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<!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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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::empty</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_empty skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>empty</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-iterator.html">iterator</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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<tbody>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-array.html"><array></a></code></div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-deque.html"><deque></a></code></div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-forward_list.html"><forward_list></a></code></div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-iterator.html"><iterator></a></code></div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-list.html"><list></a></code></div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-map.html"><map></a></code></div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-regex.html"><regex></a></code></div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-set.html"><set></a></code></div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-span.html"><span></a></code></div>
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</td>
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<td></td>
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<td>
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<div><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></div>
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</td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-string.html"><string></a></code></div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-string_view.html"><string_view></a></code></div>
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</td>
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<td></td>
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<td>
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<div><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></div>
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</td>
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</tr>
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<tr class="t-dsc-header">
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||||
<td>
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||||
<div>定义于头文件 <code><a href="cpp-header-unordered_map.html"><unordered_map></a></code></div>
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</td>
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||||
<td></td>
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||||
<td></td>
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</tr>
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||||
<tr class="t-dsc-header">
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||||
<td>
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||||
<div>定义于头文件 <code><a href="cpp-header-unordered_set.html"><unordered_set></a></code></div>
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</td>
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<td></td>
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<td></td>
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</tr>
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||||
<tr class="t-dsc-header">
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||||
<td>
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||||
<div>定义于头文件 <code><a href="cpp-header-vector.html"><vector></a></code></div>
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||||
</td>
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||||
<td></td>
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||||
<td></td>
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</tr>
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</tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(1)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-since-cxx17 t-until-cxx20">
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<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> C<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> empty<span class="br0">(</span><span class="kw4">const</span> C<span class="sy3">&</span> c<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> decltype<span class="br0">(</span>c.<span class="me1">empty</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</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-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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<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> C<span class="sy1">></span><br />
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<span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> empty<span class="br0">(</span><span class="kw4">const</span> C<span class="sy3">&</span> c<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> decltype<span class="br0">(</span>c.<span class="me1">empty</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</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>
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</tbody>
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<tbody></tbody>
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||||
<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(2)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-since-cxx17 t-until-cxx20">
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<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, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> empty<span class="br0">(</span><span class="kw4">const</span> T <span class="br0">(</span><span class="sy3">&</span>array<span class="br0">)</span><span class="br0">[</span>N<span class="br0">]</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</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-cxx17">(C++17 起)</span><br />
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||||
<span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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||||
<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, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N<span class="sy1">></span><br />
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||||
<span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> empty<span class="br0">(</span><span class="kw4">const</span> T <span class="br0">(</span><span class="sy3">&</span>array<span class="br0">)</span><span class="br0">[</span>N<span class="br0">]</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</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>
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</tbody>
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<tbody></tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(3)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-since-cxx17 t-until-cxx20">
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<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> E<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> empty<span class="br0">(</span><a href="cpp-utility-initializer_list.html"><span class="kw959">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</span>E<span class="sy1">></span> il<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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||||
</td>
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||||
<td class="t-dcl-nopad"></td>
|
||||
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
|
||||
<span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
|
||||
</tr>
|
||||
<tr class="t-dcl t-since-cxx20">
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||||
<td>
|
||||
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> E<span class="sy1">></span><br />
|
||||
<span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> empty<span class="br0">(</span><a href="cpp-utility-initializer_list.html"><span class="kw959">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</span>E<span class="sy1">></span> il<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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||||
</td>
|
||||
<td class="t-dcl-nopad"></td>
|
||||
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
|
||||
</tr>
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</tbody>
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||||
