236 lines
27 KiB
HTML
236 lines
27 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>作用域</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_scope 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">作用域</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>C 程序中出现的每个<a href="c-language-identifier.html">标识符</a>都仅在一些可能不连续的部分<i>可见</i>（即可使用），这些部分被称为其<i>作用域</i>。</p>
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<p>在作用域内，标识符仅若在不同<a href="c-language-name_space.html">命名空间</a>中，才可以指代多于一个实体。</p>
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<p>C 拥有四种作用域：</p>
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<dl>
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<dd>
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<ul>
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<li>块作用域</li>
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<li>文件作用域</li>
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<li>函数作用域</li>
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<li>函数原型作用域</li>
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</ul>
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</dd>
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</dl>
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<h3><span class="mw-headline" id=".E5.B5.8C.E5.A5.97.E4.BD.9C.E7.94.A8.E5.9F.9F">嵌套作用域</span></h3>
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<p>若相同标识符所命名的二个不同实体在同一时刻都在作用域中，且它们属于同一<a href="c-language-name_space.html">命名空间</a>，则作用域被嵌套（不允许其他形式的作用域重叠），而内层作用域中的声明隐藏外层作用域中的声明：</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="co1">// 此处的命名空间为通常标识符。</span>
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<span class="kw4">int</span> a<span class="sy4">;</span> <span class="co1">// 名称 a 的文件作用域始于此</span>
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<span class="kw4">void</span> f<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> a <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// 名称 a 的块作用域始于此；隐藏文件作用域的 a</span>
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<span class="br0">{</span>
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<span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="co1">// 内层 a 的作用域始于此，隐藏外层 a </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<span class="es1">\n</span>"</span>, a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 内层 a 在作用域中，打印 2</span>
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<span class="br0">}</span> <span class="co1">// 内层 a 的块作用域终于此</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<span class="es1">\n</span>"</span>, a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 外层 a 在作用域中，打印 1</span>
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<span class="br0">}</span> <span class="co1">// 外层 a 的作用域终于此</span>
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<span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">int</span> a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 名称 a 拥有函数原型作用域；隐藏文件作用域的 a</span></pre></div>
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</div>
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<h3><span class="mw-headline" id=".E5.9D.97.E4.BD.9C.E7.94.A8.E5.9F.9F">块作用域</span></h3>
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<p>任何在<a href="c-language-statements.html#.E5.A4.8D.E5.90.88.E8.AF.AD.E5.8F.A5">复合语句</a>，包含函数体<span class="t-rev-inl t-since-c99"><span>或出现于 <a href="c-language-if.html">if</a> 、 <a href="c-language-switch.html">switch</a> 、 <a href="c-language-for.html">for</a> 、 <a href="c-language-while.html">while</a> 或 <a href="c-language-do.html">do-while</a> 语句中的任何表达式、声明或语句</span> <span><span class="t-mark-rev t-since-c99">(C99 起)</span></span></span>，或在<a href="c-language-function_definition.html">函数定义</a>内的参数列表中声明的标识符的作用域，在声明点开始，在声明于其中的块或语句的结尾结束。</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="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span> <span class="co1">// 函数参数 'n' 的作用域开始</span>
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<span class="br0">{</span> <span class="co1">// 函数体开始</span>
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<span class="sy2">++</span>n<span class="sy4">;</span> <span class="co1">// 'n' 在作用域中并指代函数参数</span>
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<span class="co1">// int n = 2; // 错误：不能在同一作用域重声明标识符</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n<span class="sy1"><</span><span class="nu0">10</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span> <span class="br0">{</span> <span class="co1">// 循环局域的 'n' 的作用域开始</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<span class="es1">\n</span>"</span>, n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印 0 1 2 3 4 5 6 7 8 9</span>
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<span class="br0">}</span> <span class="co1">// 循环局域的 'n' 的作用域结束</span>
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<span class="co1">// 函数参数 'n' 回到作用域</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<span class="es1">\n</span>"</span>, n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印参数的值</span>
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<span class="br0">}</span> <span class="co1">// 函数参数 'n' 的作用域结束</span>
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<span class="kw4">int</span> a <span class="sy1">=</span> n<span class="sy4">;</span> <span class="co1">// 错误：名称 'n' 不在作用域中</span></pre></div>
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</div>
