125 lines
14 KiB
HTML
125 lines
14 KiB
HTML
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<title>std::optional<T>::transform</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_optional_transform skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::optional<T>::</span>transform</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> transform<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> transform<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> transform<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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<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> transform<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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<td></td>
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<td></td>
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<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 含值则返回含有 <code>f</code> 在所含值上调用结果的 <span class="t-lc"><a href="cpp-utility-optional.html">std::optional</a></span> 。否则返回这种类型的空 <span class="t-lc"><a href="cpp-utility-optional.html">std::optional</a></span> 。</p>
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<p>结果的所含值类型（以下以 <code>U</code> 代表）必须为非数组对象类型，且必须不是 <span class="t-lc"><a href="cpp-utility-in_place.html">std::in_place_t</a></span> 或 <span class="t-lc"><a href="cpp-utility-optional-nullopt_t.html">std::nullopt_t</a></span> 。否则程序非良构。</p>
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<div class="t-li1"><span class="t-li">1)</span> 令 <code>U</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><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></span> 。若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 含值，则返回所含值从 <span class="t-c"><span class="mw-geshi cpp source-cpp"><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></span> 直接非列表初始化的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span></span></span> 。否则返回空的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span></span></span> 。<br />
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若变量定义 <span class="t-c"><span class="mw-geshi cpp source-cpp">U x<span class="br0">(</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="br0">)</span><span class="sy4">;</span></span></span> 非良构则程序非良构。</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ，除了 <code>U</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><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></span> 。</div>
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<div class="t-li1"><span class="t-li">3)</span> 令 <code>U</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><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></span> 。若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 含值，则返回所含值从 <span class="t-c"><span class="mw-geshi cpp source-cpp"><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></span> 直接非列表初始化的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span></span></span> 。否则返回空的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span></span></span> 。<br />
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若变量定义 <span class="t-c"><span class="mw-geshi cpp source-cpp">U x<span class="br0">(</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="br0">)</span><span class="sy4">;</span></span></span> 非良构则程序非良构。</div>
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<div class="t-li1"><span class="t-li">4)</span> 同 <span class="t-v">(3)</span> ，除了 <code>U</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><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></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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<table class="t-par-begin">
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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</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>含有 <code>f</code> 的结果的 <span class="t-lc"><a href="cpp-utility-optional.html">std::optional</a></span> 或空 <span class="t-lc"><a href="cpp-utility-optional.html">std::optional</a></span> ，如上所属。</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>因为 <code>transform</code> 直接在正确的位置构造 <code>U</code> 对象，而非将它传递给构造函数， <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_move_constructible.html"><span class="kw539">std::<span class="me2">is_move_constructible_v</span></span></a><span class="sy1"><</span>U<span class="sy1">></span></span></span> 能为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</p>
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<p>有些语言称此操作为 <code>map</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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</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-and_then.html"><span class="t-lines"><span>and_then</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>若所含值存在则返回给定的函数在其上的结果，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 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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</div>
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<div class="visualClear"></div>
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