205 lines
16 KiB
HTML
205 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>初始化</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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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_language_initialization 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">初始化</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>对象<a href="c-language-declarations.html">声明</a>可以通过名为<i>初始化</i>的步骤提供其初始值。</p>
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<p>对于每个<a href="c-language-declarations.html">声明器</a>，若不省略初始化器，则它可以是下列之一：</p>
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<table class="t-sdsc-begin">
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<tr class="t-sdsc">
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<td><code><b>=</b></code> <span class="t-spar">表达式</span></td>
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<td>(1)</td>
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<td class="t-sdsc-nopad"></td>
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<td><code><b>=</b></code> <code><b>{</b></code> <span class="t-spar">初始化器列表</span> <code><b>}</b></code></td>
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<td>(2)</td>
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<td class="t-sdsc-nopad"></td>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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<p>其中 <span class="t-spar">初始化器列表</span> 是非空的逗号分隔 <span class="t-spar">初始化器</span> 列表（尾逗号可选），这里每个初始化器拥有三种可能形式之一：</p>
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<td colspan="10" class="t-sdsc-sep"></td>
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<td><span class="t-spar">表达式</span></td>
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<td>(1)</td>
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<td class="t-sdsc-nopad"></td>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><code><b>{</b></code> <span class="t-spar">初始化器列表</span> <code><b>}</b></code></td>
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<td>(2)</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 class="t-sdsc">
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<td><span class="t-spar">指派符列表</span> <code><b>=</b></code> <span class="t-spar">初始化器</span></td>
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<td>(3)</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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<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-rev-begin">
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<tr class="t-rev t-since-c99">
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<td>
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<p>其中 <span class="t-spar">指派符列表</span> 是形式为 <code><b>[</b></code> <span class="t-spar">常量表达式</span> <code><b>]</b></code> 的数组指派符列表，或形式为 <code><b>.</b></code> <span class="t-spar">标识符</span> 的结构体/联合体指派符列表；见<a href="c-language-array_initialization.html">数组初始化</a>及<a href="c-language-struct_initialization.html">结构体初始化</a>。</p>
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<p>注意：除了初始化器，花括号环绕的 <span class="t-spar">初始化器列表</span> 亦可出现于<a href="c-language-compound_literal.html">复合字面量</a>中，它是有下列形式的表达式：</p>
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<table class="t-sdsc-begin">
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<td colspan="10" class="t-sdsc-sep"></td>
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<td class="t-sdsc-nopad"><code><b>(</b></code> <span class="t-spar">类型</span> <code><b>)</b></code> <code><b>{</b></code> <span class="t-spar">初始化器列表</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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<td colspan="10" class="t-sdsc-sep"></td>
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</table>
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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=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p>初始化器指定存储于一个对象中的初始值。</p>
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<h4><span class="mw-headline" id=".E6.98.BE.E5.BC.8F.E5.88.9D.E5.A7.8B.E5.8C.96">显式初始化</span></h4>
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<p>若提供了初始化器，对于</p>
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<ul>
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<li>标量类型初始化，见<a href="c-language-scalar_initialization.html">标量初始化</a>。</li>
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<li>数组类型初始化，见<a href="c-language-array_initialization.html">数组初始化</a>。</li>
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<li>结构体及联合体类型初始化，见<a href="c-language-struct_initialization.html">结构体初始化</a>。</li>
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</ul>
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<h4><span class="mw-headline" id=".E9.9A.90.E5.BC.8F.E5.88.9D.E5.A7.8B.E5.8C.96">隐式初始化</span></h4>
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<p>若未提供初始化器：</p>
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<ul>
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<li>拥有自动<a href="c-language-storage_duration.html">存储期</a>的对象将被初始化为不确定值（可能是<a href="c-language-object.html">陷阱表示</a>）</li>
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<li>拥有静态及线程局域<a href="c-language-storage_duration.html">存储期</a>的对象被零初始化。</li>
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</ul>
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<h4><span class="mw-headline" id=".E9.9B.B6.E5.88.9D.E5.A7.8B.E5.8C.96">零初始化</span></h4>
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<p>一些情况下，若未显式初始化对象，则零初始化它，即：</p>
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<dl>
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<dd>
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<ul>
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<li>指针被初始化成其类型的空指针值</li>
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<li>整数类型对象被初始化成无符号的零</li>
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<li>浮点类型对象被初始化成正零</li>
