187 lines
13 KiB
HTML
187 lines
13 KiB
HTML
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<title>std::addressof</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_addressof skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>addressof</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-memory.html">memory</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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<div>定义于头文件 <code><a href="cpp-header-memory.html"><memory></a></code></div>
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<td rowspan="142">(1)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
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T<span class="sy2">*</span> addressof<span class="br0">(</span>T<span class="sy3">&</span> arg<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br />
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<span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> T<span class="sy2">*</span> addressof<span class="br0">(</span>T<span class="sy3">&</span> arg<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span><br />
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<span class="kw4">const</span> T<span class="sy2">*</span> addressof<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&&</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 获得对象或函数 <code>arg</code> 的实际地址，即使存在 <code>operator&</code> 的重载</div>
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<div class="t-li1"><span class="t-li">2)</span> 右值重载被删除，以避免取 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> 右值的地址。</div>
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<p>表达式 <code>std::addressof(E)</code> 是一个<a href="cpp-language-constant_expression.html">常量表达式</a>，若 <code>E</code> 是左值常量子表达式。</p>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<td>arg</td>
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<td>-</td>
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<td>左值对象或函数</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>指向 <code>arg</code> 的指针。</p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<table class="eq-fun-cpp-table">
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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">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw1">typename</span> <a href="cpp-types-enable_if.html"><span class="kw630">std::<span class="me2">enable_if</span></span></a><span class="sy1"><</span><a href="cpp-types-is_object.html"><span class="kw486">std::<span class="me2">is_object</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span>, T<span class="sy2">*</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span> addressof<span class="br0">(</span>T<span class="sy3">&</span> arg<span class="br0">)</span> <span class="kw1">noexcept</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> <span class="kw1">reinterpret_cast</span><span class="sy1"><</span>T<span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>
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<span class="sy3">&</span><span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">char</span><span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span>
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<span class="kw1">reinterpret_cast</span><span class="sy1"><</span><span class="kw4">const</span> <span class="kw4">volatile</span> <span class="kw4">char</span><span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span>arg<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><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">typename</span> <a href="cpp-types-enable_if.html"><span class="kw630">std::<span class="me2">enable_if</span></span></a><span class="sy1"><</span><span class="sy3">!</span><a href="cpp-types-is_object.html"><span class="kw486">std::<span class="me2">is_object</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span>, T<span class="sy2">*</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span> addressof<span class="br0">(</span>T<span class="sy3">&</span> arg<span class="br0">)</span> <span class="kw1">noexcept</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> <span class="sy3">&</span>arg<span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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<p>注：上述实现不是 <code>constexpr</code> （这要求编译器支持）。</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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<p>operator& 可以为指针封装器类重载，以获得指向指针的指针：</p>
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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="co2">#include <memory></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> Ptr <span class="br0">{</span>
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T<span class="sy2">*</span> pad<span class="sy4">;</span> <span class="co1">// 增加填充以显示‘ this ’和‘ data ’的区别</span>
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T<span class="sy2">*</span> data<span class="sy4">;</span>
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Ptr<span class="br0">(</span>T<span class="sy2">*</span> arg<span class="br0">)</span> <span class="sy4">:</span> pad<span class="br0">(</span>nullptr<span class="br0">)</span>, data<span class="br0">(</span>arg<span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Ctor this = "</span> <span class="sy1"><<</span> this <span class="sy1"><<</span> <a href="cpp-io-manip-endl.html"><span class="kw1790">std::<span class="me2">endl</span></span></a><span class="sy4">;</span>
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<span class="br0">}</span>
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~Ptr<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> delete data<span class="sy4">;</span> <span class="br0">}</span>
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T<span class="sy2">**</span> operator<span class="sy3">&</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="sy3">&</span>data<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">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> f<span class="br0">(</span>Ptr<span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> p<span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Ptr overload called with p = "</span> <span class="sy1"><<</span> p <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="sy2">**</span> p<span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"int** overload called with p = "</span> <span class="sy1"><<</span> p <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</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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Ptr<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> p<span class="br0">(</span>new <span class="kw4">int</span><span class="br0">(</span><span class="nu0">42</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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f<span class="br0">(</span><span class="sy3">&</span>p<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 int** 重载</span>
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f<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">addressof</span><span class="br0">(</span>p<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 Ptr<int>* 重载，（ = this ）</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">Ctor this = 0x7fff59ae6e88
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int** overload called with p = 0x7fff59ae6e90
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Ptr overload called with p = 0x7fff59ae6e88</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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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-allocator.html"><span class="t-lines"><span>allocator</span></span></a></div>
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</div>
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</td>
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<td>默认的分配器<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>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-pointer_traits-pointer_to.html"><span class="t-lines"><span>pointer_to</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</span></span></span></div>
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</div>
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</td>
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<td>获得指向其参数的可解引用指针<br />
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<span class="t-mark">(<code>std::pointer_traits<Ptr></code> 的公开静态成员函数)</span></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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