162 lines
16 KiB
HTML
162 lines
16 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>转型运算符</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_cast 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">转型运算符</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>进行显式类型转换。</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"><code><b>(</b></code> <span class="t-spar">类型名</span> <code><b>)</b></code> <span class="t-spar">表达式</span></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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<p>其中</p>
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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">类型名</span></td>
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<td>-</td>
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<td><span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">void</span></span></span> 类型或任何<a href="c-language-types.html#.E7.B1.BB.E5.9E.8B.E7.BB.84.E5.88.AB">标量类型</a></td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">表达式</span></td>
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<td>-</td>
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<td>任何<a href="c-language-type.html#.E7.B1.BB.E5.9E.8B.E7.BB.84.E5.88.AB" class="mw-redirect">标量类型</a><a href="c-language-expressions.html">表达式</a>（除非 <span class="t-spar">类型名</span>是 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">void</span></span></span> ，此情况下它可以是任何表达式）</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>若 <span class="t-spar">类型名</span> 是 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">void</span></span></span> ，则 <span class="t-spar">表达式</span> 为其副效应求值，而其返回值被舍弃，与单独将 <span class="t-spar">表达式</span> 用作<a href="c-language-statements.html#.E8.A1.A8.E8.BE.BE.E5.BC.8F.E8.AF.AD.E5.8F.A5">表达式语句</a>时相同。</p>
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<p>否则，若 <span class="t-spar">类型名</span> 恰是 <span class="t-spar">表达式</span> 的类型，则不做任何事（除非若 <span class="t-spar">表达式</span> 拥有浮点类型，且它表示大于其范围和精度的值——见后述）。</p>
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<p>否则，转换 <span class="t-spar">表达式</span> 的值被到 <span class="t-spar">类型名</span> 所指名的类型，如下：</p>
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<p>允许每种<a href="c-language-conversion.html">如同赋值的隐式转换</a>。</p>
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<p>另外还允许显式转换，允许下列转换规则：</p>
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<ul>
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<li>任何整数能被转型到任何指针类型。除了如同 <span class="t-lc"><a href="c-types-NULL.html">NULL</a></span> 的空指针常量（<a href="c-language-conversion.html">这不需要转型</a>），结果是实现定义的，可能对齐不准确，可能不指向所引用类型的对象，而且可能是一个<a href="c-language-object.html">陷阱表示</a>。</li>
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<li>任何指针类型可以被转型到任何整数类型。结果是实现定义的，即使是对空指针值（它们不必得到零值）。若结果不能被表示成目标类型，则行为未定义（无符号整数在从指针转型的情况下不实现模算术）。</li>
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<li>任何指向对象的指针能被转型成指向任何其他对象类型的指针。若其值未为目标类型正确对齐，则行为未定义。否则，若其值转换回原类型，则它与原值比较相等。若指向对象指针被转型到指向任何字符类型的指针，则结果指向对象的最低字节，且能自增到目标类型的 sizeof （换言之，可用于检验<a href="c-language-object.html">对象表示</a>，或通过 <span class="t-lc"><a href="c-string-byte-memcpy.html">memcpy</a></span> 或 <span class="t-lc"><a href="c-string-byte-memmove.html">memmove</a></span> 复制）。</li>
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<li>任何指向函数的指针能被转型成指向任何其他函数类型的指针。若转换结果指针回原类型，则它与原值比较相等。若将转换所得指针用作函数调用，则行为未定义（除非函数类型<a href="c-language-types.html#.E5.85.BC.E5.AE.B9.E7.B1.BB.E5.9E.8B">兼容</a>）。</li>
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<li>在指针间转换（指向对象或指向函数的）时，若原值是其类型的空指针值，则结果是目标类型的正确空指针值。</li>
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</ul>
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<p>在任何场合（执行隐式转换和同类型转型时）中，若 <span class="t-spar">表达式</span> 与 <span class="t-spar">类型名</span> 是浮点类型，且 <span class="t-spar">表达式</span> 表示大于其类型所指示的范围和精度（见 <span class="t-lc"><a href="c-types-limits-FLT_EVAL_METHOD.html">FLT_EVAL_METHOD</a></span> ），则剥除范围和精度以匹配目标类型。</p>
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<p>转型表达式的<a href="c-language-value_category.html">值类别</a>始终是非左值。</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>因为 <a href="c-language-const.html"><code>const</code></a>、 <a href="c-language-volatile.html"><code>volatile</code></a>、<a href="c-language-restrict.html"><code>restrict</code></a> 及 <a href="c-language-atomic.html"><code>_Atomic</code></a> 限定符仅对<a href="c-language-value_category.html">左值</a>有效，到 cvr 限定或原子类型的转型准确等价于到对应无无限定类型的转型。</p>
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<p>转型到 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">void</span></span></span> 在令编译器关于未使用结果的警告沉默时有用。</p>
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<p>不允许不列于此的转换。特别是，</p>
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<ul>
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<li>没有指针和浮点类型间的转换</li>
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<li>没有指向函数指针和指向对象指针（含 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">void</span><span class="sy2">*</span></span></span> ）间的转换</li>
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</ul>
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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>若实现提供 <span class="t-lc"><a href="c-types-integer.html">intptr_t</a></span> 与/或 <span class="t-lc"><a href="c-types-integer.html">uintptr_t</a></span>，则从指向对象类型（包括 <i>cv</i> <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">void</span></span></span>）的指针到这些类型的转型始终为良定义。然而这对于函数指针并不保证。</p>
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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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<p>注意函数指针与对象指针间的转换被许多编译器作为扩展接受，并且为 <a rel="nofollow" class="external text" href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/dlsym.html">POSIX <code>dlsym()</code> 函数</a>的一些用法所期待。</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">// 检验对象表示是转型的合法使用</span>
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<span class="kw4">double</span> d <span class="sy1">=</span> <span class="nu16">3.14</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"The double %.2f(%a) is: "</span>, d, d<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><a href="c-types-size_t.html"><span class="kw100">size_t</span></a> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> <span class="kw4">sizeof</span> d<span class="sy4">;</span> <span class="sy2">++</span>n<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">"0x%02x "</span>, <span class="br0">(</span><span class="br0">(</span><span class="kw4">unsigned</span> <span class="kw4">char</span><span class="sy2">*</span><span class="br0">)</span><span class="sy3">&</span>d<span class="br0">)</span><span class="br0">[</span>n<span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// edge cases</span>
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<span class="kw1">struct</span> S <span class="br0">{</span><span class="kw4">int</span> x<span class="sy4">;</span><span class="br0">}</span> s<span class="sy4">;</span>
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<span class="co1">// (struct S)s; // 错误；非标量类型</span>
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<span class="co1">// 尽管转型到相同类型什么都不做</span>
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<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>s<span class="sy4">;</span> <span class="co1">// 转换任何类型到 void 都 OK</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">The double 3.14(0x1.91eb851eb851fp+1) is: 0x1f 0x85 0xeb 0x51 0xb8 0x1e 0x09 0x40</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>6.5.4 Cast operators (p: 65-66)</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>6.5.4 Cast operators (p: 91)</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>6.5.4 Cast operators (p: 81)</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>3.3.4 Cast operators</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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