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web/cppreference-zh/chmhelp/cpp-memory-to_address.html
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<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::to_address</title>
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_to_address skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>to_address</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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<table class="t-dcl-begin">
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<tbody>
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<tr class="t-dsc-header">
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<td>
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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>
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<td></td>
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<td></td>
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<tr class="t-dcl t-since-cxx20">
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<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> Ptr <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> to_address<span class="br0">(</span><span class="kw4">const</span> Ptr<span class="sy3">&</span> p<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td>(1)</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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<tr class="t-dcl t-since-cxx20">
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<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> to_address<span class="br0">(</span>T<span class="sy2">*</span> p<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</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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</tr>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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</tbody>
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</table>
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<p>获得 <code>p</code> 所指向的地址，而不形成到被指向者的引用：</p>
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<div class="t-li1"><span class="t-li">1)</span> 缀饰指针重载：若表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-pointer_traits.html"><span class="kw756">std::<span class="me2">pointer_traits</span></span></a><span class="sy1"><</span>Ptr<span class="sy1">></span><span class="sy4">::</span><span class="me2">to_address</span><span class="br0">(</span>p<span class="br0">)</span></span></span> 为良构，则返回该表达式的结果。否则，返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">to_address</span><span class="br0">(</span>p.<span class="me1">operator</span><span class="sy2">-</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 裸指针重载：若 <code>T</code> 为函数类型，则程序为谬构，否则返回不修改的 <span class="t-c"><span class="mw-geshi cpp source-cpp">p</span></span> 。</div>
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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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<tr class="t-par">
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<td>p</td>
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<td>-</td>
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<td>缀饰或裸指针</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>表示与 <code>p</code> 所表示者相同地址的裸指针。</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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<tr>
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<td>
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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="kw4">constexpr</span> T<span class="sy2">*</span> to_address<span class="br0">(</span>T<span class="sy2">*</span> p<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><span class="sy3">!</span><a href="cpp-types-is_function.html"><span class="kw479">std::<span class="me2">is_function_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> p<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="kw4">constexpr</span> <span class="kw4">auto</span> to_address<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> p<span class="br0">)</span> <span class="kw1">noexcept</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span>requires<span class="br0">{</span> <a href="cpp-memory-pointer_traits.html"><span class="kw756">std::<span class="me2">pointer_traits</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">to_address</span><span class="br0">(</span>p<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">return</span> <a href="cpp-memory-pointer_traits.html"><span class="kw756">std::<span class="me2">pointer_traits</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">to_address</span><span class="br0">(</span>p<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span>
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<span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">to_address</span><span class="br0">(</span>p.<span class="me1">operator</span><span class="sy2">-</span><span class="sy1">></span><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="br0">}</span></pre></div>
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</div>
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</td>
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</tr>
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</table>
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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>即使在 <code>p</code> 引用的存储中无已构造的对象时，也能使用 <code>std::to_address</code> ，该情况下不能用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span><span class="sy2">*</span>p<span class="br0">)</span></span></span> ，因为无可绑定 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a></span></span> 参数的合法对象。</p>
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<p><code>to_address</code> 的缀饰指针重载审查 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-pointer_traits.html"><span class="kw756">std::<span class="me2">pointer_traits</span></span></a><span class="sy1"><</span>Ptr<span class="sy1">></span></span></span> 特化。若实例化该特化自身为谬构（往往是因为无法定义 <code>element_type</code> ），则导致立即语境外的硬错误并使得程序为谬构。</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 <memory></span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> A<span class="sy1">></span>
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<span class="kw4">auto</span> allocator_new<span class="br0">(</span>A<span class="sy3">&</span> a<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw4">auto</span> p <span class="sy1">=</span> a.<span class="me1">allocate</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">try</span> <span class="br0">{</span>
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<a href="cpp-memory-allocator_traits.html"><span class="kw701">std::<span class="me2">allocator_traits</span></span></a><span class="sy1"><</span>A<span class="sy1">></span><span class="sy4">::</span><span class="me2">construct</span><span class="br0">(</span>a, std<span class="sy4">::</span><span class="me2">to_address</span><span class="br0">(</span>p<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span> <span class="kw1">catch</span> <span class="br0">(</span>...<span class="br0">)</span> <span class="br0">{</span>
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a.<span class="me1">deallocate</span><span class="br0">(</span>p, <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">throw</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> p<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> A<span class="sy1">></span>
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<span class="kw4">void</span> allocator_delete<span class="br0">(</span>A<span class="sy3">&</span> a, <span class="kw1">typename</span> <a href="cpp-memory-allocator_traits.html"><span class="kw701">std::<span class="me2">allocator_traits</span></span></a><span class="sy1"><</span>A<span class="sy1">></span><span class="sy4">::</span><span class="me2">pointer</span> p<span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-memory-allocator_traits.html"><span class="kw701">std::<span class="me2">allocator_traits</span></span></a><span class="sy1"><</span>A<span class="sy1">></span><span class="sy4">::</span><span class="me2">destroy</span><span class="br0">(</span>a, std<span class="sy4">::</span><span class="me2">to_address</span><span class="br0">(</span>p<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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a.<span class="me1">deallocate</span><span class="br0">(</span>p, <span class="nu0">1</span><span class="br0">)</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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<a href="cpp-memory-allocator.html"><span class="kw700">std::<span class="me2">allocator</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> a<span class="sy4">;</span>
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<span class="kw4">auto</span> p <span class="sy1">=</span> allocator_new<span class="br0">(</span>a<span class="br0">)</span><span class="sy4">;</span>
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allocator_delete<span class="br0">(</span>a, p<span class="br0">)</span><span class="sy4">;</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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<table class="t-dsc-begin">
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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.html"><span class="t-lines"><span>pointer_traits</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>提供关于指针式类型的信息<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-to_address.html"><span class="t-lines"><span>to_address</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</span> <span class="t-mark-rev t-since-cxx20">(C++20)</span><span class="t-mark">(可选)</span></span></span></div>
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</div>
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</td>
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<td>从缀饰指针获得裸指针（ <code>pointer_to</code> 的反函数）<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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</div>
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</div>
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</div>
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</body>
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</html>
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