182 lines
16 KiB
HTML
182 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>条件包含</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-cpp_preprocessor_conditional 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">条件包含</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-preprocessor.html">preprocessor</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>预处理器支持有条件地编译源文件的某些部分。这一行为由 <code>#if</code>、<code>#else</code>、<code>#elif</code>、<code>#ifdef</code>、<code>#ifndef</code><span class="t-rev-inl t-since-cxx23"><span>、<code>#elifdef</code>、<code>#elifndef</code></span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span> 与 <code>#endif</code> 指令所控制。</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><code><b>#if</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></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>#ifdef</b></code> <span class="t-spar">标识符</span></td>
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<td class="t-sdsc-nopad"></td>
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<td></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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<tr class="t-sdsc">
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<td><code><b>#ifndef</b></code> <span class="t-spar">标识符</span></td>
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<td class="t-sdsc-nopad"></td>
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<td></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>#elif</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></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>#elifdef</b></code> <span class="t-spar">标识符</span></td>
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<td class="t-sdsc-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></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>#elifndef</b></code> <span class="t-spar">标识符</span></td>
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<td class="t-sdsc-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></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>#else</b></code></td>
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<td class="t-sdsc-nopad"></td>
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<td></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>#endif</b></code></td>
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<td class="t-sdsc-nopad"></td>
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<td></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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</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>条件预处理块由 <code>#if</code>、<code>#ifdef</code> 或 <code>#ifndef</code> 指令开始，然后可选地包含任意多个 <code>#elif</code><span class="t-rev-inl t-since-cxx23"><span>、<code>#elifdef</code> 或 <code>#elifndef</code></span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span> 指令，接下来是最多一个可选的 <code>#else</code> 指令，并以 <code>#endif</code> 指令结束。嵌套的条件预处理块会被单独处理。</p>
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<p>各个 <code>#if</code>、<code>#ifdef</code>、<code>#ifndef</code>、<code>#elif</code><span class="t-rev-inl t-since-cxx23"><span>、<code>#elifdef</code>、<code>#elifndef</code></span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span> 和 <code>#else</code> 指令所控制的代码块在第一个不属于内部嵌套的条件预处理块的 <code>#elif</code><span class="t-rev-inl t-since-cxx23"><span>、<code>#elifdef</code>、<code>#elifndef</code></span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span>、<code>#else</code> 或 <code>#endif</code> 指令处结束。</p>
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<p><code>#if</code>、<code>#ifdef</code> 和 <code>#ifndef</code> 指令测试其所指定的条件（见下文），如果条件求值为真，则编译其控制的代码块。此时后续的 <code>#else</code><span class="t-rev-inl t-since-cxx23"><span>、<code>#elifdef</code>、<code>#elifndef</code></span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span> 和 <code>#elif</code> 指令将被忽略。否则，如果所指定的条件求值为假，则跳过其所控制的代码块，然后处理后续的 <code>#else</code><span class="t-rev-inl t-since-cxx23"><span>、<code>#elifdef</code>、<code>#elifndef</code></span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span> 或 <code>#elif</code> 指令（如果存在）。若后续指令为 <code>#else</code>，则 <code>#else</code> 指令所控制的代码块将会无条件地进行编译。否则，<code>#elif</code><span class="t-rev-inl t-since-cxx23"><span>、<code>#elifdef</code> 或 <code>#elifndef</code></span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span> 指令按照与 <code>#if</code> 指令相同的方式执行：即测试条件是否满足，并根据其结果决定编译或跳过其所控制的代码块，并在后一种情况下继续处理后续的 <code>#elif</code><span class="t-rev-inl t-since-cxx23"><span>、<code>#elifdef</code>、<code>#elifndef</code></span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span> 和 <code>#else</code> 指令。条件预处理块以 <code>#endif</code> 指令结束。</p>
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<h3><span class="mw-headline" id=".E6.9D.A1.E4.BB.B6.E7.9A.84.E6.B1.82.E5.80.BC">条件的求值</span></h3>
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<h4><span class="mw-headline" id=".23if.2C_.23elif"><code>#if, #elif</code></span></h4>
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<p><span class="t-spar">表达式</span> 是常量表达式。</p>
