klarkxy/hustoj
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity
Files
96b9eb57e2b0c2deeeac982c17075ee85aaa7603
hustoj/web/cppreference-zh/chmhelp/cpp-utility-optional-optional.html
klarkxy 196e706fe4 上传cppreference
2025-08-01 12:57:54 +08:00

312 lines
40 KiB
HTML
Raw Blame History

<!DOCTYPE html>
<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
<head><meta http-equiv="x-ua-compatible" content="ie=edge">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>std::optional&lt;T&gt;::optional</title>
<link rel="stylesheet" href="ext.css" />
<link rel="stylesheet" href="site_modules.css" />
</head>
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_optional_optional skin-cppreference2 action-view cpp-navbar">
<div id="cpp-content-base">
<div id="content"><a id="top"></a>
<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::optional&lt;T&gt;::</span>optional</h1>
<div id="bodyContent">
<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-utility.html">utility</a>&lrm; | <a href="cpp-utility-optional.html">optional</a></span></div>
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
<table class="t-dcl-begin">
<tbody>
<tr class="t-dcl t-since-cxx17">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> optional<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span><br />
<span class="kw4">constexpr</span> optional<span class="br0">(</span> <a href="cpp-utility-optional-nullopt_t.html"><span class="kw1078">std::<span class="me2">nullopt_t</span></span></a> <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
</td>
<td>(1)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
</tr>
<tr class="t-dcl t-since-cxx17">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> optional<span class="br0">(</span> <span class="kw4">const</span> optional<span class="sy3">&amp;</span> other <span class="br0">)</span><span class="sy4">;</span></span></div>
</td>
<td>(2)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
</tr>
<tr class="t-dcl t-since-cxx17">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> optional<span class="br0">(</span> optional<span class="sy3">&amp;&amp;</span> other <span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/* see below */</span><span class="br0">)</span><span class="sy4">;</span></span></div>
</td>
<td>(3)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
</tr>
</tbody>
<tbody class="t-dcl-rev t-dcl-rev-num">
<tr class="t-dcl-rev-aux">
<td></td>
<td rowspan="142">(4)</td>
<td></td>
</tr>
<tr class="t-dcl t-since-cxx17 t-until-cxx20">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span> <span class="kw1">class</span> U <span class="sy1">&gt;</span><br />
optional<span class="br0">(</span> <span class="kw4">const</span> optional<span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span> other <span class="br0">)</span><span class="sy4">;</span></span></div>
</td>
<td class="t-dcl-nopad"></td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span><br />
<span class="t-mark-rev t-until-cxx20">(C++20 &#21069;)</span><br />
<span class="t-mark">(&#26465;&#20214;&#24615; <code>explicit</code>)</span></td>
</tr>
<tr class="t-dcl t-since-cxx20">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span> <span class="kw1">class</span> U <span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> optional<span class="br0">(</span> <span class="kw4">const</span> optional<span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span> other <span class="br0">)</span><span class="sy4">;</span></span></div>
</td>
<td class="t-dcl-nopad"></td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span><br />
<span class="t-mark">(&#26465;&#20214;&#24615; <code>explicit</code>)</span></td>
</tr>
</tbody>
<tbody></tbody>
<tbody class="t-dcl-rev t-dcl-rev-num">
<tr class="t-dcl-rev-aux">
<td></td>
<td rowspan="142">(5)</td>
<td></td>
</tr>
<tr class="t-dcl t-since-cxx17 t-until-cxx20">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span> <span class="kw1">class</span> U <span class="sy1">&gt;</span><br />
