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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>make_optional</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>&#23450;&#20041;&#20110;&#22836;&#25991;&#20214; <code><a href="cpp-header-optional.html">&lt;optional&gt;</a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> T <span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</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-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;&gt;</span> make_optional<span class="br0">(</span> T<span class="sy3">&amp;&amp;</span> value <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> T, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> <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> make_optional<span class="br0">(</span> Args<span class="sy3">&amp;&amp;</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> T, <span class="kw1">class</span> U, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> <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> make_optional<span class="br0">(</span> <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> il, Args<span class="sy3">&amp;&amp;</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(3)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> &#20174;<code>value</code> &#21019;&#24314; <code>optional</code> &#23545;&#35937;&#12290;&#31561;&#25928;&#22320;&#35843;&#29992; <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><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;&gt;</span><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1116">std::<span class="me2">forward</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="br0">(</span>value<span class="br0">)</span><span class="br0">)</span></span></span> &#12290;</div>
<div class="t-li1"><span class="t-li">2)</span> &#20174; <code>args...</code> &#21019;&#24314;&#21407;&#20301;&#26500;&#36896;&#30340; <code>optional</code> &#23545;&#35937;&#12290;&#31561;&#20215;&#20110; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> <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 class="br0">(</span><a href="cpp-utility-in_place.html"><span class="kw1080">std::<span class="me2">in_place</span></span></a>, <a href="cpp-utility-forward.html"><span class="kw1116">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 class="br0">)</span><span class="sy4">;</span></span></span> &#12290;</div>
<div class="t-li1"><span class="t-li">3)</span> &#20174; <code>il</code> &#21644; <code>args...</code> &#21019;&#24314;&#21407;&#20301;&#26500;&#36896;&#30340; <code>optional</code> &#23545;&#35937;&#12290;&#31561;&#20215;&#20110; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> <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 class="br0">(</span><a href="cpp-utility-in_place.html"><span class="kw1080">std::<span class="me2">in_place</span></span></a>, il, <a href="cpp-utility-forward.html"><span class="kw1116">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 class="br0">)</span><span class="sy4">;</span></span></span> &#12290;</div>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">&#21442;&#25968;</span></h3>
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<td>value</td>
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<td>&#26500;&#36896; <code>optional</code> &#23545;&#35937;&#25152;&#29992;&#30340;&#20540;</td>
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<td>il, args</td>
<td>-</td>
<td>&#20256;&#36882;&#32473; <code>T</code> &#26500;&#36896;&#20989;&#25968;&#30340;&#21442;&#25968;&#12290;</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">&#36820;&#22238;&#20540;</span></h3>
<p>&#26500;&#36896;&#30340; <code>optional</code> &#23545;&#35937;&#12290;</p>
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">&#24322;&#24120;</span></h3>
<p>&#25243;&#20986;&#20219;&#20309; <code>T</code> &#30340;&#26500;&#36896;&#20989;&#25968;&#25152;&#25243;&#30340;&#24322;&#24120;&#12290;</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">&#27880;&#35299;</span></h3>
<p>&#23545;&#20110;&#37325;&#36733; <span class="t-v">(2-3)</span> <code>T</code> &#19981;&#38656;&#35201;&#21487;&#31227;&#21160;&#65292;&#22240;&#20026;&#21463;&#20445;&#35777;&#30340;&#22797;&#21046;&#28040;&#38500;&#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="co2">#include &lt;iomanip&gt;</span>
<span class="co2">#include &lt;vector&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>
<span class="kw4">auto</span> op1 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">make_optional</span><span class="sy1">&lt;</span><a href="cpp-container-vector.html"><span class="kw1268">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;&gt;</span><span class="br0">(</span><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>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;op1: &quot;</span><span class="sy4">;</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">char</span> c<span class="sy4">:</span> op1.<span class="me1">value</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">{</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> c <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;,&quot;</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">auto</span> op2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">make_optional</span><span class="sy1">&lt;</span><a href="cpp-container-vector.html"><span class="kw1268">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;&gt;</span><span class="br0">(</span><span class="nu0">5</span>, <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;<span class="es1">\n</span>op2: &quot;</span><span class="sy4">;</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i<span class="sy4">:</span> <span class="sy2">*</span>op2<span class="br0">)</span><span class="br0">{</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> i <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;,&quot;</span><span class="sy4">;</span>
<span class="br0">}</span>
<a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a> str<span class="br0">{</span><span class="st0">&quot;hello world&quot;</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">auto</span> op3 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">make_optional</span><span class="sy1">&lt;</span><a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span><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="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;<span class="es1">\n</span>op3: &quot;</span> <span class="sy1">&lt;&lt;</span> quoted<span class="br0">(</span>op3.<span class="me1">value_or</span><span class="br0">(</span><span class="st0">&quot;empty value&quot;</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;str: &quot;</span> <span class="sy1">&lt;&lt;</span> <a href="cpp-io-manip-quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>str<span class="br0">)</span> <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>
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<p>&#21487;&#33021;&#30340;&#36755;&#20986;&#65306;</p>
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<pre class="de1">op1: a,b,c,
op2: 2,2,2,2,2,
op3: &quot;hello world&quot;
str: &quot;&quot;</pre></div>
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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-optional.html"><span class="t-lines"><span>(&#26500;&#36896;&#20989;&#25968;)</span></span></a></div>
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<td>&#26500;&#36896;<code>optional</code>&#23545;&#35937;<br />
<span class="t-mark">(&#20844;&#24320;&#25104;&#21592;&#20989;&#25968;)</span></td>
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