146 lines
16 KiB
HTML
146 lines
16 KiB
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::future<T>::</span>get</h1>
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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><span class="mw-geshi cpp source-cpp">T get<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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<td><span class="t-mark">(仅为泛型 <code>future</code> 模板的成员)</span><br />
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<span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> get<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td><span class="t-mark">(仅为 <code>future<T&></code> 模板特化的成员)</span><br />
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<span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> get<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark">(仅为 <code>future<void></code> 模板特化的成员)</span><br />
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<span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<p><code>get</code> 方法等待直至 <code>future</code> 拥有合法结果并（依赖于使用哪个模板）获取它。它等效地调用 <span class="t-lc"><a href="cpp-thread-future-wait.html">wait()</a></span> 等待结果。</p>
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<p>泛型模板和二个模板特化各含单个 <code>get</code> 版本。 <code>get</code> 的三个版本仅在返回类型有别。</p>
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<p>若调用此函数前 <span class="t-lc"><a href="cpp-thread-future-valid.html">valid()</a></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 则行为未定义。</p>
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<p>释放任何共享状态。调用此方法后 <span class="t-lc"><a href="cpp-thread-future-valid.html">valid()</a></span> 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=".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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<div class="t-li1"><span class="t-li">1)</span> 存储于共享状态的值 <code>v</code> ，如同 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>v<span class="br0">)</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 存储于共享状态的值的引用。</div>
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<div class="t-li1"><span class="t-li">3)</span> 无。</div>
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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>若 future 所引用的共享状态中存储异常（例如，通过调用 <a href="cpp-thread-promise-set_exception.html"><tt>std::promise::set_exception()</tt></a> ），则抛出该异常。</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>鼓励实现在调用前检测 <span class="t-lc"><a href="cpp-thread-future-valid.html">valid()</a></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><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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<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 class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <thread></span>
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<span class="co2">#include <future></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <chrono></span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> time<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">static</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">steady_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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<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> d <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">steady_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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<span class="kw1">return</span> <span class="st0">"["</span> <span class="sy2">+</span> <a href="cpp-string-basic_string-to_string.html"><span class="kw1265">std::<span class="me2">to_string</span></span></a><span class="br0">(</span>d.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy2">+</span> <span class="st0">"s]"</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> <span class="br0">{</span>
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<span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">chrono_literals</span><span class="sy4">;</span>
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<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> time<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" launching thread<span class="es1">\n</span>"</span><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">int</span><span class="sy1">></span> f <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>
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<a href="cpp-thread-sleep_for.html"><span class="kw2149">std::<span class="me2">this_thread</span><span class="sy4">::</span><span class="me2">sleep_for</span></span></a><span class="br0">(</span>1s<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="nu0">7</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> time<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" waiting for the future, f.valid() == "</span>
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<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="kw4">int</span> n <span class="sy1">=</span> 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> time<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" future.get() returned with "</span> <span class="sy1"><<</span> n <span class="sy1"><<</span> <span class="st0">". f.valid() = "</span>
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<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>
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<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> time<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" launching thread<span class="es1">\n</span>"</span><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">int</span><span class="sy1">></span> f <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>
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<a href="cpp-thread-sleep_for.html"><span class="kw2149">std::<span class="me2">this_thread</span><span class="sy4">::</span><span class="me2">sleep_for</span></span></a><span class="br0">(</span>1s<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="kw2">true</span> <span class="sy4">?</span> <span class="kw1">throw</span> <a href="cpp-error-runtime_error.html"><span class="kw770">std::<span class="me2">runtime_error</span></span></a><span class="br0">(</span><span class="st0">"7"</span><span class="br0">)</span> <span class="sy4">:</span> <span class="nu0">7</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> time<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" waiting for the future, f.valid() == "</span>
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<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="kw1">try</span> <span class="br0">{</span>
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<span class="kw4">int</span> n <span class="sy1">=</span> 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> time<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" future.get() returned with "</span> <span class="sy1"><<</span> n
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<span class="sy1"><<</span> <span class="st0">" f.valid() = "</span> <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> <span class="kw1">catch</span><span class="br0">(</span><span class="kw4">const</span> <a href="cpp-error-exception.html"><span class="kw764">std::<span class="me2">exception</span></span></a><span class="sy3">&</span> e<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> time<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" caught exception "</span> <span class="sy1"><<</span> e.<span class="me1">what</span><span class="br0">(</span><span class="br0">)</span>
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<span class="sy1"><<</span> <span class="st0">", f.valid() == "</span> <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>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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<p>可能的输出：</p>
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<div class="text source-text">
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<pre class="de1">[0.000004s] launching thread
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[0.000461s] waiting for the future, f.valid() == 1
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[1.001156s] future.get() returned with 7. f.valid() = 0
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[1.001192s] launching thread
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[1.001275s] waiting for the future, f.valid() == 1
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[2.002356s] caught exception 7, f.valid() == 0</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><a href="cpp-thread-future-valid.html"><span class="t-lines"><span>valid</span></span></a></div>
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</td>
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<td>检查 future 是否拥有共享状态<br />
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<span class="t-mark">(公开成员函数)</span></td>
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