optional<span class="br0">(</span> optional<span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;&amp;</span> other <span class="br0">)</span><span class="sy4">;</span></span></div>
</td>
<td class="t-dcl-nopad"></td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span><br />
<span class="t-mark-rev t-until-cxx20">(C++20 &#21069;)</span><br />
<span class="t-mark">(&#26465;&#20214;&#24615; <code>explicit</code>)</span></td>
</tr>
<tr class="t-dcl t-since-cxx20">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span> <span class="kw1">class</span> U <span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> optional<span class="br0">(</span> optional<span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;&amp;</span> other <span class="br0">)</span><span class="sy4">;</span></span></div>
</td>
<td class="t-dcl-nopad"></td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span><br />
<span class="t-mark">(&#26465;&#20214;&#24615; <code>explicit</code>)</span></td>
</tr>
</tbody>
<tbody>
<tr class="t-dcl t-since-cxx17">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> <span class="kw1">explicit</span> optional<span class="br0">(</span> <a href="cpp-utility-in_place.html"><span class="kw1082">std::<span class="me2">in_place_t</span></span></a>, Args<span class="sy3">&amp;&amp;</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></span></div>
</td>
<td>(6)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
</tr>
<tr class="t-dcl t-since-cxx17">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> U, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">&gt;</span><br /></span>
<p><span class="kw4">constexpr</span> <span class="kw1">explicit</span> optional<span class="br0">(</span> <a href="cpp-utility-in_place.html"><span class="kw1082">std::<span class="me2">in_place_t</span></span></a>,<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;<a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span> ilist,<br /></p>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Args<span class="sy3">&amp;&amp;</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></div>
</td>
<td>(7)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
</tr>
<tr class="t-dcl t-since-cxx17">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span> <span class="kw1">class</span> U <span class="sy1">=</span> T <span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> optional<span class="br0">(</span> U<span class="sy3">&amp;&amp;</span> value <span class="br0">)</span><span class="sy4">;</span></span></div>
</td>
<td>(8)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span><br />
<span class="t-mark">(&#26465;&#20214;&#24615; <code>explicit</code>)</span></td>
</tr>
<tr class="t-dcl-sep">
<td></td>
<td></td>
<td></td>
</tr>
</tbody>
</table>
<p>&#26500;&#36896;&#26032;&#30340; <code>optional</code> &#23545;&#35937;&#12290;</p>
<div class="t-li1"><span class="t-li">1)</span> &#26500;&#36896;<i>&#19981;&#21547;&#20540;</i>&#30340;&#23545;&#35937;&#12290;</div>
<div class="t-li1"><span class="t-li">2)</span> &#22797;&#21046;&#26500;&#36896;&#20989;&#25968;&#65306;&#33509; <code>other</code> &#21547;&#20540;&#65292;&#21017;&#21021;&#22987;&#21270;&#25152;&#21547;&#20540;&#65292;&#22914;&#21516;&#20197;&#34920;&#36798;&#24335; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>other</span></span> <a href="cpp-language-direct_initialization.html">&#30452;&#25509;&#21021;&#22987;&#21270;</a>&#65288;&#20294;&#19981;&#26159;&#30452;&#25509;&#21015;&#34920;&#21021;&#22987;&#21270;&#65289; <code>T</code> &#31867;&#22411;&#23545;&#35937;&#12290;&#33509; <code>other</code> &#19981;&#21547;&#20540;&#65292;&#21017;&#26500;&#36896;&#19968;&#20010;<i>&#19981;&#21547;&#20540;</i>&#30340;&#23545;&#35937;&#12290;
<ul>
