245 lines
16 KiB
HTML
245 lines
16 KiB
HTML
<!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::pointer_traits</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_pointer_traits 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>pointer_traits</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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<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-cxx11">
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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> <span class="kw1">struct</span> pointer_traits<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-cxx11">(C++11 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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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> <span class="kw1">struct</span> pointer_traits<span class="sy1"><</span>T<span class="sy2">*</span><span class="sy1">></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-cxx11">(C++11 起)</span></td>
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<tr class="t-dcl-sep">
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<td></td>
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<p><code>pointer_traits</code> 类模板提供标准化方法，访问类指针类型（<a href="cpp-named_req-Allocator.html#.E7.BC.80.E9.A5.B0.E6.8C.87.E9.92.88">缀饰指针</a>，如 <a rel="nofollow" class="external text" href="http://www.boost.org/doc/libs/release/doc/html/interprocess/offset_ptr.html"><code>boost::interprocess::offset_ptr</code></a> ）的某些属性。标准模板 <span class="t-lc"><a href="cpp-memory-allocator_traits.html">std::allocator_traits</a></span> 依靠 <code>pointer_traits</code> 确定多种<a href="cpp-named_req-Allocator.html"><span style="font-style:italic">分配器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Allocator)</span></a> 所要求的 typedef 的默认行为。</p>
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<div class="t-li1"><span class="t-li">1)</span> 非特化 <code>pointer_traits</code> 声明下列类型：
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<div class="t-member">
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">成员类型</span></h3>
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<table class="t-dsc-begin">
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<tr class="t-dsc-hitem">
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<td>类型</td>
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<td>定义</td>
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</tr>
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<tr class="t-dsc">
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<td><code>pointer</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">Ptr</span></span></td>
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</tr>
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<tr class="t-dsc">
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<td><code>element_type</code></td>
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<td>若存在则为 <span class="t-c"><span class="mw-geshi cpp source-cpp">Ptr<span class="sy4">::</span><span class="me2">element_type</span></span></span> 。否则若 <code>Ptr</code> 是模板特化 <span class="t-c"><span class="mw-geshi cpp source-cpp">Template<span class="sy1"><</span>T, Args...<span class="sy1">></span></span></span> 则为 <code>T</code> 。否则该 <code>pointer_traits</code> 特化为谬构</td>
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</tr>
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<tr class="t-dsc">
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<td><code>difference_type</code></td>
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<td>若存在则为 <span class="t-c"><span class="mw-geshi cpp source-cpp">Ptr<span class="sy4">::</span><span class="me2">difference_type</span></span></span> ，否则为 <span class="t-lc"><a href="cpp-types-ptrdiff_t.html">std::ptrdiff_t</a></span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.88.AB.E5.90.8D.E6.A8.A1.E6.9D.BF">成员别名模板</span></h3>
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<table class="t-dsc-begin">
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<tr class="t-dsc-hitem">
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<td>模板</td>
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<td>定义</td>
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</tr>
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<tr class="t-dsc">
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> U<span class="sy1">></span> <span class="kw1">using</span> rebind</span></span></td>
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<td>若存在则为 <span class="t-c"><span class="mw-geshi cpp source-cpp">Ptr<span class="sy4">::</span><span class="me2">rebind</span><span class="sy1"><</span>U<span class="sy1">></span></span></span> ，否则若 <code>Ptr</code> 是模板特化 <span class="t-c"><span class="mw-geshi cpp source-cpp">Template<span class="sy1"><</span>T, Args...<span class="sy1">></span></span></span> 则为 <span class="t-c"><span class="mw-geshi cpp source-cpp">Template<span class="sy1"><</span>U, Args...<span class="sy1">></span></span></span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</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-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">(公开静态成员函数)</span></td>
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</tr>
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<tr>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E7.A8.8B.E5.BA.8F.E5.AE.9A.E4.B9.89.E7.89.B9.E5.8C.96.E7.9A.84.E5.8F.AF.E9.80.89.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">程序定义特化的可选成员函数</span></h5>
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</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">(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>
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<div class="t-li1"><span class="t-li">2)</span> 为指针类型 <code>T*</code> 提供的特化，它声明下列类型：
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<div class="t-member">
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B_2">成员类型</span></h3>
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<table class="t-dsc-begin">
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<tr class="t-dsc-hitem">
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<td>类型</td>
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<td>定义</td>
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</tr>
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<tr class="t-dsc">
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<td><code>pointer</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy2">*</span></span></span></td>