<tbody>
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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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</tr>
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</tbody>
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</table>
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<p>返回给定的范围是否为空。</p>
|
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<div class="t-li1"><span class="t-li">1)</span> 返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">c.<span class="me1">empty</span><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> 返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span></div>
|
||||
<div class="t-li1"><span class="t-li">3)</span> 返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">il.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span></span></span></div>
|
||||
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
|
||||
<table class="t-par-begin">
|
||||
<tr class="t-par">
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<td>c</td>
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<td>-</td>
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||||
<td>拥有 <code>empty</code> 成员函数的容器或视图</td>
|
||||
</tr>
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||||
<tr class="t-par">
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||||
<td>array</td>
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<td>-</td>
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<td>任意类型的数组</td>
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||||
</tr>
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||||
<tr class="t-par">
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<td>il</td>
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<td>-</td>
|
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<td>一个 initializer_list</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
|
||||
<p>若范围不含有任何元素则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</p>
|
||||
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
|
||||
<div class="t-li1"><span class="t-li">1)</span> 可能抛出实现定义的异常。</div>
|
||||
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
||||
<p>需要对 <span class="t-lc"><a href="cpp-utility-initializer_list.html">std::initializer_list</a></span> 的重载，因为它无成员函数 <code>empty</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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||||
<tr>
|
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<th>版本一</th>
|
||||
</tr>
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<tr>
|
||||
<td>
|
||||
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
||||
<div class="cpp source-cpp">
|
||||
<pre class="de1"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> C<span class="sy1">></span>
|
||||
<span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> empty<span class="br0">(</span><span class="kw4">const</span> C<span class="sy3">&</span> c<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> decltype<span class="br0">(</span>c.<span class="me1">empty</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>
|
||||
<span class="br0">{</span>
|
||||
<span class="kw1">return</span> c.<span class="me1">empty</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
||||
<span class="br0">}</span></pre></div>
|
||||
</div>
|
||||
</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<th>版本二</th>
|
||||
</tr>
|
||||
<tr>
|
||||
<td>
|
||||
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
||||
<div class="cpp source-cpp">
|
||||
<pre class="de1"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N<span class="sy1">></span>
|
||||
<span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> empty<span class="br0">(</span><span class="kw4">const</span> T <span class="br0">(</span><span class="sy3">&</span>array<span class="br0">)</span><span class="br0">[</span>N<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">false</span><span class="sy4">;</span>
|
||||
<span class="br0">}</span></pre></div>
|
||||
</div>
|
||||
</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<th>版本三</th>
|
||||
</tr>
|
||||
<tr>
|
||||
<td>
|
||||
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
||||
<div class="cpp source-cpp">
|
||||
<pre class="de1"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> E<span class="sy1">></span>
|
||||
<span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> empty<span class="br0">(</span><a href="cpp-utility-initializer_list.html"><span class="kw959">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</span>E<span class="sy1">></span> il<span class="br0">)</span> <span class="kw1">noexcept</span>
|
||||
<span class="br0">{</span>
|
||||
<span class="kw1">return</span> il.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span><span class="sy4">;</span>
|
||||
<span class="br0">}</span></pre></div>
|
||||
</div>
|
||||
</td>
|
||||
</tr>
|
||||
</table>
|
||||
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
||||
<div class="t-example">
|
||||
<div class="t-example-live-link"></div>
|
||||
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
||||
<div class="cpp source-cpp">
|
||||
<pre class="de1"><span class="co2">#include <iostream></span>
|
||||
<span class="co2">#include <vector></span>
|
||||
|
||||
<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
|
||||
<span class="kw4">void</span> print<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> container<span class="br0">)</span>
|
||||
<span class="br0">{</span>
|
||||
<span class="kw1">if</span> <span class="br0">(</span> <span class="sy3">!</span>std<span class="sy4">::</span><span class="me2">empty</span><span class="br0">(</span>container<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"><<</span> <span class="st0">"Elements:<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
||||
<span class="kw1">for</span> <span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&</span> element <span class="sy4">:</span> container <span class="br0">)</span>
|
||||
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> element <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
||||
<span class="br0">}</span>
|
||||
<span class="kw1">else</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"><<</span> <span class="st0">"Empty<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
||||
<span class="br0">}</span>
|
||||
<span class="br0">}</span>
|
||||
|
||||
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
||||
<span class="br0">{</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> c <span class="sy1">=</span> <span class="br0">{</span> <span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span> <span class="br0">}</span><span class="sy4">;</span>
|
||||
print<span class="br0">(</span>c<span class="br0">)</span><span class="sy4">;</span>
|
||||
c.<span class="me1">clear</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
||||
print<span class="br0">(</span>c<span class="br0">)</span><span class="sy4">;</span>
|
||||
|
||||
<span class="kw4">int</span> array<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span> <span class="nu0">4</span>, <span class="nu0">5</span>, <span class="nu0">6</span> <span class="br0">}</span><span class="sy4">;</span>
|
||||
print<span class="br0">(</span>array<span class="br0">)</span><span class="sy4">;</span>
|
||||
|
||||
<span class="kw4">auto</span> il <span class="sy1">=</span> <span class="br0">{</span> <span class="nu0">7</span>, <span class="nu0">8</span>, <span class="nu0">9</span> <span class="br0">}</span><span class="sy4">;</span>
|
||||
print<span class="br0">(</span>il<span class="br0">)</span><span class="sy4">;</span>
|
||||
<span class="br0">}</span></pre></div>
|
||||
</div>
|
||||
<p>输出：</p>
|
||||
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
||||
<div class="text source-text">
|
||||
<pre class="de1">Elements:
|
||||
1
|
||||
2
|
||||
3
|
||||
Empty
|
||||
Elements:
|
||||
4
|
||||
5
|
||||
6
|
||||
Elements:
|
||||
7
|
||||
8
|
||||
9</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
||||
<table class="t-dsc-begin">
|
||||
<tr class="t-dsc">
|
||||
<td>
|
||||
<div class="t-dsc-member-div">
|
||||
<div><a href="cpp-ranges-empty.html"><span class="t-lines"><span>ranges::empty</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>检查范围是否为空<br />
|
||||
<span class="t-mark">(定制点对象)</span></td>
|
||||
</tr>
|
||||
</table>
|
||||
</div>
|
||||
<div class="visualClear"></div>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
Reference in New Issue
Block a user