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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::optional&lt;T&gt;::</span>reset</h1>
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<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>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> reset<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<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></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> <span class="kw4">void</span> reset<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span></td>
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<p>&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> &#21547;&#20540;&#65292;&#21017;&#22914;&#21516;&#29992; <span class="t-c"><span class="mw-geshi cpp source-cpp">value<span class="br0">(</span><span class="br0">)</span>.<span class="me1">T</span><span class="sy4">::</span>~T<span class="br0">(</span><span class="br0">)</span></span></span> &#38144;&#27585;&#27492;&#20540;&#12290;&#21542;&#21017;&#26080;&#25928;&#26524;&#12290;</p>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> &#22312;&#27492;&#35843;&#29992;&#21518;&#19981;&#21547;&#20540;&#12290;</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
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<pre class="de1"><span class="co2">#include &lt;optional&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="kw1">struct</span> A <span class="br0">{</span>
<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span>
A<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> str<span class="br0">)</span> <span class="sy4">:</span> s<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>str<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</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="st0">&quot; constructed<span class="es1">\n</span>&quot;</span><span class="sy4">;</span> <span class="br0">}</span>
~A<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</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="st0">&quot; destructed<span class="es1">\n</span>&quot;</span><span class="sy4">;</span> <span class="br0">}</span>
A<span class="br0">(</span><span class="kw4">const</span> A<span class="sy3">&amp;</span> o<span class="br0">)</span> <span class="sy4">:</span> s<span class="br0">(</span>o.<span class="me1">s</span><span class="br0">)</span> <span class="br0">{</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="st0">&quot; copy constructed<span class="es1">\n</span>&quot;</span><span class="sy4">;</span> <span class="br0">}</span>
A<span class="br0">(</span>A<span class="sy3">&amp;&amp;</span> o<span class="br0">)</span> <span class="sy4">:</span> s<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>o.<span class="me1">s</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</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="st0">&quot; move constructed<span class="es1">\n</span>&quot;</span><span class="sy4">;</span> <span class="br0">}</span>
A<span class="sy3">&amp;</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> A<span class="sy3">&amp;</span> other<span class="br0">)</span> <span class="br0">{</span>
s <span class="sy1">=</span> other.<span class="me1">s</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="st0">&quot; copy assigned<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span>
<span class="br0">}</span>
A<span class="sy3">&amp;</span> operator<span class="sy1">=</span><span class="br0">(</span>A<span class="sy3">&amp;&amp;</span> other<span class="br0">)</span> <span class="br0">{</span>
s <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>other.<span class="me1">s</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="st0">&quot; move assigned<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</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="st0">&quot;Create empty optional:<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1">&lt;</span>A<span class="sy1">&gt;</span> opt<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="st0">&quot;Construct and assign value:<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
opt <span class="sy1">=</span> A<span class="br0">(</span><span class="st0">&quot;Lorem ipsum dolor sit amet, consectetur adipiscing elit nec.&quot;</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="st0">&quot;Reset optional:<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
opt.<span class="me1">reset</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="st0">&quot;End example<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
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<p>&#36755;&#20986;&#65306;</p>
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<pre class="de1">Create empty optional:
Construct and assign value:
constructed
move constructed
destructed
Reset optional:
destructed
End example</pre></div>
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<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>
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<th>DR</th>
<th>&#24212;&#29992;&#20110;</th>
<th>&#20986;&#29256;&#26102;&#30340;&#34892;&#20026;</th>
<th>&#27491;&#30830;&#34892;&#20026;</th>
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<td><a rel="nofollow" class="external text" href="https://wg21.link/P2231R1">P2231R1</a></td>
<td>C++20</td>
<td><code>reset</code> &#19981;&#26159; constexpr &#32780; C++20 &#20013;&#20801;&#35768;&#38750;&#24179;&#20961;&#26512;&#26500;&#22312; constexpr &#20013;</td>
<td>&#20351;&#20043;&#20026; constexpr</td>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">&#21442;&#38405;</span></h3>
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<div><a href="cpp-utility-optional-operator=.html"><span class="t-lines"><span>operator=</span></span></a></div>
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<td>&#23545;&#20869;&#23481;&#36171;&#20540;<br />
<span class="t-mark">(&#20844;&#24320;&#25104;&#21592;&#20989;&#25968;)</span></td>
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