215 lines
15 KiB
HTML
215 lines
15 KiB
HTML
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<title>static_assert 声明</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_static_assert skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading">static_assert 声明</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-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>进行编译时断言检查。</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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<td><code><b>static_assert</b></code> <code><b>(</b></code> <span class="t-spar">布尔常量表达式</span> <code><b>,</b></code> <span class="t-spar">消息</span> <code><b>)</b></code></td>
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<td class="t-sdsc-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<td><code><b>static_assert</b></code> <code><b>(</b></code> <span class="t-spar">布尔常量表达式</span> <code><b>)</b></code></td>
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<td class="t-sdsc-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<table class="t-par-begin">
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<td><span class="t-spar">布尔常量表达式</span></td>
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<td>-</td>
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<table class="t-rev-begin">
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<tr class="t-rev t-until-cxx23">
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<p><a href="cpp-language-constant_expression.html#.E7.BB.8F.E8.BD.AC.E6.8D.A2.E7.9A.84.E5.B8.B8.E9.87.8F.E8.A1.A8.E8.BE.BE.E5.BC.8F">按语境转换成 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 类型的常量表达式</a></p>
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</td>
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<td><span class="t-mark-rev t-until-cxx23">(C++23 前)</span></td>
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<tr class="t-rev t-since-cxx23">
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<p><a href="cpp-language-implicit_conversion.html#.E6.8C.89.E8.AF.AD.E5.A2.83.E8.BD.AC.E6.8D.A2">按语境转换成 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span></a> 的表达式，其中转换为<a href="cpp-language-constant_expression.html">常量表达式</a></p>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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</td>
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<tr class="t-par">
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<td><span class="t-spar">消息</span></td>
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<td>-</td>
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<td>当 <span class="t-spar">布尔常量表达式</span> 为 <code>false</code> 时将出现的<span class="t-rev-inl t-since-cxx17"><span>可选的</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>字符串字面量</td>
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<p><code>static_assert</code> 声明可以出现在命名空间和块作用域中（作为<a href="cpp-language-declarations.html">块声明</a>），也可以在类体中（作为<a href="cpp-language-class.html">成员声明</a>）。</p>
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<p>如果 <span class="t-spar">布尔常量表达式</span> 返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span>，那么该声明没有效果。否则将发布编译时错误，且当存在 <span class="t-spar">消息</span>时诊断消息中会包含其文本。</p>
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<table class="t-rev-begin">
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<p><span class="t-spar">消息</span> 可以省略。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<table class="t-rev-begin">
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<p>除了到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 的非<a href="cpp-language-list_initialization.html#.E7.AA.84.E5.8C.96.E8.BD.AC.E6.8D.A2">窄化</a><a href="cpp-language-implicit_conversion.html#.E6.95.B4.E5.9E.8B.E8.BD.AC.E6.8D.A2">整型转换</a>以外，不允许内建转换。</p>
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</td>
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<td><span class="t-mark-rev t-until-cxx23">(C++23 前)</span></td>
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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 <span class="t-spar">消息</span> 必须是字符串字面量，所以它不能容纳动态信息，乃至自身并非字符串字面量的<a href="cpp-language-constant_expression.html">常量表达式</a>。特别是它不能容纳<a href="cpp-language-template_parameters.html">模板类型实参</a>的<a href="cpp-language-identifiers.html#.E5.90.8D.E5.AD.97">名字</a>。</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-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">#include <type_traits></span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">void</span> swap<span class="br0">(</span>T<span class="sy3">&</span> a, T<span class="sy3">&</span> b<span class="br0">)</span> <span class="kw1">noexcept</span>
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<span class="br0">{</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_copy_constructible.html"><span class="kw532">std::<span class="me2">is_copy_constructible</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span>,
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<span class="st0">"交换需要可复制"</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_copy_constructible.html"><span class="kw536">std::<span class="me2">is_nothrow_copy_constructible</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span>
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<span class="sy3">&&</span> <a href="cpp-types-is_copy_assignable.html"><span class="kw554">std::<span class="me2">is_nothrow_copy_assignable</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span>,
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<span class="st0">"交换需要可复制及可赋值，且无异常抛出"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> c <span class="sy1">=</span> b<span class="sy4">;</span>
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b <span class="sy1">=</span> a<span class="sy4">;</span>
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a <span class="sy1">=</span> c<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw1">struct</span> data_structure
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<span class="br0">{</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_default_constructible.html"><span class="kw526">std::<span class="me2">is_default_constructible</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span>,
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<span class="st0">"数据结构要求元素可默认构造"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> no_copy
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<span class="br0">{</span>
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no_copy <span class="br0">(</span> <span class="kw4">const</span> no_copy<span class="sy3">&</span> <span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span>
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no_copy <span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> no_default
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<span class="br0">{</span>
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no_default <span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</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="kw4">int</span> a, b<span class="sy4">;</span>
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swap<span class="br0">(</span>a, b<span class="br0">)</span><span class="sy4">;</span>
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no_copy nc_a, nc_b<span class="sy4">;</span>
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swap<span class="br0">(</span>nc_a, nc_b<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 1</span>
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data_structure<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> ds_ok<span class="sy4">;</span>
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data_structure<span class="sy1"><</span>no_default<span class="sy1">></span> ds_error<span class="sy4">;</span> <span class="co1">// 2</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">1：错误：静态断言失败：交换需要可复制
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2：错误：静态断言失败：数据结构要求元素可默认构造</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/issue2039">CWG 2039</a></td>
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<td>C++11</td>
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<td>只要求转换前的表达式是常量</td>
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<td>转换本身必须也在常量表达式中合法</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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<td><b><a href="cpp-preprocessor-error.html"><tt>error</tt></a></b></td>
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<td>显示给定的错误信息并使程序非良构</td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-error-assert.html"><span class="t-lines"><span>assert</span></span></a></div>
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</div>
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</td>
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<td>若用户指定的条件非 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 则中断程序。可以在发行构建中禁用<br />
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<span class="t-mark">(宏函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-program-abort.html"><span class="t-lines"><span>abort</span></span></a></div>
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</div>
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</td>
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<td>导致非正常的程序终止（不进行清理）<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-program-exit.html"><span class="t-lines"><span>exit</span></span></a></div>
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</div>
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</td>
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<td>导致正常的程序终止并进行清理<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-types-enable_if.html"><span class="t-lines"><span>enable_if</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>条件性地从重载决议<a href="cpp-language-sfinae.html">移除</a>函数重载或模板特化<br />
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<span class="t-mark">(类模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<td><a href="cpp-types.html#.E7.B1.BB.E5.9E.8B.E7.89.B9.E5.BE.81.28C.2B.2B11_.E8.B5.B7.29"><b>类型特征</b></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td>
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<td>定义了一个编译期基于模板的用来查询或修改类型性质的接口</td>
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</tr>
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</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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