<li>&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_copy_constructible.html"><span class="kw533">std::<span class="me2">is_copy_constructible_v</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span></span></span> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> &#21017;&#23450;&#20041;&#27492;&#26500;&#36896;&#20989;&#25968;&#20026;&#34987;&#21024;&#38500;&#12290;</li>
<li>&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_copy_constructible.html"><span class="kw535">std::<span class="me2">is_trivially_copy_constructible_v</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span></span></span> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> &#65292;&#21017;&#23427;&#26159;&#24179;&#20961;&#26500;&#36896;&#20989;&#25968;&#12290;</li>
</ul>
</div>
<div class="t-li1"><span class="t-li">3)</span> &#31227;&#21160;&#26500;&#36896;&#20989;&#25968;&#65306;&#33509; <code>other</code> &#21547;&#20540;&#65292;&#21017;&#21021;&#22987;&#21270;&#25152;&#21547;&#20540;&#65292;&#22914;&#21516;&#20197;&#34920;&#36798;&#24335; <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><span class="sy2">*</span>other<span class="br0">)</span></span></span> <a href="cpp-language-direct_initialization.html">&#30452;&#25509;&#21021;&#22987;&#21270;</a>&#65288;&#20294;&#19981;&#26159;&#30452;&#25509;&#21015;&#34920;&#21021;&#22987;&#21270;&#65289; <code>T</code> &#31867;&#22411;&#23545;&#35937;&#65292;&#19988;<i>&#19981;</i>&#20196; <code>other</code> &#20026;&#31354;&#65306;&#34987;&#31227;&#21160;&#28304;&#30340; <code>optional</code> &#20173;&#28982;<i>&#21547;&#20540;</i>&#65292;&#20294;&#35813;&#20540;&#33258;&#36523;&#26159;&#34987;&#31227;&#21160;&#30340;&#12290;
<ul>
<li>&#33509; <code>other</code> &#19981;&#21547;&#20540;&#65292;&#21017;&#26500;&#36896;&#19968;&#20010;<i>&#19981;&#21547;&#20540;</i>&#30340;&#23545;&#35937;&#12290;&#27492;&#26500;&#36896;&#20989;&#25968;&#19981;&#21442;&#19982;&#37325;&#36733;&#20915;&#35758;&#65292;&#38500;&#38750; <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">&lt;</span>T<span class="sy1">&gt;</span></span></span> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span></li>
<li>&#12290;&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_move_constructible.html"><span class="kw541">std::<span class="me2">is_trivially_move_constructible_v</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span></span></span> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> &#65292;&#21017;&#23427;&#26159;&#24179;&#20961;&#26500;&#36896;&#20989;&#25968;&#12290;</li>
</ul>
</div>
<div class="t-li1"><span class="t-li">4)</span> &#36716;&#25442;&#22797;&#21046;&#26500;&#36896;&#20989;&#25968;&#65306;&#33509; <code>other</code> &#19981;&#21547;&#20540;&#65292;&#21017;&#26500;&#36896;&#19981;&#21547;&#20540;&#30340; <code>optional</code> &#23545;&#35937;&#12290;&#21542;&#21017;&#65292;&#26500;&#36896;&#21547;&#20540;&#30340; <code>optional</code> &#23545;&#35937;&#65292;&#22914;&#21516;&#20197;&#34920;&#36798;&#24335; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>other</span></span> <a href="cpp-language-direct_initialization.html">&#30452;&#25509;&#21021;&#22987;&#21270;</a>&#65288;&#20294;&#19981;&#26159;&#30452;&#25509;&#21015;&#34920;&#21021;&#22987;&#21270;&#65289; <code>T</code> &#31867;&#22411;&#23545;&#35937;&#19968;&#33324;&#21021;&#22987;&#21270;&#12290;
<ul>
<li>&#27492;&#26500;&#36896;&#20989;&#25968;&#19981;&#21442;&#19982;&#37325;&#36733;&#20915;&#35758;&#65292;&#38500;&#38750;&#28385;&#36275;&#19979;&#21015;&#26465;&#20214;&#65306;
<ul>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, <span class="kw4">const</span> U<span class="sy3">&amp;</span><span class="sy1">&gt;</span></span></span> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> &#12290;</li>
<li><code>T</code> &#19981;&#21487;&#20174;&#20219;&#20309; <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">&lt;</span>U<span class="sy1">&gt;</span></span></span> &#65288;&#21487;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> &#65289;&#31867;&#22411;&#34920;&#36798;&#24335;&#26500;&#36896;&#25110;&#36716;&#25442;&#65292;&#21363;&#19979;&#21015; 8 &#20010;&#31867;&#22411;&#29305;&#24449;&#22343;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> &#65306;
