323 lines
19 KiB
HTML
323 lines
19 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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<title>std::ranges::views::empty, std::ranges::empty_view</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_empty_view 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><span style="font-size:0.7em; line-height:130%">views::</span>empty, <span style="font-size:0.7em; line-height:130%">std::ranges::</span>empty_view</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-ranges.html">ranges</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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<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 /></span>
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<p> requires <a href="cpp-types-is_object.html"><span class="kw487">std::<span class="me2">is_object_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><br /></p>
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<span class="kw1">class</span> empty_view <span class="sy4">:</span> <span class="kw1">public</span> ranges<span class="sy4">::</span><span class="me2">view_interface</span><span class="sy1"><</span>empty_view<span class="sy1"><</span>T<span class="sy1">>></span></div>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">namespace</span> views <span class="br0">{</span><br /></span>
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<p> <span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw1">inline</span> <span class="kw4">constexpr</span> empty_view<span class="sy1"><</span>T<span class="sy1">></span> empty<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span><br /></p>
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<span class="br0">}</span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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<div class="t-li1"><span class="t-li">1)</span> 产生无特定类型元素的 <a href="cpp-ranges-view.html"><code>view</code></a> 的范围工厂。</div>
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<div class="t-li1"><span class="t-li">2)</span> <code>empty_view</code> 的变量模板。</div>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
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<table class="t-dsc-begin">
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<div><span class="t-lines"><span>begin</span></span></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</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>返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">nullptr</span></span><br />
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<span class="t-mark">(公开静态成员函数)</span></td>
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</tr>
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<div class="t-dsc-member-div">
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<div><span class="t-lines"><span>end</span></span></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</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>返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">nullptr</span></span><br />
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<span class="t-mark">(公开静态成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><span class="t-lines"><span>data</span></span></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</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>返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">nullptr</span></span><br />
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<span class="t-mark">(公开静态成员函数)</span></td>
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<div class="t-dsc-member-div">
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<div><span class="t-lines"><span>size</span></span></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</span> <span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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<td>返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> （零）<br />
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<span class="t-mark">(公开静态成员函数)</span></td>
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<div><span class="t-lines"><span>empty</span></span></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</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>返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span><br />
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<span class="t-mark">(公开静态成员函数)</span></td>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E7.BB.A7.E6.89.BF.E8.87.AA_std::ranges::view_interface">继承自 <span class="t-lc">std::ranges::view_interface</span></span></h5>
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<div><a href="cpp-ranges-view_interface-operator_bool.html"><span class="t-lines"><span>operator bool</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>返回派生视图是否为非空。若 <span class="t-lc">ranges::empty</span> 可应用于它则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</span></td>
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-view_interface-front.html"><span class="t-lines"><span>front</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>返回派生视图中的首元素。若视图满足 <a href="cpp-ranges-forward_range.html"><code>forward_range</code></a> 则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</span></td>
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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-ranges-view_interface-back.html"><span class="t-lines"><span>back</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>返回派生视图中的末元素。若视图满足 <a href="cpp-ranges-bidirectional_range.html"><code>bidirectional_range</code></a> 与 <a href="cpp-ranges-common_range.html"><code>common_range</code></a> 则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-view_interface-operator_at.html"><span class="t-lines"><span>operator[]</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>返回派生视图中的第 n 个元素。若视图满足 <a href="cpp-ranges-random_access_range.html"><code>random_access_range</code></a> 则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</span></td>
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</table>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::ranges::empty_view::begin"><span style="font-size:0.7em; line-height:130%">std::ranges::empty_view::</span>begin</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span> <span class="kw4">constexpr</span> T<span class="sy2">*</span> begin<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> nullptr<span class="sy4">;</span> <span class="br0">}</span></span></div>
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</td>
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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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<tr class="t-dcl-sep">
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<td></td>
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<p><code>empty_view</code> 不引用任何元素。</p>
