176 lines
14 KiB
HTML
176 lines
14 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<title>std::ranges::construct_at</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_ranges_construct_at 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::ranges::</span>construct_at</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-memory.html">memory</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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<div>定义于头文件 <code><a href="cpp-header-memory.html"><memory></a></code></div>
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<td></td>
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<div>调用签名</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="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> T<span class="sy2">*</span> construct_at<span class="br0">(</span> T<span class="sy2">*</span> p, 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 class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<td></td>
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<p>在给定地址 <code>p</code> 创建以参数 <code>args...</code> 初始化的 <code>T</code> 对象。 <code>construct_at</code> 仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span><span class="me2">new</span><span class="br0">(</span><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span><span class="kw4">void</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> T<span class="br0">(</span><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>...<span class="br0">)</span></span></span> 在不求值语境中为良构才参与重载决议。</p>
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<p>等价于</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">return</span> <span class="sy4">::</span><span class="me2">new</span> <span class="br0">(</span><span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">void</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span><span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">const</span> <span class="kw4">volatile</span> <span class="kw4">void</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>p<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span>
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T<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>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...<span class="br0">)</span><span class="sy4">;</span></pre></div>
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<p>除了 <code>construct_at</code> 可用于<a href="cpp-language-constant_expression.html">常量表达式</a>的求值。</p>
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<p>在某常量表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 的求值中调用 <code>construct_at</code> 时，参数 <code>p</code> 必须指向用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-allocator.html"><span class="kw700">std::<span class="me2">allocator</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">allocate</span></span></span> 获得的存储或生存期始于 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 的求值内的对象。</p>
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<p>此页面上描述的仿函数实体是 <i>niebloid</i> ，即：</p>
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<ul>
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<li>调用它们中的任何一者时不可指定显式模板实参。</li>
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<li>它们无一对<a href="cpp-language-adl.html">实参依赖查找</a>可见。</li>
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<li>在<a href="cpp-language-unqualified_lookup.html">通常无限定查找</a>寻找函数调用运算符左侧的名称找到它们之一时，它禁止<a href="cpp-language-adl.html">实参依赖查找</a>。</li>
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</ul>
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<p>实际上，它们能以函数对象，或以某些特殊编译器扩展实现。</p>
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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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<tr class="t-par">
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<td>p</td>
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<td>-</td>
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<td>指向将在其上构造 <code>T</code> 对象的未初始化存储的指针</td>
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</tr>
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<tr class="t-par">
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<td>args...</td>
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<td>-</td>
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<td>用于初始化的参数</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><code>p</code></p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<table class="eq-fun-cpp-table">
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">struct</span> construct_at_fn <span class="br0">{</span>
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<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>
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requires
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requires <span class="br0">(</span><span class="kw4">void</span><span class="sy2">*</span> vp, Args<span class="sy3">&&</span>... <span class="me1">args</span><span class="br0">)</span> <span class="br0">{</span> <span class="sy4">::</span><span class="me2">new</span> <span class="br0">(</span>vp<span class="br0">)</span> T<span class="br0">(</span><span class="kw1">static_cast</span><span class="sy1"><</span>Args<span class="sy3">&&</span><span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">constexpr</span> T<span class="sy2">*</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>T<span class="sy2">*</span> p, Args<span class="sy3">&&</span>... <span class="me1">args</span><span class="br0">)</span> <span class="kw4">const</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">construct_at</span><span class="br0">(</span>p, <span class="kw1">static_cast</span><span class="sy1"><</span>Args<span class="sy3">&&</span><span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> construct_at_fn construct_at<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></pre></div>
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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><code>std::ranges::construct_at</code> 与 <code>std::construct_at</code> 表现准确相同，除了它对实参依赖查找不可见。</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="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <memory></span>
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<span class="kw1">struct</span> S <span class="br0">{</span>
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<span class="kw4">int</span> x<span class="sy4">;</span>
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<span class="kw4">float</span> y<span class="sy4">;</span>
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<span class="kw4">double</span> z<span class="sy4">;</span>
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S<span class="br0">(</span><span class="kw4">int</span> x, <span class="kw4">float</span> y, <span class="kw4">double</span> z<span class="br0">)</span> <span class="sy4">:</span> x<span class="br0">{</span>x<span class="br0">}</span>, y<span class="br0">{</span>y<span class="br0">}</span>, z<span class="br0">{</span>z<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"><<</span> <span class="st0">"S::S();<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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~S<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"><<</span> <span class="st0">"S::~S();<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">void</span> print<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"S { x="</span> <span class="sy1"><<</span> x <span class="sy1"><<</span> <span class="st0">"; y="</span> <span class="sy1"><<</span> y <span class="sy1"><<</span> <span class="st0">"; z="</span> <span class="sy1"><<</span> z <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>
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<span class="br0">}</span><span class="sy4">;</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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alignas<span class="br0">(</span>S<span class="br0">)</span> <span class="kw4">unsigned</span> <span class="kw4">char</span> buf<span class="br0">[</span>sizeof<span class="br0">(</span>S<span class="br0">)</span><span class="br0">]</span><span class="sy4">;</span>
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S<span class="sy2">*</span> ptr <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">construct_at</span><span class="br0">(</span><span class="kw1">reinterpret_cast</span><span class="sy1"><</span>S<span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>buf<span class="br0">)</span>, <span class="nu0">42</span>, <span class="nu17">2.71828f</span>, <span class="nu16">3.1415</span><span class="br0">)</span><span class="sy4">;</span>
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ptr<span class="sy2">-</span><span class="sy1">></span>print<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">destroy_at</span><span class="br0">(</span>ptr<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>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">S::S();
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S { x=42; y=2.71828; z=3.1415; };
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S::~S();</pre></div>
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</div>
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</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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<table class="t-dsc-begin">
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-ranges-destroy_at.html"><span class="t-lines"><span>ranges::destroy_at</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>销毁位于给定地址的元素<br />
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<span class="t-mark">(niebloid)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-construct_at.html"><span class="t-lines"><span>construct_at</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>在给定地址创建对象<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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