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web/cppreference-zh/chmhelp/cpp-memory-weak_ptr-use_count.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::weak_ptr<T>::use_count</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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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_weak_ptr_use_count 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::weak_ptr<T>::</span>use_count</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>‎ | <a href="cpp-memory-weak_ptr.html">weak ptr</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-dcl t-since-cxx11">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> use_count<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><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>返回共享被管理对象所有权的 <code>shared_ptr</code> 实例数量，或 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> ，若被管理对象已被删除，即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 为空。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<p>（无）</p>
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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>在调用的瞬间共享被管理对象所有权的 <code>shared_ptr</code> 实例数量。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p><span class="t-lc"><a href="cpp-memory-weak_ptr-expired.html">expired()</a></span> 可能快于 <code>use_count()</code> 。此函数固有地有不稳，若被管理对象在可能创建并销毁 <code>shared_ptr</code> 副本的线程间共享：则结果仅若匹配调用方线程所独占的副本数或零才可靠；任何其他值可能在能使用前就变得过时了。</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="co2">#include <memory></span>
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<a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> gwp<span class="sy4">;</span>
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<span class="kw4">void</span> observe_gwp<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</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">"use_count(): "</span> <span class="sy1"><<</span> gwp.<span class="me1">use_count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\t</span> id: "</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span><span class="kw4">auto</span> sp <span class="sy1">=</span> gwp.<span class="me1">lock</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</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="sy2">*</span>sp <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw1">else</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">"??<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> share_recursively<span class="br0">(</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> sp, <span class="kw4">int</span> depth<span class="br0">)</span> <span class="br0">{</span>
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observe_gwp<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// : 2 3 4</span>
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<span class="kw1">if</span> <span class="br0">(</span><span class="nu0">1</span> <span class="sy1"><</span> depth<span class="br0">)</span>
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share_recursively<span class="br0">(</span>sp, depth <span class="sy2">-</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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observe_gwp<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// : 4 3 2</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> <span class="br0">{</span>
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observe_gwp<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="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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gwp <span class="sy1">=</span> sp<span class="sy4">;</span>
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observe_gwp<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// : 1</span>
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share_recursively<span class="br0">(</span>sp, <span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// : 2 3 4 4 3 2</span>
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observe_gwp<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// : 1</span>
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<span class="br0">}</span>
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observe_gwp<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// : 0</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(): 0 id: ??
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use_count(): 1 id: 42
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use_count(): 2 id: 42
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use_count(): 3 id: 42
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use_count(): 4 id: 42
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use_count(): 4 id: 42
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use_count(): 3 id: 42
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use_count(): 2 id: 42
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use_count(): 1 id: 42
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use_count(): 0 id: ??</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-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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</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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