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web/cppreference-zh/chmhelp/cpp-string-basic_string-resize.html
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<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::basic_string<CharT,Traits,Allocator>::resize</title>
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_string_basic_string_resize skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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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>resize</h1>
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<div id="bodyContent">
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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 id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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<tbody></tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(1)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-until-cxx20">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> resize<span class="br0">(</span> size_type count <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> <span class="kw4">void</span> resize<span class="br0">(</span> size_type count <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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</tbody>
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<tbody></tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(2)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-until-cxx20">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> resize<span class="br0">(</span> size_type count, CharT ch <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> <span class="kw4">void</span> resize<span class="br0">(</span> size_type count, CharT ch <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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</tbody>
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<tbody>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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</tr>
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</tbody>
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</table>
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<p>重设 string 大小以含 <code>count</code> 个字符。</p>
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<p>若当前大小小于 <code>count</code> ，则后附额外的字符。</p>
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<p>若当前大小大于 <code>count</code> ，则缩减 string 到为其首 <code>count</code> 个元素。</p>
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<p>第一版本初始化新字符为 <span class="t-c"><span class="mw-geshi cpp source-cpp">CharT<span class="br0">(</span><span class="br0">)</span></span></span> ，第二版本初始化新字符为 <code>ch</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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<tr class="t-par">
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<td>count</td>
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<td>-</td>
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<td>string 的新大小</td>
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</tr>
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<tr class="t-par">
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<td>ch</td>
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<td>-</td>
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<td>用以初始化新字符的字符</td>
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</tr>
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</table>
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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>（无）</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 <span class="t-c"><span class="mw-geshi cpp source-cpp">count <span class="sy1">></span> max_size<span class="br0">(</span><span class="br0">)</span></span></span> 则为 <span class="t-lc"><a href="cpp-error-length_error.html">std::length_error</a></span> 。对应 <code>Allocator</code> 所抛的任何异常。</p>
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<p><span class="t-rev-inl t-since-cxx11"><span>若因任何原因抛出异常，则此函数无效果（强异常保证）。</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span></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 <stdexcept></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-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Basic functionality:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">unsigned</span> desired_length<span class="br0">(</span><span class="nu0">8</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> long_string<span class="br0">(</span> <span class="st0">"Where is the end?"</span> <span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> short_string<span class="br0">(</span> <span class="st0">"Ha"</span> <span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 缩短</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">"Before: <span class="es1">\"</span>"</span> <span class="sy1"><<</span> long_string <span class="sy1"><<</span> <span class="st0">"<span class="es1">\"</span><span class="es1">\n</span>"</span><span class="sy4">;</span>
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long_string.<span class="me1">resize</span><span class="br0">(</span> desired_length <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">"After: <span class="es1">\"</span>"</span> <span class="sy1"><<</span> long_string <span class="sy1"><<</span> <span class="st0">"<span class="es1">\"</span><span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co1">// 加长</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">"Before: <span class="es1">\"</span>"</span> <span class="sy1"><<</span> short_string <span class="sy1"><<</span> <span class="st0">"<span class="es1">\"</span><span class="es1">\n</span>"</span><span class="sy4">;</span>
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short_string.<span class="me1">resize</span><span class="br0">(</span> desired_length, <span class="st0">'a'</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> <span class="st0">"After: <span class="es1">\"</span>"</span> <span class="sy1"><<</span> short_string <span class="sy1"><<</span> <span class="st0">"<span class="es1">\"</span><span class="es1">\n</span>"</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>Errors:<span class="es1">\n</span>"</span><span class="sy4">;</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> s<span class="sy4">;</span>
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<span class="kw1">try</span> <span class="br0">{</span>
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<span class="co1">// 大小 OK ，无 length_error</span>
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<span class="co1">// （可能抛 bad_alloc ）</span>
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s.<span class="me1">resize</span><span class="br0">(</span>s.<span class="me1">max_size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span> <span class="nu0">1</span>, <span class="st0">'x'</span><span class="br0">)</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-memory-new-bad_alloc.html"><span class="kw693">std::<span class="me2">bad_alloc</span></span></a><span class="sy3">&</span><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">"1. bad alloc<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">try</span> <span class="br0">{</span>
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<span class="co1">// 大小 OK ，无 length_error</span>
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<span class="co1">// （可能抛 bad_alloc ）</span>
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s.<span class="me1">resize</span><span class="br0">(</span>s.<span class="me1">max_size</span><span class="br0">(</span><span class="br0">)</span>, <span class="st0">'x'</span><span class="br0">)</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-memory-new-bad_alloc.html"><span class="kw693">std::<span class="me2">bad_alloc</span></span></a><span class="sy3">&</span> exc<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">"2. bad alloc<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">try</span> <span class="br0">{</span>
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<span class="co1">// 大小错误，抛出 length_error</span>
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s.<span class="me1">resize</span><span class="br0">(</span>s.<span class="me1">max_size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">+</span> <span class="nu0">1</span>, <span class="st0">'x'</span><span class="br0">)</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-length_error.html"><span class="kw767">std::<span class="me2">length_error</span></span></a><span class="sy3">&</span><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">"3. length error<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>
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<span class="br0">}</span></pre></div>
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</div>
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<p>可能的输出：</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">Basic functionality:
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Before: "Where is the end?"
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After: "Where is"
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Before: "Ha"
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After: "Haaaaaaa"
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Errors:
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1. bad alloc
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2. bad alloc
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3. length error</pre></div>
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</div>
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</div>
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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 string 大小成线性。</p>
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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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<tr class="t-dsc">
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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>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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</html>
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