110 lines
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110 lines
12 KiB
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<title>const_cast 转换</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_const_cast skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">const_cast 转换</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>在有不同 cv 限定的类型间转换。</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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<td class="t-sdsc-nopad"><code><b>const_cast</b></code> <code><b><</b></code> <span class="t-spar">新类型</span> <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 class="t-sdsc-nopad"></td>
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<td class="t-sdsc-nopad"></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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<p>返回 <span class="t-spar">新类型</span> 类型的值。</p>
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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>只有下列转换能用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">const_cast</span></span></span> 进行。尤其是只有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">const_cast</span></span></span> 可用于移除常量性或易变性。</p>
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<div class="t-li1"><span class="t-li">1)</span> 两个指向同一类型的可能多级的指针可以互相转换，不受每个层级的 cv 限定符影响。</div>
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<div class="t-li1"><span class="t-li">2)</span> 任何 <code>T</code> 类型的左值可转换为到同一类型 <code>T</code> 的左值或右值引用，cv 限定可更多或更少。同样地，类类型纯右值或任何类型的亡值可转换成具有更多或更少 cv 限定的右值引用。引用 <code>const_cast</code> 的结果<span class="t-rev-inl t-until-cxx17"><span>指代原对象</span> <span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span><span class="t-rev-inl t-since-cxx17"><span>在 <span class="t-spar">表达式</span> 是泛左值时指代原对象，否则指代<a href="cpp-language-implicit_conversion.html#.E4.B8.B4.E6.97.B6.E9.87.8F.E5.AE.9E.E8.B4.A8.E5.8C.96">实质化的临时量</a></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>。</div>
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<div class="t-li1"><span class="t-li">3)</span> 同样的规则适用于可能多层的到数据成员的指针<span class="t-rev-inl t-since-cxx17"><span>，及可能多层的到已知和未知边界数组（cv 限定元素的数组自身也被认为有 cv 限定）</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>。</div>
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<div class="t-li1"><span class="t-li">4)</span> 空指针值可转换成 <span class="t-spar">新类型</span> 的空指针值。</div>
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<p>同所有转型表达式，结果是：</p>
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<ul>
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<li>左值，如果 <span class="t-spar">新类型</span> 是左值引用或到函数类型的右值引用；</li>
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<li>亡值，如果 <span class="t-spar">新类型</span> 是到对象类型的右值引用；</li>
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<li>否则为纯右值。</li>
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</ul>
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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>函数指针和成员函数指针不可用于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">const_cast</span></span></span></p>
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<p><code>const_cast</code> 使得到非 const 类型的引用或指针能够实际指代 <a href="cpp-language-cv.html">const 对象</a>，或到非 volatile 类型的引用或指针能够实际指代 <a href="cpp-language-cv.html">volatile 对象</a>。通过非 const 访问路径修改 const 对象和通过非 volatile <a href="cpp-language-value_category.html#.E6.B3.9B.E5.B7.A6.E5.80.BC">泛左值</a>涉指 volatile 对象是未定义行为。</p>
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<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
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<p><a href="cpp-keyword-const_cast.html"><tt>const_cast</tt></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">
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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 <iostream></span>
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<span class="kw1">struct</span> type <span class="br0">{</span>
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<span class="kw4">int</span> i<span class="sy4">;</span>
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type<span class="br0">(</span><span class="br0">)</span><span class="sy4">:</span> i<span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> v<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
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<span class="co1">// this->i = v; // 编译错误：this 是指向 const 的指针</span>
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<span class="kw1">const_cast</span><span class="sy1"><</span>type<span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>this<span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>i <span class="sy1">=</span> v<span class="sy4">;</span> <span class="co1">// 只要该对象不是 const 就 OK</span>
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<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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<span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span> <span class="co1">// 不声明 i 为 const</span>
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<span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&</span> rci <span class="sy1">=</span> i<span class="sy4">;</span>
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<span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span>rci<span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">4</span><span class="sy4">;</span> <span class="co1">// OK：修改 i</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">"i = "</span> <span class="sy1"><<</span> i <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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type t<span class="sy4">;</span> <span class="co1">// 假如这是 const type t，则 t.f(4) 会是未定义行为</span>
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t.<span class="me1">f</span><span class="br0">(</span><span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</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">"type::i = "</span> <span class="sy1"><<</span> t.<span class="me1">i</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">int</span> j <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span> <span class="co1">// 声明 j 为 const</span>
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<span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span>
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<span class="kw4">int</span><span class="sy2">*</span> pj <span class="sy1">=</span> <span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span><span class="sy3">&</span>j<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// *pj = 4; // 未定义行为</span>
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<span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span>
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<span class="kw4">void</span> <span class="br0">(</span>type<span class="sy4">::</span><span class="sy2">*</span> pmf<span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="kw4">const</span> <span class="sy1">=</span> <span class="sy3">&</span>type<span class="sy4">::</span><span class="me2">f</span><span class="sy4">;</span> <span class="co1">// 指向成员函数的指针</span>
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<span class="co1">// const_cast<void(type::*)(int)>(pmf); // 编译错误：const_cast 不能用于成员函数指针</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">i = 4
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type::i = 4</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-static_cast.html"><code>static_cast</code></a></li>
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<li><a href="cpp-language-dynamic_cast.html"><code>dynamic_cast</code></a></li>
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<li><a href="cpp-language-reinterpret_cast.html"><code>reinterpret_cast</code></a></li>
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<li><a href="cpp-language-explicit_cast.html">显式转型</a></li>
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<li><a href="cpp-language-implicit_conversion.html">隐式转换</a></li>
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</ul>
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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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