107 lines
6.6 KiB
HTML
107 lines
6.6 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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<title>std::weak_ptr<T>::expired</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_weak_ptr_expired 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>expired</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><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> expired<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>等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">use_count<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span></span></span> 。可能仍未对被管理对象调用析构函数，但此对象的析构已经临近（或可能已发生）。</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>若被管理对象已被删除则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</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>若被管理对象在线程间共享，则此函数内在地不可靠，通常 false 结果可能在能用之前就变得过时。 true 结果可靠。</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>演示如何用 expired 检查指针的合法性。</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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<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> gw<span class="sy4">;</span>
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<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="br0">(</span><span class="sy3">!</span>gw.<span class="me1">expired</span><span class="br0">(</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="st0">"gw is valid<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="kw1758">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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f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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f<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">gw is valid
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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=".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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<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-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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</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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