<ul>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, <span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;&amp;</span><span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, <span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;&amp;</span><span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1">&lt;</span><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span>, T<span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1">&lt;</span><span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span>, T<span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1">&lt;</span><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;&amp;</span>, T<span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1">&lt;</span><span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;&amp;</span>, T<span class="sy1">&gt;</span></span></span></li>
</ul>
</li>
</ul>
</li>
<li>&#27492;&#26500;&#36896;&#20989;&#25968;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> &#33509;&#19988;&#21807;&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1">&lt;</span><span class="kw4">const</span> U<span class="sy3">&amp;</span>, T<span class="sy1">&gt;</span></span></span> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> &#12290;</li>
</ul>
</div>
<div class="t-li1"><span class="t-li">5)</span> &#36716;&#25442;&#31227;&#21160;&#26500;&#36896;&#20989;&#25968;&#65306;&#33509; <code>other</code> &#19981;&#21547;&#20540;&#65292;&#21017;&#26500;&#36896;&#19981;&#21547;&#20540;&#30340; <code>optional</code> &#23545;&#35937;&#12290;&#21542;&#21017;&#65292;&#26500;&#36896;&#21547;&#20540;&#30340; <code>optional</code> &#23545;&#35937;&#65292;&#22914;&#21516;&#20197;&#34920;&#36798;&#24335; <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><span class="sy2">*</span>other<span class="br0">)</span></span></span> <a href="cpp-language-direct_initialization.html">&#30452;&#25509;&#21021;&#22987;&#21270;</a>&#65288;&#20294;&#19981;&#26159;&#30452;&#25509;&#21015;&#34920;&#21021;&#22987;&#21270;&#65289; <code>T</code> &#31867;&#22411;&#23545;&#35937;&#19968;&#33324;&#21021;&#22987;&#21270;&#12290;
<ul>
<li>&#27492;&#26500;&#36896;&#20989;&#25968;&#19981;&#21442;&#19982;&#37325;&#36733;&#20915;&#35758;&#65292;&#38500;&#38750;&#28385;&#36275;&#19979;&#21015;&#26465;&#20214;&#65306;
<ul>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, U<span class="sy3">&amp;&amp;</span><span class="sy1">&gt;</span></span></span> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> &#12290;</li>
<li><code>T</code> &#19981;&#21487;&#20174;&#20219;&#20309;<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">&lt;</span>U<span class="sy1">&gt;</span></span></span> &#65288;&#21487;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> &#65289;&#31867;&#22411;&#34920;&#36798;&#24335;&#26500;&#36896;&#25110;&#36716;&#25442;&#65292;&#21363;&#19979;&#21015; 8 &#20010;&#31867;&#22411;&#29305;&#24449;&#22343;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> &#65306;
<ul>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, <span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;&amp;</span><span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, <span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;&amp;</span><span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1">&lt;</span><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span>, T<span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1">&lt;</span><span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span>, T<span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1">&lt;</span><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;&amp;</span>, T<span class="sy1">&gt;</span></span></span></li>
