157 lines
19 KiB
HTML
157 lines
19 KiB
HTML
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<title>std::ranges::borrowed_range, std::ranges::enable_borrowed_range</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ranges::</span>borrowed_range, <span style="font-size:0.7em; line-height:130%">std::ranges::</span>enable_borrowed_range</h1>
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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>定义于头文件 <code><a href="cpp-header-ranges.html"><ranges></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> R<span class="sy1">></span><br /></span>
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<p>concept borrowed_range <span class="sy1">=</span><br />
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ranges<span class="sy4">::</span><span class="me2">range</span><span class="sy1"><</span>R<span class="sy1">></span> <span class="sy3">&&</span><br />
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<span class="br0">(</span><a href="cpp-types-is_lvalue_reference.html"><span class="kw465">std::<span class="me2">is_lvalue_reference_v</span></span></a><span class="sy1"><</span>R<span class="sy1">></span> <span class="sy3">||</span><br /></p>
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ranges<span class="sy4">::</span><span class="me2">enable_borrowed_range</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>R<span class="sy1">>></span><span class="br0">)</span><span class="sy4">;</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">template</span><span class="sy1"><</span><span class="kw1">class</span> R<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> <span class="kw2">false</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 概念 <code>borrowed_range</code> 定义范围的要求，使得函数能按值接收它，并返回从它获得的迭代器，而无悬垂之虞。</div>
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<div class="t-li1"><span class="t-li">2)</span> <code>enable_borrowed_range</code> 变量模板用于指示一个 <a href="cpp-ranges-range.html"><code>range</code></a> 是否为 <code>borrowed_range</code> 。主模板定义为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</div>
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<h3><span class="mw-headline" id=".E8.AF.AD.E4.B9.89.E8.A6.81.E6.B1.82">语义要求</span></h3>
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<p>若 <code>T</code> 为右值引用类型则令 <code>U</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> ，否则为 <code>T</code> 。给定 <code>U</code> 类型变量 <code>u</code> ， <code>T</code> 实现 <code>borrowed_range</code> 仅若从 <code>u</code> 获得的迭代器的合法性不绑定到该变量的生存期。</p>
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<h3><span class="mw-headline" id=".E7.89.B9.E5.8C.96">特化</span></h3>
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<p><code>enable_borrowed_range</code> 对下列标准模板的所有特化定义为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ：</p>
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<ul>
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<li><a href="cpp-string-basic_string_view.html"><tt>std::basic_string_view</tt></a></li>
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<li><a href="cpp-container-span.html"><tt>std::span</tt></a></li>
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<li><a href="cpp-ranges-subrange.html"><tt>std::ranges::subrange</tt></a></li>
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<li><a href="cpp-ranges-ref_view.html"><tt>std::ranges::ref_view</tt></a></li>
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<li><a href="cpp-ranges-empty_view.html"><tt>std::ranges::empty_view</tt></a></li>
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<li><a href="cpp-ranges-iota_view.html"><tt>std::ranges::iota_view</tt></a></li>
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</ul>
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<p><code>enable_borrowed_range</code> 对下列标准范围适配器定义为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 当且仅当 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">enable_borrowed_range</span><span class="sy1"><</span>V<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，其中 <code>V</code> 是底层视图类型：</p>
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<ul>
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<li><a href="cpp-ranges-owning_view.html"><tt>std::ranges::owning_view</tt></a></li>
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<li><a href="cpp-ranges-take_view.html"><tt>std::ranges::take_view</tt></a></li>
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<li><a href="cpp-ranges-drop_view.html"><tt>std::ranges::drop_view</tt></a></li>
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<li><a href="cpp-ranges-drop_while_view.html"><tt>std::ranges::drop_while_view</tt></a></li>
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<li><a href="cpp-ranges-common_view.html"><tt>std::ranges::common_view</tt></a></li>
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<li><a href="cpp-ranges-reverse_view.html"><tt>std::ranges::reverse_view</tt></a></li>
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<li><a href="cpp-ranges-elements_view.html"><tt>std::ranges::elements_view</tt></a></li>
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</ul>
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<ul>
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<li><a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/ranges/adjacent_view&action=edit&redlink=1" class="new" title="cpp/ranges/adjacent view（页面不存在）"><tt>std::ranges::adjacent_view</tt></a></li>
