exceeds string size</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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<div class="t-dsc-member-div">
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<div><a href="cpp-string-basic_string-copy.html"><span class="t-lines"><span>copy</span></span></a></div>
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</div>
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</td>
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<td>复制字符<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-string-basic_string-size.html"><span class="t-lines"><span>size</span><span>length</span></span></a></div>
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</div>
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</td>
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<td>返回字符数<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-string-basic_string-find.html"><span class="t-lines"><span>find</span></span></a></div>
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</div>
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</td>
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<td>于字符串中寻找字符<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-string-basic_string-npos.html"><span class="t-lines"><span>npos</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</span></span></span></div>
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</div>
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</td>
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<td>特殊值。准确含义依赖语境<br />
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<span class="t-mark">(公开静态成员常量)</span></td>
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</tr>
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<div class="visualClear"></div>
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