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1">&lt;</span><span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;&amp;</span>, T<span class="sy1">&gt;</span></span></span></li>
</ul>
</li>
</ul>
</li>
<li>&#27492;&#26500;&#36896;&#20989;&#25968;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> &#65292;&#33509;&#19988;&#21807;&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1">&lt;</span>U<span class="sy3">&amp;&amp;</span>, T<span class="sy1">&gt;</span></span></span> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> &#12290;</li>
</ul>
</div>
<div class="t-li1"><span class="t-li">6)</span> &#26500;&#36896;&#19968;&#20010;<i>&#21547;&#20540;</i>&#30340;&#23545;&#35937;&#65292;&#22914;&#21516;&#20174;&#21442;&#25968; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1">&lt;</span>Args<span class="sy1">&gt;</span><span class="br0">(</span>args<span class="br0">)</span>...</span></span> <a href="cpp-language-direct_initialization.html">&#30452;&#25509;&#21021;&#22987;&#21270;</a>&#65288;&#20294;&#19981;&#26159;&#30452;&#25509;&#21015;&#34920;&#21021;&#22987;&#21270;&#65289; <code>T</code> &#31867;&#22411;&#23545;&#35937;&#19968;&#33324;&#21021;&#22987;&#21270;&#12290;
<ul>
<li>&#33509;&#36873;&#25321;&#30340; <code>T</code> &#26500;&#36896;&#20989;&#25968;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> &#26500;&#36896;&#20989;&#25968;&#65292;&#21017;&#27492;&#26500;&#36896;&#20989;&#25968;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> &#26500;&#36896;&#20989;&#25968;&#12290;</li>
<li>&#35813;&#20989;&#25968;&#20165;&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, Args...<span class="sy1">&gt;</span></span></span> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> &#25165;&#21442;&#19982;&#37325;&#36733;&#20915;&#35758;&#12290;</li>
</ul>
</div>
<div class="t-li1"><span class="t-li">7)</span> &#26500;&#36896;&#19968;&#20010;<i>&#21547;&#20540;</i>&#30340;&#23545;&#35937;&#65292;&#22914;&#21516;&#20174;&#21442;&#25968; <span class="t-c"><span class="mw-geshi cpp source-cpp">ilist, <a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1">&lt;</span>Args<span class="sy1">&gt;</span><span class="br0">(</span>args<span class="br0">)</span>...</span></span> <a href="cpp-language-direct_initialization.html">&#30452;&#25509;&#21021;&#22987;&#21270;</a>&#65288;&#20294;&#19981;&#26159;&#30452;&#25509;&#21015;&#34920;&#21021;&#22987;&#21270;&#65289; <code>T</code> &#31867;&#22411;&#23545;&#35937;&#19968;&#33324;&#21021;&#22987;&#21270;&#12290;
<ul>
<li>&#33509;&#36873;&#25321;&#30340; <code>T</code> &#26500;&#36896;&#20989;&#25968;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> &#26500;&#36896;&#20989;&#25968;&#65292;&#21017;&#27492;&#26500;&#36896;&#20989;&#25968;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> &#26500;&#36896;&#20989;&#25968;&#12290;</li>
<li>&#35813;&#20989;&#25968;&#20165;&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, <a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span>, Args<span class="sy3">&amp;&amp;</span>...<span class="sy1">&gt;</span></span></span> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> &#25165;&#21442;&#19982;&#37325;&#36733;&#20915;&#35758;&#12290;</li>
</ul>
</div>
<div class="t-li1"><span class="t-li">8)</span> &#26500;&#36896;&#19968;&#20010;<i>&#21547;&#20540;</i>&#30340;&#23545;&#35937;&#65292;&#22914;&#21516;&#20174;&#21442;&#25968; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="br0">(</span>value<span class="br0">)</span></span></span> <a href="cpp-language-direct_initialization.html">&#30452;&#25509;&#21021;&#22987;&#21270;</a>&#65288;&#20294;&#19981;&#26159;&#30452;&#25509;&#21015;&#34920;&#21021;&#22987;&#21270;&#65289; <code>T</code> &#31867;&#22411;&#23545;&#35937;&#19968;&#33324;&#21021;&#22987;&#21270;&#12290;
<ul>