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</tr>
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<tr class="t-dsc">
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<td><code>element_type</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">T</span></span></td>
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</tr>
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<tr class="t-dsc">
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<td><code>difference_type</code></td>
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<td><span class="t-lc"><a href="cpp-types-ptrdiff_t.html">std::ptrdiff_t</a></span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.88.AB.E5.90.8D.E6.A8.A1.E7.89.88">成员别名模版</span></h3>
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<table class="t-dsc-begin">
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<tr class="t-dsc-hitem">
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<td>模板</td>
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<td>定义</td>
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</tr>
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<tr class="t-dsc">
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> U<span class="sy1">></span> <span class="kw1">using</span> rebind</span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">U<span class="sy2">*</span></span></span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0_2">成员函数</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-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">(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>
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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>重绑定成员模板别名，使得可以由指向 <code>T</code> 的类指针类型，获取指向 <code>U</code> 的相同类指针类型。例如，</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">typedef</span> std<span class="sy4">::</span><span class="me2">pointer_traits</span><span class="sy1"><</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">>></span><span class="sy4">::</span><span class="me2">rebind</span><span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span> another_pointer<span class="sy4">;</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw588">std::<span class="me2">is_same</span></span></a><span class="sy1"><</span>another_pointer, <a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span><span class="kw4">double</span><span class="sy1">>></span><span class="sy4">::</span><span class="me2">value</span><span class="br0">)</span><span class="sy4">;</span></pre></div>
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</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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<td>
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<p>对用户定义指针类型的特化可提供额外的静态成员函数 <code>to_address</code> 以定制 <span class="t-lc">std::to_address</span> 的行为。</p>
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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="co2">#include <memory></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> Ptr<span class="sy1">></span>
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<span class="kw1">struct</span> BlockList
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<span class="br0">{</span>
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<span class="co1">// 预定义内存块</span>
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<span class="kw1">struct</span> block<span class="sy4">;</span>
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<span class="co1">// 从指针类别 Ptr 定义指向内存块的指针</span>
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<span class="co1">// 若 Ptr 是任何 T* 类型，则 block_ptr_t 为 block*</span>
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<span class="co1">// 若 Ptr 是 smart_ptr<T> ，则 block_ptr_t 为 smart_ptr<block></span>
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<span class="kw1">using</span> block_ptr_t <span class="sy1">=</span> <span class="kw1">typename</span> std<span class="sy4">::</span><span class="me2">pointer_traits</span><span class="sy1"><</span>Ptr<span class="sy1">></span><span class="sy4">::</span><span class="kw1">template</span> rebind<span class="sy1"><</span>block<span class="sy1">></span><span class="sy4">;</span>
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<span class="kw1">struct</span> block
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<span class="br0">{</span>
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<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> size<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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block_ptr_t next_block<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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block_ptr_t free_blocks<span class="sy4">;</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="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> BlockList<span class="sy1"><</span><span class="kw4">int</span><span class="sy2">*</span><span class="sy1">></span> bl1<span class="sy4">;</span>
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<span class="co1">// bl1.free_blocks 的类型是 BlockList<int*>::block*</span>
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BlockList<span class="sy1"><</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span><span class="kw4">char</span><span class="sy1">>></span> bl2<span class="sy4">;</span>
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<span class="co1">// bl2.free_blocks 的类型是 std::shared_ptr< BlockList<std::shared_ptr<char> >::block></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> bl2.<span class="me1">free_blocks</span>.<span class="me1">use_count</span><span class="br0">(</span><span class="br0">)</span> <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></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">​0​</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-allocator_traits.html"><span class="t-lines"><span>allocator_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>提供关于分配器类型的信息<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-addressof.html"><span class="t-lines"><span>addressof</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>获得对象的实际地址，即使其重载了 <i>&</i> 运算符<br />
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<span class="t-mark">(函数模板)</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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