176 lines
11 KiB
HTML
176 lines
11 KiB
HTML
<!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>goto 语句</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_language_goto 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">goto 语句</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-language.html">language</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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<p>将控制无条件转移到所欲位置。</p>
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<p>在无法用约定的构造将控制转移到所欲位置时使用。</p>
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<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<table class="t-sdsc-begin">
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td class="t-sdsc-nopad"><span class="t-spar">属性说明符序列</span><span class="t-mark">(可选)</span> <code><b>goto</b></code> <span class="t-spar">标号</span> <code><b>;</b></code></td>
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<td class="t-sdsc-nopad"></td>
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<td class="t-sdsc-nopad"></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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</table>
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<table class="t-par-begin">
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<tr class="t-par">
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<td><span class="t-spar">标号</span></td>
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<td>-</td>
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<td><code>goto</code> 语句的目标<a href="c-language-statements.html#.E6.A0.87.E5.8F.B7">标号</a></td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">属性说明符序列</span></td>
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<td>-</td>
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<td><span class="t-mark-rev t-since-c23">(C23)</span>可选的<a href="c-language-attributes.html">属性</a>列表，应用到 <code>goto</code> 语句</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p><code>goto</code> 语句导致无条件跳转（控制的转移）到前附具名 <span class="t-spar">标号</span> （必须与 goto 语句出现于同一函数中）的语句<span class="t-rev-inl t-since-c99"><span>，除非此跳转会进入<a href="c-language-array.html">非常量长度数组</a>或另一<a href="c-language-declarations.html">可变修改类型</a>的作用域</span> <span><span class="t-mark-rev t-since-c99">(C99 起)</span></span></span>。</p>
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<p><span class="t-spar">标号</span> 是一个后随冒号（ <code><b>:</b></code> ）<span class="t-rev-inl t-until-c23"><span>和一条语句</span> <span><span class="t-mark-rev t-until-c23">(C23 前)</span></span></span>的标识符。标号是仅有的拥有<i>函数作用域</i>的标识符：能在其所出现于的函数中的任何位置使用它们（在 goto 语句中）。任何语句前可以有多个标号。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-c99">
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<td>
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<p>容许进入非可变修改对象的作用域：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="c source-c">
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<pre class="de1"><span class="kw1">goto</span> lab1<span class="sy4">;</span> <span class="co1">// OK ：进入常规变量的作用域</span>
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<span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">5</span><span class="sy4">;</span>
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lab1<span class="sy4">:;</span> <span class="co1">// 注意未初始化 n ，如同以 int n; 声明</span>
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<span class="co1">// goto lab2; // 错误：进入二个 VM 类型的作用域</span>
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<span class="kw4">double</span> a<span class="br0">[</span>n<span class="br0">]</span><span class="sy4">;</span> <span class="co1">// VLA</span>
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<span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>p<span class="br0">)</span><span class="br0">[</span>n<span class="br0">]</span><span class="sy4">;</span> <span class="co1">// VM 指针</span>
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lab2<span class="sy4">:</span></pre></div>
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</div>
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<p>若 <code>goto</code> 离开 VLA 的作用域，则 VLA 会被解分配（而且可能会被再分配，若再度执行其初始化）：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="c source-c">
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<pre class="de1"><span class="br0">{</span>
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<span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
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label<span class="sy4">:;</span>
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<span class="kw4">int</span> a<span class="br0">[</span>n<span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 重分配 10 次，每次拥有不同的大小</span>
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<span class="kw1">if</span> <span class="br0">(</span>n<span class="sy2">++</span> <span class="sy1"><</span> <span class="nu0">10</span><span class="br0">)</span> <span class="kw1">goto</span> label<span class="sy4">;</span> <span class="co1">// 离开 VM 的作用域</span>
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<span class="br0">}</span></pre></div>
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</div>
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</td>
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<td><span class="t-mark-rev t-since-c99">(C99 起)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
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<p><a href="c-keyword-goto.html"><tt>goto</tt></a></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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<table class="t-rev-begin">
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<tr class="t-rev t-until-c23">
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<td>
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<p>因为声明不是语句，在声明前的标号必须使用空语句（紧随冒号后的分号）。同样的规则适用于块结尾前的标号。</p>
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</td>
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<td><span class="t-mark-rev t-until-c23">(C23 前)</span></td>
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</tr>
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</table>
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<p>C++ 在 <code>goto</code> 语句上加上了另外的限制，不过允许标号在声明（它们在 C++ 中是语句）前。</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="c source-c">
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<pre class="de1"><span class="co2">#include <stdio.h></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co1">// goto 可用于简单地离开多层循环</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> x <span class="sy1"><</span> <span class="nu0">3</span><span class="sy4">;</span> x<span class="sy2">++</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> y <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> y <span class="sy1"><</span> <span class="nu0">3</span><span class="sy4">;</span> y<span class="sy2">++</span><span class="br0">)</span> <span class="br0">{</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"(%d;%d)<span class="es1">\n</span>"</span>,x,y<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>x <span class="sy2">+</span> y <span class="sy1">>=</span> <span class="nu0">3</span><span class="br0">)</span> <span class="kw1">goto</span> endloop<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span>
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endloop<span class="sy4">:;</span>
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<span class="br0">}</span></pre></div>
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</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">(0;0)
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(0;1)
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(0;2)
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(1;0)
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(1;1)
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(1;2)</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
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<div class="t-ref-std-17">
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<ul>
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<li>C17 标准（ISO/IEC 9899:2018）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>6.8.6.1 The goto statement (p: 110-111)</li>
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</ul>
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</dd>
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</dl>
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</div>
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<div class="t-ref-std-11">
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<ul>
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<li>C11 标准（ISO/IEC 9899:2011）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>6.8.6.1 The goto statement (p: 152-153)</li>
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</ul>
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</dd>
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</dl>
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</div>
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<div class="t-ref-std-c99">
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<ul>
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<li>C99 标准（ISO/IEC 9899:1999）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>6.8.6.1 The goto statement (p: 137-138)</li>
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</ul>
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</dd>
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</dl>
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</div>
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<div class="t-ref-std-c89">
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<ul>
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<li>C89/C90 标准（ISO/IEC 9899:1990）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>3.6.6.1 The goto statement</li>
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</ul>
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</dd>
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</dl>
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</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"></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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