120 lines
11 KiB
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120 lines
11 KiB
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<title>std::shared_future<T>::wait</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_thread_shared_future_wait 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::shared_future<T>::</span>wait</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-shared_future.html">shared future</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> wait<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</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>阻塞直至结果变得可用。调用后 <span class="t-c"><span class="mw-geshi cpp source-cpp">valid<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">true</span></span></span> 。</p>
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<p>若调用此函数前 <span class="t-c"><a href="cpp-thread-shared_future-valid.html"><code>valid</code></a><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">false</span></span></span> 则行为未定义。</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>（无）</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>可能抛出实现定义的异常。</p>
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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>鼓励实现检测调用前 <span class="t-c"><span class="mw-geshi cpp source-cpp">valid<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">false</span></span></span> 的情况并抛出以 <span class="t-lc"><a href="cpp-thread-future_errc.html">std::future_errc::no_state</a></span> 为 error_condition 的 <span class="t-lc"><a href="cpp-thread-future_error.html">std::future_error</a></span> 。</p>
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<p>在同一 <code>std::shared_future</code> 上从多个线程调调用 wait 不安全；有意图的使用是令每个等待于同一共享状态上的线程拥有一个 <code>std::shared_future</code> 的副本。</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 <chrono></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <future></span>
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<span class="co2">#include <thread></span>
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<span class="kw4">int</span> fib<span class="br0">(</span><span class="kw4">int</span> n<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>n <span class="sy1"><</span> <span class="nu0">3</span><span class="br0">)</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span>
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<span class="kw1">else</span> <span class="kw1">return</span> fib<span class="br0">(</span>n<span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span> <span class="sy2">+</span> fib<span class="br0">(</span>n<span class="sy2">-</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</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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<a href="cpp-thread-shared_future.html"><span class="kw2181">std::<span class="me2">shared_future</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> f1 <span class="sy1">=</span> <a href="cpp-thread-async.html"><span class="kw2184">std::<span class="me2">async</span></span></a><span class="br0">(</span><a href="cpp-thread-launch.html"><span class="kw2186">std::<span class="me2">launch</span><span class="sy4">::</span><span class="me2">async</span></span></a>, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="br0">)</span><span class="br0">{</span>
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<span class="kw1">return</span> fib<span class="br0">(</span><span class="nu0">40</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-thread-shared_future.html"><span class="kw2181">std::<span class="me2">shared_future</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> f2 <span class="sy1">=</span> <a href="cpp-thread-async.html"><span class="kw2184">std::<span class="me2">async</span></span></a><span class="br0">(</span><a href="cpp-thread-launch.html"><span class="kw2186">std::<span class="me2">launch</span><span class="sy4">::</span><span class="me2">async</span></span></a>, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="br0">)</span><span class="br0">{</span>
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<span class="kw1">return</span> fib<span class="br0">(</span><span class="nu0">43</span><span class="br0">)</span><span class="sy4">;</span>
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<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> <span class="st0">"waiting... "</span> <span class="sy1"><<</span> <a href="cpp-io-manip-flush.html"><span class="kw1790">std::<span class="me2">flush</span></span></a><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">auto</span> start <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">system_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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f1.<span class="me1">wait</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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f2.<span class="me1">wait</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">auto</span> diff <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">system_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span> start<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-chrono-duration.html"><span class="kw993">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">duration</span></span></a><span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span><span class="br0">(</span>diff<span class="br0">)</span>.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" seconds<span class="es1">\n</span>"</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> <span class="st0">"f1: "</span> <span class="sy1"><<</span> f1.<span class="me1">get</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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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"f2: "</span> <span class="sy1"><<</span> f2.<span class="me1">get</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>可能的输出：</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">waiting... 1.61803 seconds
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f1: 102334155
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f2: 433494437</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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<div class="t-dsc-member-div">
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<div><a href="cpp-thread-shared_future-wait_for.html"><span class="t-lines"><span>wait_for</span></span></a></div>
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</div>
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<td>等待结果，如果在指定的超时间隔后仍然无法得到结果，则返回。<br />
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<span class="t-mark">(公开成员函数)</span></td>
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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-thread-shared_future-wait_until.html"><span class="t-lines"><span>wait_until</span></span></a></div>
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</div>
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</td>
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<td>等待结果，如果在已经到达指定的时间点时仍然无法得到结果，则返回。<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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