135 lines
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135 lines
13 KiB
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<title>std::make_optional</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_optional_make_optional 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::</span>make_optional</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>定义于头文件 <code><a href="cpp-header-optional.html"><optional></a></code></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> T <span class="sy1">></span><br />
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<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"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>T<span class="sy1">>></span> make_optional<span class="br0">(</span> T<span class="sy3">&&</span> value <span class="br0">)</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-cxx17">(C++17 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> T, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br />
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<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"><</span>T<span class="sy1">></span> make_optional<span class="br0">(</span> Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</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-cxx17">(C++17 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> T, <span class="kw1">class</span> U, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br />
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<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"><</span>T<span class="sy1">></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"><</span>U<span class="sy1">></span> il, Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</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-cxx17">(C++17 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 从<code>value</code> 创建 <code>optional</code> 对象。等效地调用 <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><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>T<span class="sy1">>></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"><</span>T<span class="sy1">></span><span class="br0">(</span>value<span class="br0">)</span><span class="br0">)</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 从 <code>args...</code> 创建原位构造的 <code>optional</code> 对象。等价于 <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"><</span>T<span class="sy1">></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"><</span>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...<span class="br0">)</span><span class="sy4">;</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">3)</span> 从 <code>il</code> 和 <code>args...</code> 创建原位构造的 <code>optional</code> 对象。等价于 <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"><</span>T<span class="sy1">></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"><</span>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...<span class="br0">)</span><span class="sy4">;</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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<td>value</td>
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<td>-</td>
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<td>构造 <code>optional</code> 对象所用的值</td>
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<td>il, args</td>
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<td>-</td>
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<td>传递给 <code>T</code> 构造函数的参数。</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>构造的 <code>optional</code> 对象。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>抛出任何 <code>T</code> 的构造函数所抛的异常。</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>对于重载 <span class="t-v">(2-3)</span> <code>T</code> 不需要可移动，因为受保证的复制消除。</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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<div class="t-example">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <optional></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <iomanip></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <string></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw4">auto</span> op1 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">make_optional</span><span class="sy1"><</span><a href="cpp-container-vector.html"><span class="kw1268">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">char</span><span class="sy1">>></span><span class="br0">(</span><span class="br0">{</span><span class="st0">'a'</span>,<span class="st0">'b'</span>,<span class="st0">'c'</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"op1: "</span><span class="sy4">;</span>
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<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>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> c <span class="sy1"><<</span> <span class="st0">","</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">auto</span> op2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">make_optional</span><span class="sy1"><</span><a href="cpp-container-vector.html"><span class="kw1268">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">>></span><span class="br0">(</span><span class="nu0">5</span>, <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>op2: "</span><span class="sy4">;</span>
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<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>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> i <span class="sy1"><<</span> <span class="st0">","</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<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">"hello world"</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> op3 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">make_optional</span><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">></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>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>op3: "</span> <span class="sy1"><<</span> quoted<span class="br0">(</span>op3.<span class="me1">value_or</span><span class="br0">(</span><span class="st0">"empty value"</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"str: "</span> <span class="sy1"><<</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"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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<p>可能的输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">op1: a,b,c,
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op2: 2,2,2,2,2,
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op3: "hello world"
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str: ""</pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-utility-optional-optional.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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</td>
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<td>构造<code>optional</code>对象<br />
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<span class="t-mark">(公开成员函数)</span></td>
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