127 lines
12 KiB
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127 lines
12 KiB
HTML
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<title>explicit 说明符</title>
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<h1 id="firstHeading" class="firstHeading">explicit 说明符</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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<table class="t-sdsc-begin">
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<td><code><b>explicit</b></code></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>explicit (</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><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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<table class="t-par-begin">
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<td><span class="t-spar">表达式</span></td>
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<td>-</td>
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<td><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">经按语境转换为 <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></td>
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<div class="t-li1"><span class="t-li">1)</span> 指定构造函数<span class="t-rev-inl t-since-cxx11"><span>或转换函数</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span><span class="t-rev-inl t-since-cxx17"><span>或<a href="cpp-language-class_template_argument_deduction.html">推导指引</a></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>为显式，即它不能用于<a href="cpp-language-implicit_conversion.html">隐式转换</a>和<a href="cpp-language-copy_initialization.html">复制初始化</a>。</div>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx20">
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<div class="t-li1"><span class="t-li">2)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> 说明符可以与常量表达式一同使用。函数在且只会在该常量表达式求值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 时是显式的。</div>
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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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</table>
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<p>explicit 说明符只能在类定义之内的构造函数<span class="t-rev-inl t-since-cxx11"><span>或转换函数</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>的 <span class="t-spar">声明说明符序列</span> 中出现。</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>声明时不带函数说明符 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> 的<span class="t-rev-inl t-until-cxx11"><span>拥有单个无默认值形参的</span> <span><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></span></span>构造函数被称作<a href="cpp-language-converting_constructor.html">转换构造函数</a>。</p>
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<p>构造函数（除了<a href="cpp-language-copy_constructor.html">复制</a>或<a href="cpp-language-move_constructor.html">移动</a>）和用户定义转换函数都可以是函数模板；<code>explicit</code> 的含义不变。</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>跟随 <code>explicit</code> 的 <code>(</code> 记号被解析成 explicit 指定符的一部分：</p>
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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="kw1">struct</span> S <span class="br0">{</span>
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<span class="kw1">explicit</span> <span class="br0">(</span>S<span class="br0">)</span><span class="br0">(</span><span class="kw4">const</span> S<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// C++20 中错误，C++17 中 OK</span>
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<span class="kw1">explicit</span> <span class="br0">(</span>operator <span class="kw4">int</span><span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// C++20 中错误，C++17 中 OK</span>
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<span class="br0">}</span><span class="sy4">;</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-cxx20">(C++20 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="kw1">struct</span> A
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<span class="br0">{</span>
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A<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="co1">// 转换构造函数</span>
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A<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="co1">// 转换构造函数（C++11）</span>
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operator <span class="kw4">bool</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> B
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<span class="br0">{</span>
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<span class="kw1">explicit</span> B<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
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<span class="kw1">explicit</span> B<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
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<span class="kw1">explicit</span> operator <span class="kw4">bool</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span> <span class="br0">}</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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A a1 <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// OK：复制初始化选择 A::A(int)</span>
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A a2<span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：直接初始化选择 A::A(int)</span>
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A a3 <span class="br0">{</span><span class="nu0">4</span>, <span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK：直接列表初始化选择 A::A(int, int)</span>
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A a4 <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">4</span>, <span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK：复制列表初始化选择 A::A(int, int)</span>
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A a5 <span class="sy1">=</span> <span class="br0">(</span>A<span class="br0">)</span><span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// OK：显式转型进行 static_cast</span>
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<span class="kw1">if</span> <span class="br0">(</span>a1<span class="br0">)</span> <span class="sy4">;</span> <span class="co1">// OK：A::operator bool()</span>
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<span class="kw4">bool</span> na1 <span class="sy1">=</span> a1<span class="sy4">;</span> <span class="co1">// OK：复制初始化选择 A::operator bool()</span>
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<span class="kw4">bool</span> na2 <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">bool</span><span class="sy1">></span><span class="br0">(</span>a1<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：static_cast 进行直接初始化</span>
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<span class="co1">// B b1 = 1; // 错误：复制初始化不考虑 B::B(int)</span>
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B b2<span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：直接初始化选择 B::B(int)</span>
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B b3 <span class="br0">{</span><span class="nu0">4</span>, <span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK：直接列表初始化选择 B::B(int, int)</span>
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<span class="co1">// B b4 = {4, 5}; // 错误：复制列表初始化不考虑 B::B(int,int)</span>
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B b5 <span class="sy1">=</span> <span class="br0">(</span>B<span class="br0">)</span><span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// OK：显式转型进行 static_cast</span>
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<span class="kw1">if</span> <span class="br0">(</span>b2<span class="br0">)</span> <span class="sy4">;</span> <span class="co1">// OK：B::operator bool()</span>
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<span class="co1">// bool nb1 = b2; // 错误：复制初始化不考虑 B::operator bool()</span>
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<span class="kw4">bool</span> nb2 <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">bool</span><span class="sy1">></span><span class="br0">(</span>b2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：static_cast 进行直接初始化</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=".E5.8F.82.E9.98.85">参阅</span></h3>
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<ul>
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<li><a href="cpp-language-converting_constructor.html">转换构造函数</a></li>
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<li><a href="cpp-language-initialization.html">初始化</a></li>
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<li><a href="cpp-language-copy_initialization.html">复制初始化</a></li>
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<li><a href="cpp-language-direct_initialization.html">直接初始化</a></li>
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</ul>
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</div>
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<div class="visualClear"></div>
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