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</div>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::ranges::empty_view::end"><span style="font-size:0.7em; line-height:130%">std::ranges::empty_view::</span>end</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span> <span class="kw4">constexpr</span> T<span class="sy2">*</span> end<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> nullptr<span class="sy4">;</span> <span class="br0">}</span></span></div>
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</td>
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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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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<p><code>empty_view</code> 不引用任何元素。</p>
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</div>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::ranges::empty_view::data"><span style="font-size:0.7em; line-height:130%">std::ranges::empty_view::</span>data</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span> <span class="kw4">constexpr</span> T<span class="sy2">*</span> data<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> nullptr<span class="sy4">;</span> <span class="br0">}</span></span></div>
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</td>
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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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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<p><code>empty_view</code> 不引用任何元素。</p>
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</div>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::ranges::empty_view::size"><span style="font-size:0.7em; line-height:130%">std::ranges::empty_view::</span>size</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span> <span class="kw4">constexpr</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> size<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="br0">}</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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<tr class="t-dcl-sep">
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<td></td>
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<p><code>empty_view</code> 始终为空。</p>
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</div>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::ranges::empty_view::empty"><span style="font-size:0.7em; line-height:130%">std::ranges::empty_view::</span>empty</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> empty<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span> <span class="br0">}</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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<td></td>
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<p><code>empty_view</code> 始终为空。</p>
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</div>
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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E6.A8.A1.E6.9D.BF">辅助模板</span></h3>
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<table class="t-dcl-begin">
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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="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> enable_borrowed_range<span class="sy1"><</span>ranges<span class="sy4">::</span><span class="me2">empty_view</span><span class="sy1"><</span>T<span class="sy1">>></span> <span class="sy1">=</span> <span class="kw2">true</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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<p><a href="cpp-ranges-borrowed_range.html"><tt>std::ranges::enable_borrowed_range</tt></a> 的此特化使得 <code>empty_view</code> 满足 <a href="cpp-ranges-borrowed_range.html"><code>borrowed_range</code></a> 。</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>尽管 <code>empty_view</code> 从 <code>view_interface</code> 获得成员函数 <code>front</code>、 <code>back</code> 及 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="br0">[</span><span class="br0">]</span></span></span> ，对它们的调用始终导致未定义行为，因为 <code>empty_view</code> 始终为空。</p>
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<p>继承的 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator <span class="kw4">bool</span></span></span> 转换函数始终返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</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 <ranges></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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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">empty_view</span><span class="sy1"><</span><span class="kw4">long</span><span class="sy1">></span> e<span class="sy4">;</span>
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static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">empty</span><span class="br0">(</span>e<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span><span class="nu0">0</span> <span class="sy1">==</span> e.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>nullptr <span class="sy1">==</span> e.<span class="me1">data</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>nullptr <span class="sy1">==</span> e.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>nullptr <span class="sy1">==</span> e.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><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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</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-ranges-single_view.html"><span class="t-lines"><span>ranges::single_view</span><span>views::single</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>
|
|
</td>
|
|
<td>含有具有指定值的单个元素的 <a href="cpp-ranges-view.html"><code>view</code></a><br />
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|
<span class="t-mark">(类模板)</span> <span class="t-mark">(定制点对象)</span></td>
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|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
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|
<div class="t-dsc-member-div">
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|
<div><a href="cpp-ranges-all_view.html"><span class="t-lines"><span>views::all_t</span><span>views::all</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>
|
|
</td>
|
|
<td>包含 <a href="cpp-ranges-range.html"><code>range</code></a> 的所有元素的 <a href="cpp-ranges-view.html"><code>view</code></a><br />
|
|
<span class="t-mark">(别名模板)</span> <span class="t-mark">(范围适配器对象)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-ranges-ref_view.html"><span class="t-lines"><span>ranges::ref_view</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>某个其他 <a href="cpp-ranges-range.html"><code>range</code></a> 的元素的 <a href="cpp-ranges-view.html"><code>view</code></a><br />
|
|
<span class="t-mark">(类模板)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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</html>
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