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<p>表达式可含有形如“<code><b>defined</b></code> <span class="t-spar">标识符</span>”或“<code><b>defined (</b></code><span class="t-spar">标识符</span><code><b>)</b></code>”的一元运算符。当此 <span class="t-spar">标识符</span> 已经<a href="cpp-preprocessor-replace.html">被定义为宏名</a><span class="t-rev-inl t-since-cxx17"><span>，或者此 <span class="t-spar">标识符</span> 为 <a href="cpp-preprocessor-include.html"><tt>__has_include</tt></a></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>时结果为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span></span></span>，否则结果为 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span>。</p>
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<p>在进行所有宏展开和 <code>defined</code> <span class="t-rev-inl t-since-cxx17"><span>及 <code>__has_include</code></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span> 表达式的求值后，任何非<a href="cpp-language-bool_literal.html">布尔字面量</a>的标识符都被替换成数字 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span>（这包含词法上为关键字的标识符，但不包括如 <code>and</code> 之类的代用记号）。</p>
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<p>当 <span class="t-spar">表达式</span> 求值为非零值时，包含其所控制的代码块，否则跳过该代码块。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx20">
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<td>
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<p>预处理器语境中， <code>__has_cpp_attribute</code> 表达式检测给定属性记号是否受支持及其受支持版本。见<a href="cpp-feature_test.html#.E5.B1.9E.E6.80.A7">属性测试</a>。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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</table>
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<p>注： CWG 1955 前，“<code>#if cond1</code> ... <code>#elif cond2</code>”和“<code>#if cond1</code> ... <code>#else</code>”后面跟着“<code>#if cond3</code>”是不同的，因为当 <code>cond1</code> 为真时第二个 <code>#if</code> 将被跳过，且 <code>cond3</code> 并不需要是良构的，而 #elif 中的 <code>cond2</code> 则必须是合法的表达式。 CWG 1955 开始，引领跳过的代码块的 #elif 也被跳过。</p>
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<h4><span class="mw-headline" id=".E7.BB.84.E5.90.88.E6.8C.87.E4.BB.A4">组合指令</span></h4>
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<p>检查标识符是否<a href="cpp-preprocessor-replace.html">被定义为宏名</a>。</p>
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<p>“<code>#ifdef</code> <span class="t-spar">标识符</span>”与“<code>#if defined</code> <span class="t-spar">标识符</span>”实质上等价。</p>
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<p>“<code>#ifndef</code> <span class="t-spar">标识符</span>”与“<code>#if !defined</code> <span class="t-spar">标识符</span>”实质上等价。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx23">
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<td>
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<p>“<code>#elifdef</code> <span class="t-spar">标识符</span>”与“<code>#elif defined</code> <span class="t-spar">标识符</span>”实质上等价。</p>
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<p>“<code>#elifndef</code> <span class="t-spar">标识符</span>”与“<code>#elif !defined</code> <span class="t-spar">标识符</span>”实质上等价。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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</tr>
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</table>
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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="cpp source-cpp">
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<pre class="de1"><span class="co2">#define ABCD 2</span>
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<span class="co2">#include <iostream></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co2">#ifdef ABCD</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"1: yes<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co2">#else</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"1: no<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co2">#endif</span>
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<span class="co2">#ifndef ABCD</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"2: no1<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co2">#elif ABCD == 2</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"2: yes<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co2">#else</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"2: no2<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co2">#endif</span>
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<span class="co2">#if !defined(DCBA) && (ABCD < 2*4-3)</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"3: yes<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co2">#endif</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">1: yes
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2: yes
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3: yes</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue1955">CWG 1955</a></td>
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<td>C++14</td>
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<td>要求失败的 <code>#elif</code> 中的表达式合法</td>
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<td>跳过失败的 <code>#elif</code></td>
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</tr>
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</table>
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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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