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<li>数组的所有元素、结构体的所有成员及联合体的首个成员递归地零初始化，外加初始化所有填充位为零</li>
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</ul>
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</dd>
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<dd>（在空指针值和浮点零拥有全零位表示的平台上，静态对象的这种初始化形式普遍以将其分配到程序映像的 .bss 段实现）</dd>
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</dl>
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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>在初始化静态或线程局域<a href="c-language-storage_duration.html">存储期</a>的对象时，每个初始化器中的 <span class="t-spar">表达式</span> 都必须是<a href="c-language-constant_expression.html">常量表达式</a>或<a href="c-language-string_literal.html">字符串字面量</a>。</p>
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<p>初始化器不能用于不完整类型的对象、 VLA 及拥有链接的块作用域对象。</p>
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<p>函数形参的初值如同用从<a href="c-language-operator_other.html#.E5.87.BD.E6.95.B0.E8.B0.83.E7.94.A8">函数调用</a>实参赋值，而非初始化一样建立。</p>
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<p>若将不确定值用于任何标准库调用的参数，则行为未定义。另外，任意牵涉到不确定值的表达式的值是不确定值（例如 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span> n<span class="sy4">;</span></span></span> ， <span class="t-c"><span class="mw-geshi c source-c">n</span></span> 可能与自身比较不相等，并且它在后续读取中的值可能出现更改）。</p>
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<p>C 标准不使用术语<i>零初始化</i>。该术语采纳自 <a href="cpp-language-zero_initialization.html">C++</a> ，并为方便解释而用于此。</p>
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<p>C 中无对应 C++ 中<a href="cpp-language-value_initialization.html">值初始化</a>的特殊构造，然而能用 <span class="t-c"><span class="mw-geshi c source-c"><span class="sy1">=</span> <span class="br0">{</span><span class="nu0">0</span><span class="br0">}</span></span></span> <span class="t-rev-inl t-since-c99"><span>（或复合字面量中的 <span class="t-c"><span class="mw-geshi c source-c"><span class="br0">(</span>T<span class="br0">)</span><span class="br0">{</span><span class="nu0">0</span><span class="br0">}</span></span></span> ）</span> <span><span class="t-mark-rev t-since-c99">(C99 起)</span></span></span>代替，因为 C 标准不允许空结构体、空联合体或零长度数组。</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 <stdlib.h></span>
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<span class="kw4">int</span> a<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">//初始化a为{0, 0}</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="kw4">int</span> i<span class="sy4">;</span> <span class="co1">// 初始化 i 为不确定值</span>
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<span class="kw4">static</span> <span class="kw4">int</span> j<span class="sy4">;</span> <span class="co1">// 初始化 j 为 0</span>
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<span class="kw4">int</span> k <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// 初始化 k 为 1</span>
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<span class="co1">// 初始化 int x[3] 为 1,3,5</span>
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<span class="co1">// 初始化 int* p 为 &x[0]</span>
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<span class="kw4">int</span> x<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span> <span class="nu0">1</span>, <span class="nu0">3</span>, <span class="nu0">5</span> <span class="br0">}</span>, <span class="sy2">*</span>p <span class="sy1">=</span> x<span class="sy4">;</span>
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<span class="co1">// 初始化 w （二个结构体的数组）为</span>
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<span class="co1">// { { {1,0,0}, 0}, { {2,0,0}, 0} }</span>
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<span class="kw1">struct</span> <span class="br0">{</span><span class="kw4">int</span> a<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span>, b<span class="sy4">;</span><span class="br0">}</span> w<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span>.<span class="me1">a</span> <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span><span class="br0">}</span>, <span class="br0">[</span><span class="nu0">1</span><span class="br0">]</span>.<span class="me1">a</span><span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// 函数调用表达式可用于局部变量初始化</span>
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<span class="kw4">char</span><span class="sy2">*</span> ptr <span class="sy1">=</span> <a href="c-memory-malloc.html"><span class="kw403">malloc</span></a><span class="br0">(</span><span class="nu0">10</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-memory-free.html"><span class="kw406">free</span></a><span class="br0">(</span>ptr<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 错误：拥有静态存储期的对象要求常量初始化器</span>
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<span class="co1">// static char* ptr = malloc(10);</span>
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<span class="co1">// 错误：不能初始化 VLA</span>
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<span class="co1">// int vla[n] = {0};</span>
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<span class="br0">}</span></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-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>6.7.9 Initialization (p: 139-144)</li>
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</ul>
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</dd>
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</dl>
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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>6.7.8 Initialization (p: 125-130)</li>
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</ul>
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</dd>
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</dl>
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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>3.5.7 Initialization</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>
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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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