90 lines
6.9 KiB
HTML
90 lines
6.9 KiB
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::future<T>::valid</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_thread_future_valid skin-cppreference2 action-view cpp-page-template cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::future<T>::</span>valid</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-thread.html">thread</a>‎ | <a href="cpp-thread-future.html">future</a></span></div>
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<div class="t-page-template" style="">
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<p></p>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> valid<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><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<p>检查期货是否指代共享状态。</p>
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<p>这是非默认构造或被移动的期货（例如为 <span class="t-lc"><a href="cpp-thread-promise-get_future.html">std::promise::get_future()</a></span> 、 <span class="t-lc"><a href="cpp-thread-packaged_task-get_future.html">std::packaged_task::get_future()</a></span> 或 <span class="t-lc"><a href="cpp-thread-async.html">std::async()</a></span> 所返回）在首次调用 <span class="t-lc"><a href="cpp-thread-future-get.html">get()</a></span> 或 <span class="t-lc"><a href="cpp-thread-future-share.html">share()</a></span> 前的唯一情况。</p>
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<p>若在不指代共享状态的 <code>future</code> 上调用任何析构函数、移动赋值运算符或 <code>valid</code> 以外的成员函数，则行为未定义（尽管鼓励实现在此情况下抛出指示 <code>no_state</code> 的 <span class="t-lc"><a href="cpp-thread-future_error.html">std::future_error</a></span> ）。从 <code>valid()</code> 为 <code>false</code> 的 future 对象移动是合法的。</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 *this 指代共享状态则为 <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=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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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 <future></span>
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<span class="co2">#include <iostream></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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<a href="cpp-thread-promise.html"><span class="kw2179">std::<span class="me2">promise</span></span></a><span class="sy1"><</span><span class="kw4">void</span><span class="sy1">></span> p<span class="sy4">;</span>
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<a href="cpp-thread-future.html"><span class="kw2180">std::<span class="me2">future</span></span></a><span class="sy1"><</span><span class="kw4">void</span><span class="sy1">></span> f <span class="sy1">=</span> p.<span class="me1">get_future</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</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> <a href="cpp-io-manip-boolalpha.html"><span class="kw1764">std::<span class="me2">boolalpha</span></span></a><span class="sy4">;</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> f.<span class="me1">valid</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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p.<span class="me1">set_value</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</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> f.<span class="me1">valid</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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f.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</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> f.<span class="me1">valid</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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<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">true
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true
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false</pre></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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<div class="t-dsc-member-div">
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<div><a href="cpp-thread-future-wait.html"><span class="t-lines"><span>wait</span></span></a></div>
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</td>
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<td>等待结果变得可用<br />
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<span class="t-mark">(公开成员函数)</span></td>
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