125 lines
9.6 KiB
HTML
125 lines
9.6 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>标量初始化</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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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_language_scalar_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">标量初始化</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>在<a href="c-language-initialization.html">初始化</a><a href="c-language-types.html#.E7.B1.BB.E5.9E.8B.E7.BB.84.E5.88.AB">标量类型</a>对象时，初始化器必须是单个表达式。</p>
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<p>标量（包含布尔和枚举类型的整数类型，包含复数和虚数的浮点类型，以及包含指向函数指针的指针类型对象）的初始化器必须是单个表达式，可选地以花括号环绕：</p>
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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><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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</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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<tr class="t-sdsc">
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<td><code><b>=</b></code> <code><b>{</b></code> <span class="t-spar">表达式</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>
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</table>
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<p>求值该表达式，而其值在<a href="c-language-conversion.html">如同赋值般转换</a>到对象类型后，成为被初始化对象的初值。</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>因为应用到转换到规则如同赋值，在确定 <span class="t-spar">表达式</span> 所转换到的类型时，忽略被声明类型上的 <a href="c-language-const.html"><code>const</code></a> 与 <a href="c-language-volatile.html"><code>volatile</code></a> 限定符。</p>
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<p>不使用初始化器时应用的规则见<a href="c-language-initialization.html">初始化</a>。</p>
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<p>同所有其他初始化，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， <span class="t-spar">表达式</span> 必须为<a href="c-language-constant_expression.html">常量表达式</a>。</p>
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<p><span class="t-spar">表达式</span> 不能为<a href="c-language-operator_other.html#.E9.80.97.E5.8F.B7.E8.BF.90.E7.AE.97.E7.AC.A6">逗号运算符</a>（除非加括号），因为顶层的逗号会被转译为下个声明器的开始。</p>
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<p>在初始化浮点类型对象时，对所有拥有自动<a href="c-language-storage_duration.html">存储期</a>的对象所作的计算如同在执行时进行，并受<a href="c-numeric-fenv-FE_round.html">当前舍入</a>影响；报告 <a href="c-numeric-math-math_errhandling.html"><tt>math_errhandling</tt></a> 中指定的浮点错误。对于拥有静态和线程局域存储期的对象，计算如同在编译时进行，而且不引发异常：</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">void</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co2">#pragma STDC FENV_ACCESS ON</span>
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<span class="kw4">static</span> <span class="kw4">float</span> v <span class="sy1">=</span> <span class="nu19">1.1e75</span><span class="sy4">;</span> <span class="co1">// 不引发异常：静态初始化</span>
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<span class="kw4">float</span> u<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span> <span class="nu19">1.1e75</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 引发 FE_INEXACT</span>
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<span class="kw4">float</span> w <span class="sy1">=</span> <span class="nu19">1.1e75</span><span class="sy4">;</span> <span class="co1">// 引发 FE_INEXACT</span>
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<span class="kw4">double</span> x <span class="sy1">=</span> <span class="nu19">1.1e75</span><span class="sy4">;</span> <span class="co1">// 可能引发 FE_INEXACT （取决于 FLT_EVAL_METHOD ）</span>
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<span class="kw4">float</span> y <span class="sy1">=</span> 1.1e75f<span class="sy4">;</span> <span class="co1">// 可能引发 FE_INEXACT （取决于 FLT_EVAL_METHOD ）</span>
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<span class="kw4">long</span> <span class="kw4">double</span> z <span class="sy1">=</span> <span class="nu19">1.1e75</span><span class="sy4">;</span> <span class="co1">// 不引发异常（转换是准确的）</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=".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 <stdbool.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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_Bool b <span class="sy1">=</span> <span class="kw2">true</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">double</span> d <span class="sy1">=</span> <span class="nu16">3.14</span><span class="sy4">;</span>
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<span class="kw4">int</span> k <span class="sy1">=</span> <span class="nu16">3.15</span><span class="sy4">;</span> <span class="co1">// 从 double 转换到 int</span>
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<span class="kw4">int</span> n <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">12</span><span class="br0">}</span>, <span class="co1">// 可选地花括号</span>
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<span class="sy2">*</span>p <span class="sy1">=</span> <span class="sy3">&</span>n, <span class="co1">// 对自动对象，非常量表达式 OK</span>
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<span class="br0">(</span><span class="sy2">*</span>fp<span class="br0">)</span><span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="sy1">=</span> main<span class="sy4">;</span>
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<span class="kw2">enum</span> <span class="br0">{</span>RED, BLUE<span class="br0">}</span> e <span class="sy1">=</span> RED<span class="sy4">;</span> <span class="co1">// 枚举亦是标量类型</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>6.7.9/11 Initialization (p: 140)</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>6.7.8/11 Initialization (p: 126)</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>6.5.7 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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</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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