260 lines
15 KiB
HTML
260 lines
15 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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<title>std::weak_ptr</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_weak_ptr 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>weak_ptr</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">
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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">class</span> weak_ptr<span class="sy4">;</span></span></div>
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</td>
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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></td>
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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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<p><code>std::weak_ptr</code> 是一种智能指针，它对被 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 管理的对象存在非拥有性（「弱」）引用。在访问所引用的对象前必须先转换为 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span>。</p>
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<p><code>std::weak_ptr</code> 用来表达临时所有权的概念：当某个对象只有存在时才需要被访问，而且随时可能被他人删除时，可以使用 <code>std::weak_ptr</code> 来跟踪该对象。需要获得临时所有权时，则将其转换为 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span>，此时如果原来的 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 被销毁，则该对象的生命期将被延长至这个临时的 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 同样被销毁为止。</p>
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<p><code>std::weak_ptr</code> 的另一用法是打断 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 所管理的对象组成的环状引用。若这种环被孤立（例如无指向环中的外部共享指针），则 <code>shared_ptr</code> 引用计数无法抵达零，而内存被泄露。能令环中的指针之一为弱指针以避免此情况。</p>
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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>成员类型</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><span class="t-c"><span class="mw-geshi cpp source-cpp">element_type</span></span></td>
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<td>
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<table class="t-rev-begin">
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<tr class="t-rev t-until-cxx17">
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<td>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">T</span></span></p>
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</td>
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<td><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
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</tr>
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<tr class="t-rev t-since-cxx17">
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<td>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_extent.html"><span class="kw621">std::<span class="me2">remove_extent_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span></p>
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</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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</table>
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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=".E6.88.90.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 t-dsc-member-nobold-div">
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<div><a href="cpp-memory-weak_ptr-weak_ptr.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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</td>
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<td>构造新的<code>weak_ptr</code><br />
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<span class="t-mark">(公开成员函数)</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 t-dsc-member-nobold-div">
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<div><a href="cpp-memory-weak_ptr-~weak_ptr.html"><span class="t-lines"><span>(析构函数)</span></span></a></div>
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</div>
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</td>
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<td>销毁 <code>weak_ptr</code><br />
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<span class="t-mark">(公开成员函数)</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-weak_ptr-operator=.html"><span class="t-lines"><span>operator=</span></span></a></div>
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</div>
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</td>
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<td>为<code>weak_ptr</code>赋值<br />
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<span class="t-mark">(公开成员函数)</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=".E4.BF.AE.E6.94.B9.E5.99.A8">修改器</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-weak_ptr-reset.html"><span class="t-lines"><span>reset</span></span></a></div>
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</div>
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</td>
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<td>释放被管理对象的所有权<br />
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<span class="t-mark">(公开成员函数)</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-weak_ptr-swap.html"><span class="t-lines"><span>swap</span></span></a></div>
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</div>
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</td>
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<td>交换被管理对象<br />
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<span class="t-mark">(公开成员函数)</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=".E8.A7.82.E5.AF.9F.E5.99.A8">观察器</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-weak_ptr-use_count.html"><span class="t-lines"><span>use_count</span></span></a></div>
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</div>
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</td>
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<td>返回管理该对象的 <code>shared_ptr</code> 对象数量<br />
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<span class="t-mark">(公开成员函数)</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-weak_ptr-expired.html"><span class="t-lines"><span>expired</span></span></a></div>
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</div>
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</td>
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<td>检查被引用的对象是否已删除<br />
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<span class="t-mark">(公开成员函数)</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-weak_ptr-lock.html"><span class="t-lines"><span>lock</span></span></a></div>
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</div>
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</td>
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<td>创建管理被引用的对象的<code>shared_ptr</code><br />
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<span class="t-mark">(公开成员函数)</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-weak_ptr-owner_before.html"><span class="t-lines"><span>owner_before</span></span></a></div>
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</div>
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</td>
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<td>提供弱指针的基于拥有者顺序<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E9.9D.9E.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">非成员函数</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-weak_ptr-swap2.html"><span class="t-lines"><span>std::swap<span class="t-dsc-small">(std::weak_ptr)</span></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>特化 <span class="t-lc"><a href="cpp-algorithm-swap.html">std::swap</a></span> 算法<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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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E7.B1.BB">辅助类</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-weak_ptr-atomic2.html"><span class="t-lines"><span>std::atomic<span class="t-dsc-small"><std::weak_ptr></span></span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>原子弱指针<br />
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<span class="t-mark">(类模板特化)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.8E.A8.E5.AF.BC.E6.8C.87.E5.BC.95.28C.2B.2B17_.E8.B5.B7.29"><a href="cpp-memory-weak_ptr-deduction_guides.html">推导指引</a><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></h3>
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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>同 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> ， <code>weak_ptr</code> 的典型实现存储二个指针：</p>
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<ul>
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<li>指向控制块的指针；和</li>
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<li>作为构造来源的 <code>shared_ptr</code> 的存储指针。</li>
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</ul>
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<p>需要用分离的存储指针确保 <code>shared_ptr</code> 和 <code>weak_ptr</code> 间的来回转化正确进行，即使对于别名使用的 <code>shared_ptr</code> 。不可能不经将 <code>weak_ptr</code> 中的存储指针锁入 <code>shared_ptr</code> 就访问它。</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>演示如何通过锁来保证指针的有效性</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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std<span class="sy4">::</span><span class="me2">weak_ptr</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> gw<span class="sy4">;</span>
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<span class="kw4">void</span> observe<span class="br0">(</span><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">"use_count == "</span> <span class="sy1"><<</span> gw.<span class="me1">use_count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">": "</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span><span class="kw4">auto</span> spt <span class="sy1">=</span> gw.<span class="me1">lock</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// 使用之前必须复制到 shared_ptr</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="sy2">*</span>spt <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="kw1">else</span> <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">"gw is expired<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</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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<span class="br0">{</span>
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<span class="kw4">auto</span> sp <span class="sy1">=</span> <a href="cpp-memory-shared_ptr-make_shared.html"><span class="kw736">std::<span class="me2">make_shared</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span><span class="nu0">42</span><span class="br0">)</span><span class="sy4">;</span>
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gw <span class="sy1">=</span> sp<span class="sy4">;</span>
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observe<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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observe<span class="br0">(</span><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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<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">use_count == 1: 42
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use_count == 0: gw is expired</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3001">LWG 3001</a></td>
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<td>C++17</td>
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<td>未为数组支持更新 <code>element_type</code></td>
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<td>已更新</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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