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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>C99 前，选择和迭代语句不建立其自身的块作用域（尽管若在语句中使用复合语句，则它拥有其通常的块作用域）：</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="kw2">enum</span> <span class="br0">{</span>a, b<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">int</span> different<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="kw1">if</span> <span class="br0">(</span><span class="kw4">sizeof</span><span class="br0">(</span><span class="kw2">enum</span> <span class="br0">{</span>b, a<span class="br0">}</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> <span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span>
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<span class="kw1">return</span> a<span class="sy4">;</span> <span class="co1">// a == 1</span>
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<span class="kw1">return</span> b<span class="sy4">;</span> <span class="co1">// C89 中 b == 0 ， C99 中 b == 1</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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<p>块作用域对象默认<a href="c-language-storage_duration.html">无链接</a>并拥有<a href="c-language-storage_duration.html">自动存储期</a>。注意非 VLA 局部对象的存储期在进入块时开始，但在见到声明前，该标识符不在作用域中且不能访问。</p>
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<h3><span class="mw-headline" id=".E6.96.87.E4.BB.B6.E4.BD.9C.E7.94.A8.E5.9F.9F">文件作用域</span></h3>
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<p>在任何块或参数列表外声明的任何标识符的作用域，在声明点开始，翻译单元尾结束。</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="kw4">int</span> i<span class="sy4">;</span> <span class="co1">// i 的作用域开始</span>
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<span class="kw4">static</span> <span class="kw4">int</span> g<span class="br0">(</span><span class="kw4">int</span> a<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> a<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// g 的作用域开始（注意 "a" 拥有块作用域）</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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i <span class="sy1">=</span> g<span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// i 和 g 在作用域中</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>文件作用域的标识符默认拥有<a href="c-language-storage_duration.html">外部链接</a>和<a href="c-language-storage_duration.html">静态存储期</a>。</p>
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<h3><span class="mw-headline" id=".E5.87.BD.E6.95.B0.E4.BD.9C.E7.94.A8.E5.9F.9F">函数作用域</span></h3>
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<p>声明于函数内部的<a href="c-language-statements.html#Labels">标号（且只有标号）</a>，在该函数中的所有位置（所有嵌套块中，其自身声明前后）都在作用域内。注意：任何语句前的冒号字符前的标识符，若不用于其他用途，则隐式声明一个标号。</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="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="br0">{</span>
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<span class="kw1">goto</span> label<span class="sy4">;</span> <span class="co1">// label 在作用域中，尽管之后才声明</span>
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label<span class="sy4">:;</span>
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<span class="br0">}</span>
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<span class="kw1">goto</span> label<span class="sy4">;</span> <span class="co1">// 标号忽略块作用域</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">goto</span> label<span class="sy4">;</span> <span class="co1">// 错误： g() 中 label 不在作用域中</span>
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<span class="br0">}</span></pre></div>
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</div>
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<h3><span class="mw-headline" id=".E5.87.BD.E6.95.B0.E5.8E.9F.E5.9E.8B.E4.BD.9C.E7.94.A8.E5.9F.9F">函数原型作用域</span></h3>
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<p>非函数定义的<a href="c-language-function_declaration.html">函数声明</a>的参数列表中引入的名称的作用域，在函数<a href="c-language-declarations.html">声明器</a>的结尾结束。</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="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> n,
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<span class="kw4">int</span> a<span class="br0">[</span>n<span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// n 在作用域中并指代第一参数</span></pre></div>
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</div>
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<p>注意，若声明中有多个或嵌套声明器，则作用域在最近的外围函数声明器的结尾结束：</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="kw4">void</span> f <span class="br0">(</span> <span class="co1">// 函数名 'f' 在文件作用域</span>
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<span class="kw4">long</span> <span class="kw4">double</span> f, <span class="co1">// 标识符 'f' 现在在作用域中，隐藏文件作用域的 'f'</span>
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<span class="kw4">char</span> <span class="br0">(</span><span class="sy2">**</span>a<span class="br0">)</span><span class="br0">[</span><span class="nu0">10</span> <span class="sy2">*</span> <span class="kw4">sizeof</span> f<span class="br0">]</span> <span class="co1">// 'f' 指代第一参数，它在作用域中</span>
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<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw2">enum</span><span class="br0">{</span> n <span class="sy1">=</span> <span class="nu0">3</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span><span class="br0">(</span><span class="sy2">*</span>g<span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span><span class="br0">)</span><span class="br0">[</span>n<span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 函数参数 'n' 的作用域在其函数声明器的结尾结束</span>
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<span class="co1">// 数组声明器中，全局 n 在作用域</span>
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<span class="co1">// （这声明指向返回 3 个 int 的数组的指针的函数的指针）</span></pre></div>
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</div>
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<h3><span class="mw-headline" id=".E5.A3.B0.E6.98.8E.E7.82.B9">声明点</span></h3>
