138 lines
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138 lines
12 KiB
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<title>std::optional<T>::and_then</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::optional<T>::</span>and_then</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a>‎ | <a href="cpp-utility-optional.html">optional</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> F <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> and_then<span class="br0">(</span> F<span class="sy3">&&</span> f <span class="br0">)</span> <span class="sy3">&</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> F <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> and_then<span class="br0">(</span> F<span class="sy3">&&</span> f <span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> F <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> and_then<span class="br0">(</span> F<span class="sy3">&&</span> f <span class="br0">)</span> <span class="sy3">&&</span><span class="sy4">;</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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<tr class="t-dcl t-since-cxx23">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> F <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> and_then<span class="br0">(</span> F<span class="sy3">&&</span> f <span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&&</span><span class="sy4">;</span></span></div>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 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>若所含值存在则返回在其上调用 <code>f</code> 的结果。否则，返回返回类型的空值。</p>
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<p>返回类型（见后述）必须为 <span class="t-lc"><a href="cpp-utility-optional.html">std::optional</a></span> 的特化。否则程序非良构。</p>
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<div class="t-li1"><span class="t-li">1)</span> 等价于<br />
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<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">if</span> <span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span><br /></span></span>
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<p> <span class="kw1">return</span> <a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>F<span class="sy1">></span><span class="br0">(</span>f<span class="br0">)</span>, this<span class="sy2">-</span><span class="sy1">></span>value<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span><br />
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<span class="kw1">else</span><br /></p>
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<span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span>F, T<span class="sy3">&</span><span class="sy1">>></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 等价于<br />
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<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">if</span> <span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span><br /></span></span>
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<p> <span class="kw1">return</span> <a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>F<span class="sy1">></span><span class="br0">(</span>f<span class="br0">)</span>, this<span class="sy2">-</span><span class="sy1">></span>value<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span><br />
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<span class="kw1">else</span><br /></p>
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<span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span>F, <span class="kw4">const</span> T<span class="sy3">&</span><span class="sy1">>></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> 。</div>
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<div class="t-li1"><span class="t-li">3)</span> 等价于<br />
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<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">if</span> <span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span><br /></span></span>
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<p> <span class="kw1">return</span> <a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>F<span class="sy1">></span><span class="br0">(</span>f<span class="br0">)</span>, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>this<span class="sy2">-</span><span class="sy1">></span>value<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span><br />
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<span class="kw1">else</span><br /></p>
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<span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span>F, T<span class="sy1">>></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> 。</div>
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<div class="t-li1"><span class="t-li">4)</span> 等价于<br />
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<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">if</span> <span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span><br /></span></span>
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<p> <span class="kw1">return</span> <a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>F<span class="sy1">></span><span class="br0">(</span>f<span class="br0">)</span>, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>this<span class="sy2">-</span><span class="sy1">></span>value<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span><br />
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<span class="kw1">else</span><br /></p>
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<span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span>F, <span class="kw4">const</span> T<span class="sy1">>></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> 。</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<td>f</td>
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<td>-</td>
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<td>适合的函数或<a href="cpp-named_req-Callable.html"><span style="font-style:italic">可调用</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span></a> 对象，返回 <span class="t-lc"><a href="cpp-utility-optional.html">std::optional</a></span></td>
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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><code>f</code> 的结果或空的 <span class="t-lc"><a href="cpp-utility-optional.html">std::optional</a></span> ，如上所述。</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>有些语言称此操作为 <code>flatmap</code> 。</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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<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
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<td class="mbox-empty-cell"></td>
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<td class="mbox-text" style="">本节未完成<br />
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原因：暂无示例</td>
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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-utility-optional-value_or.html"><span class="t-lines"><span>value_or</span></span></a></div>
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</div>
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</td>
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<td>若所含值可用则返回它，否则返回另一个值<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-optional-transform.html"><span class="t-lines"><span>transform</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
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</div>
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</td>
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<td>若所含值存在则返回含有变换后的所含值的 <code>optional</code> ，否则返回空的 <code>optional</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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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-optional-or_else.html"><span class="t-lines"><span>or_else</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
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<td>若 <code>optional</code> 含值则返回其自身，否则返回给定函数的结果<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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