<li>&#33509;&#36873;&#25321;&#30340; <code>T</code> &#26500;&#36896;&#20989;&#25968;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> &#26500;&#36896;&#20989;&#25968;&#65292;&#21017;&#27492;&#26500;&#36896;&#20989;&#25968;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> &#26500;&#36896;&#20989;&#25968;&#12290;</li>
<li>&#27492;&#26500;&#36896;&#20989;&#25968;&#20165;&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, U<span class="sy3">&amp;&amp;</span><span class="sy1">&gt;</span></span></span> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> &#19988; <span class="t-rev-inl t-until-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span></span></span></span> <span><span class="t-mark-rev t-until-cxx20">(C++20 &#21069;)</span></span></span><span class="t-rev-inl t-since-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span></span></span></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span></span></span> &#26082;&#38750; <span class="t-lc"><a href="cpp-utility-in_place.html">std::in_place_t</a></span> &#20134;&#38750; <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">&lt;</span>T<span class="sy1">&gt;</span></span></span> &#25165;&#21442;&#19982;&#37325;&#36733;&#20915;&#35758;&#12290;</li>
<li>&#27492;&#26500;&#36896;&#20989;&#25968;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> &#33509;&#19988;&#21807;&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1">&lt;</span>U<span class="sy3">&amp;&amp;</span>, T<span class="sy1">&gt;</span></span></span> &#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> &#12290;</li>
</ul>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">&#21442;&#25968;</span></h3>
<table class="t-par-begin">
<tr class="t-par">
<td>other</td>
<td>-</td>
<td>&#35201;&#22797;&#21046;&#20854;&#25152;&#21547;&#20540;&#30340;&#21478;&#19968; <code>optional</code> &#23545;&#35937;</td>
</tr>
<tr class="t-par">
<td>value</td>
<td>-</td>
<td>&#21021;&#22987;&#21270;&#25152;&#21547;&#20540;&#25152;&#29992;&#30340;&#20540;</td>
</tr>
<tr class="t-par">
<td>args...</td>
<td>-</td>
<td>&#21021;&#22987;&#21270;&#25152;&#21547;&#20540;&#25152;&#29992;&#30340;&#21442;&#25968;</td>
</tr>
<tr class="t-par">
<td>ilist</td>
<td>-</td>
<td>&#21021;&#22987;&#21270;&#25152;&#21547;&#20540;&#25152;&#29992;&#30340; initializer_list</td>
</tr>
</table>
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">&#24322;&#24120;</span></h3>
<div class="t-li1"><span class="t-li">2)</span> &#25243;&#20986;&#20219;&#20309; <code>T</code> &#30340;&#26500;&#36896;&#20989;&#25968;&#25152;&#25243;&#30340;&#24322;&#24120;&#12290;</div>
<div class="t-li1"><span class="t-li">3)</span> &#25243;&#20986;&#20219;&#20309; <code>T</code> &#30340;&#26500;&#36896;&#20989;&#25968;&#25152;&#25243;&#30340;&#24322;&#24120;&#12290;&#25317;&#26377;&#19979;&#21015;
<div class="t-noexcept-full"><a href="cpp-language-noexcept_spec.html"><tt>noexcept</tt></a> &#35828;&#26126;&#65306;&nbsp;&nbsp;
<div class="t-noexcept-box"><span class="t-cc"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span><a href="cpp-types-is_move_constructible.html"><span class="kw542">std::<span class="me2">is_nothrow_move_constructible</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span><span class="br0">)</span></span></span></div>
</div>
</div>
<div class="t-li1"><span class="t-li">4-8)</span> &#25243;&#20986;&#20219;&#20309; <code>T</code> &#30340;&#26500;&#36896;&#20989;&#25968;&#25152;&#25243;&#30340;&#24322;&#24120;&#12290;</div>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
<div class="t-example">
<div class="t-example-live-link"></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="cpp source-cpp">
<pre class="de1"><span class="co2">#include &lt;optional&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> o1, <span class="co1">// &#31354;</span>
o2 <span class="sy1">=</span> <span class="nu0">1</span>, <span class="co1">// &#20174;&#21491;&#20540;&#21021;&#22987;&#21270;</span>
o3 <span class="sy1">=</span> o2<span class="sy4">;</span> <span class="co1">// &#22797;&#21046;&#26500;&#36896;&#20989;&#25968;</span>