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<p>结构体、联合体及枚举标签的作用域，在声明该标签的类型指定符中的标签出现后立即开始。</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">struct</span> Node <span class="br0">{</span>
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<span class="kw1">struct</span> Node<span class="sy2">*</span> next<span class="sy4">;</span> <span class="co1">// Node 在作用域中并指代此 struct</span>
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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<p>枚举常量的作用域，在枚举项列表中其定义枚举项的出现后立即开始。</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="kw2">enum</span> <span class="br0">{</span> x <span class="sy1">=</span> <span class="nu0">12</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">{</span>
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<span class="kw2">enum</span> <span class="br0">{</span> x <span class="sy1">=</span> x <span class="sy2">+</span> <span class="nu0">1</span>, <span class="co1">// 新 x 在逗号前不在作用域中，初始化 x 为 13</span>
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y <span class="sy1">=</span> x <span class="sy2">+</span> <span class="nu0">1</span> <span class="co1">// 新枚举项 x 现在在作用域中，初始化 y 为 14</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>任何其他标识符的作用域，正好在其声明器结束后和初始化器前开始，若存在初始化器：</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="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="co1">// 第一个 'x' 的作用域开始</span>
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<span class="br0">{</span>
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<span class="kw4">int</span> x<span class="br0">[</span>x<span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 新声明的 x 的作用域在声明器 （ x[x] ）后开始。</span>
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<span class="co1">// 在声明器内，外层 'x' 仍在作用域中。</span>
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<span class="co1">// 这声明 2 个 int 的 VLA 数组。</span>
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<span class="br0">}</span>
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<span class="kw4">unsigned</span> <span class="kw4">char</span> y <span class="sy1">=</span> <span class="nu0">32</span><span class="sy4">;</span> <span class="co1">// 外层 'y' 的作用域开始</span>
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<span class="br0">{</span>
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<span class="kw4">unsigned</span> <span class="kw4">char</span> y <span class="sy1">=</span> y<span class="sy4">;</span>
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<span class="co1">// 内层 'y' 的作用域在初始化器（ = y ）前开始</span>
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<span class="co1">// 这不会以值 32 初始化内层 'y' ，</span>
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<span class="co1">// 这以其自身的不确定值初始化内层 'y'</span>
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<span class="br0">}</span>
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<span class="kw4">unsigned</span> <span class="kw4">long</span> factorial<span class="br0">(</span><span class="kw4">unsigned</span> <span class="kw4">long</span> n<span class="br0">)</span>
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<span class="co1">// 声明器结束， 'factorial' 从此点开始在作用域中</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> n <span class="sy1"><</span> <span class="nu0">2</span> <span class="sy4">?</span> <span class="nu0">1</span> <span class="sy4">:</span> n <span class="sy2">*</span> factorial<span class="br0">(</span>n <span class="sy2">-</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 递归调用</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>作为特殊情况，非标识符声明的<a href="c-language-types.html">类型名</a>的作用域，被认为正好假如在类型名内未省略标识符，则标识符会出现的位置之后开始。</p>
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|
<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
|
|
<p>C89 前，拥有外部链接的标识符在块中引入时，拥有文件作用域，因此不要求 C89 编译器诊断已离开作用域的 extern 标识符的使用（这种使用是未定义行为）。</p>
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|
<p>C 中，循环体内的局部对象，能隐藏声明于 <a href="c-language-for.html">for</a> 循环的初始化子句中的对象（其作用域为嵌套的），但 C++ 中不能如此。</p>
|
|
<p>不同于 C++ ， C 无结构体作用域：声明于 struct/union/enum 声明内的名称在结构体声明所在的相同作用域（除了数据成员在其<a href="c-language-name_space.html">成员命名空间</a>中）：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="c source-c">
|
|
<pre class="de1"><span class="kw1">struct</span> foo <span class="br0">{</span>
|
|
<span class="kw1">struct</span> baz <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="kw2">enum</span> color <span class="br0">{</span>RED, BLUE<span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="kw1">struct</span> baz b<span class="sy4">;</span> <span class="co1">// baz 在作用域中</span>
|
|
<span class="kw2">enum</span> color x <span class="sy1">=</span> RED<span class="sy4">;</span> <span class="co1">// color 和 RED 在作用域中</span></pre></div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
|
|
<div class="t-ref-std-11">
|
|
<ul>
|
|
<li>C11 标准（ISO/IEC 9899:2011）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>6.2.1 Scopes of identifiers (p: 35-36)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<div class="t-ref-std-c99">
|
|
<ul>
|
|
<li>C99 标准（ISO/IEC 9899:1999）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>6.2.1 Scopes of identifiers (p: 29-30)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<div class="t-ref-std-c89">
|
|
<ul>
|
|
<li>C89/C90 标准（ISO/IEC 9899:1990）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>3.1.2.1 Scopes of identifiers</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
<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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</div>
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</body>
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</html>
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