65 lines
8.0 KiB
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65 lines
8.0 KiB
HTML
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<title>转换构造函数</title>
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<h1 id="firstHeading" class="firstHeading">转换构造函数</h1>
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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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<p>不以说明符 <a href="cpp-language-explicit.html">explicit</a> 声明<span class="t-rev-inl t-until-cxx11"><span>且可以用单个参数调用</span> <span><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></span></span>的构造函数被称为<i>转换构造函数（converting constructor）</i>。</p>
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<p>与只在<a href="cpp-language-direct_initialization.html">直接初始化</a>（包括如 <a href="cpp-language-static_cast.html">static_cast</a> 这样的<a href="cpp-language-explicit_cast.html">显式转换</a>）中被考虑的显式构造函数不同，转换构造函数也会作为<a href="cpp-language-implicit_conversion.html">用户定义的转换序列</a>中的一部分而在<a href="cpp-language-copy_initialization.html">复制初始化</a>的考虑范围内。</p>
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<p>通常说法是转换构造函数指定了一个从它的实参类型（如果存在）到它的类类型的隐式转换。注意非显式的<a href="cpp-language-cast_operator.html">用户定义转换函数</a>也指定了一个隐式转换。</p>
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<p>隐式声明的及用户定义的非显式<a href="cpp-language-copy_constructor.html">复制构造函数</a>与<a href="cpp-language-move_constructor.html">移动构造函数</a>也是转换构造函数。</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="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="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="co1">// 转换构造函数 (C++11 起) </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 起)</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="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="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="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，是直接初始化</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：复制列表初始化选择了显式构造函数 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，是直接初始化</span>
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B b6<span class="sy4">;</span> <span class="co1">// OK：默认初始化</span>
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B b7<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK：直接列表初始化</span>
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<span class="co1">// B b8 = {}; // 错误：复制列表初始化选择了显式构造函数 B::B()</span>
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<span class="br0">}</span></pre></div>
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