<span class="co1">// &#35843;&#29992; std::string( initializer_list&lt;CharT&gt; ) &#26500;&#36896;&#20989;&#25968;</span>
<a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span> o4<span class="br0">(</span><a href="cpp-utility-in_place.html"><span class="kw1081">std::<span class="me2">in_place</span></span></a>, <span class="br0">{</span><span class="st0">&#39;a&#39;</span>, <span class="st0">&#39;b&#39;</span>, <span class="st0">&#39;c&#39;</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// &#35843;&#29992; std::string( size_type count, CharT ch ) &#26500;&#36896;&#20989;&#25968;</span>
<a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span> o5<span class="br0">(</span><a href="cpp-utility-in_place.html"><span class="kw1081">std::<span class="me2">in_place</span></span></a>, <span class="nu0">3</span>, <span class="st0">&#39;A&#39;</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// &#20174; std::string &#31227;&#21160;&#26500;&#36896;&#65292;&#29992;&#25512;&#23548;&#25351;&#24341;&#25342;&#21462;&#31867;&#22411;</span>
<a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a> o6<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="br0">{</span><span class="st0">&quot;deduction&quot;</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="sy2">*</span>o2 <span class="sy1">&lt;&lt;</span> <span class="st0">&#39; &#39;</span> <span class="sy1">&lt;&lt;</span> <span class="sy2">*</span>o3 <span class="sy1">&lt;&lt;</span> <span class="st0">&#39; &#39;</span> <span class="sy1">&lt;&lt;</span> <span class="sy2">*</span>o4 <span class="sy1">&lt;&lt;</span> <span class="st0">&#39; &#39;</span> <span class="sy1">&lt;&lt;</span> <span class="sy2">*</span>o5 <span class="sy1">&lt;&lt;</span> <span class="st0">&#39; &#39;</span> <span class="sy1">&lt;&lt;</span> <span class="sy2">*</span>o6 <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
</div>
<p>&#36755;&#20986;&#65306;</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="text source-text">
<pre class="de1">1 1 abc AAA deduction</pre></div>
</div>
</div>
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">&#32570;&#38519;&#25253;&#21578;</span></h3>
<p>&#19979;&#21015;&#26356;&#25913;&#34892;&#20026;&#30340;&#32570;&#38519;&#25253;&#21578;&#36861;&#28335;&#22320;&#24212;&#29992;&#20110;&#20197;&#21069;&#20986;&#29256;&#30340; C++ &#26631;&#20934;&#12290;</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th>DR</th>
<th>&#24212;&#29992;&#20110;</th>
<th>&#20986;&#29256;&#26102;&#30340;&#34892;&#20026;</th>
<th>&#27491;&#30830;&#34892;&#20026;</th>
</tr>
<tr>
<td><a rel="nofollow" class="external text" href="https://wg21.link/P0602R4">P0602R4</a></td>
<td>C++17</td>
<td>&#21363;&#20351;&#24213;&#23618;&#26500;&#36896;&#20989;&#25968;&#24179;&#20961;&#65292;&#22797;&#21046;/&#31227;&#21160;&#26500;&#36896;&#20989;&#25968;&#20134;&#21487;&#33021;&#19981;&#24179;&#20961;</td>
<td>&#35201;&#27714;&#20256;&#25773;&#24179;&#20961;&#24615;</td>
</tr>
<tr>
<td><a rel="nofollow" class="external text" href="https://wg21.link/P2231R1">P2231R1</a></td>
<td>C++20</td>
<td>&#28304;&#33258;&#21478;&#19968; <code>optional</code> &#30340;&#36716;&#25442;&#26500;&#36896;&#20989;&#25968;&#19981;&#26159; constexpr &#32780;&#35201;&#27714;&#30340;&#25805;&#20316;&#22312; C++20 &#20013;&#33021;&#26159;</td>
<td>&#20351;&#20043;&#20026; constexpr</td>
</tr>
</table>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">&#21442;&#38405;</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-utility-optional-make_optional.html"><span class="t-lines"><span>make_optional</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
</div>
</td>
<td>&#21019;&#24314;&#19968;&#20010; <code>optional</code> &#23545;&#35937;<br />
<span class="t-mark">(&#20989;&#25968;&#27169;&#26495;)</span></td>
</tr>
</table>
</div>
<div class="visualClear"></div>
</div>
</div>
</div>
</body>
</html>
Reference in New Issue View Git Blame Copy Permalink