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</ul>
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</td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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<p>对 <a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/ranges/zip_view&action=edit&redlink=1" class="new" title="cpp/ranges/zip view（页面不存在）"><tt>std::ranges::zip_view</tt></a> 的特化定义为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 当且仅当 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">enable_borrowed_range</span><span class="sy1"><</span>Vs<span class="sy1">></span> <span class="sy3">&&</span> ...<span class="br0">)</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，其中 <code>Vs...</code> 为其所适配的所有视图类型。</p>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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<p>用户可对要实现 <code>borrowed_range</code> 的无 cv 限定的程序定义类型特化 <code>enable_borrowed_range</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，而对不实现的类型特化为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。这些特化应当可用于<a href="cpp-language-constant_expression.html">常量表达式</a>并拥有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">bool</span></span></span> 类型。</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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<p>演示 <code>enable_borrowed_range</code> 对程序定义类型的特化。这种特化防御潜在悬垂的结果。</p>
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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 <algorithm></span>
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<span class="co2">#include <array></span>
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<span class="co2">#include <cstddef></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <ranges></span>
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<span class="co2">#include <span></span>
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<span class="co2">#include <type_traits></span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">typename</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>
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<span class="kw1">struct</span> MyRange <span class="sy4">:</span> <a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a><span class="sy1"><</span>T, N<span class="sy1">></span> <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">typename</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>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">enable_borrowed_range</span><span class="sy1"><</span>MyRange<span class="sy1"><</span>T, N<span class="sy1">>></span> <span class="sy1">=</span> <span class="kw2">false</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">typename</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>
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<span class="kw1">struct</span> MyBorrowedRange <span class="sy4">:</span> std<span class="sy4">::</span><span class="me2">span</span><span class="sy1"><</span>T, N<span class="sy1">></span> <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">typename</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>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">enable_borrowed_range</span><span class="sy1"><</span>MyBorrowedRange<span class="sy1"><</span>T, N<span class="sy1">>></span> <span class="sy1">=</span> <span class="kw2">true</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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static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">range</span><span class="sy1"><</span>MyRange<span class="sy1"><</span><span class="kw4">int</span>, <span class="nu0">8</span><span class="sy1">>></span><span class="br0">)</span><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">borrowed_range</span><span class="sy1"><</span>MyRange<span class="sy1"><</span><span class="kw4">int</span>, <span class="nu0">8</span><span class="sy1">>></span> <span class="sy1">==</span> <span class="kw2">false</span><span class="br0">)</span><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">range</span><span class="sy1"><</span>MyBorrowedRange<span class="sy1"><</span><span class="kw4">int</span>, <span class="nu0">8</span><span class="sy1">>></span><span class="br0">)</span><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">borrowed_range</span><span class="sy1"><</span>MyBorrowedRange<span class="sy1"><</span><span class="kw4">int</span>, <span class="nu0">8</span><span class="sy1">>></span> <span class="sy1">==</span> <span class="kw2">true</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> getMyRangeByValue <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span> <span class="br0">{</span> <span class="kw1">return</span> MyRange<span class="sy1"><</span><span class="kw4">int</span>, <span class="nu0">4</span><span class="sy1">></span><span class="br0">{</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">42</span>, <span class="nu0">3</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> dangling_iter <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">max_element</span><span class="br0">(</span>getMyRangeByValue<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><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">dangling</span>, decltype<span class="br0">(</span>dangling_iter<span class="br0">)</span><span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// *dangling_iter; // 编译错误（即悬垂保护生效。）</span>
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<span class="kw4">auto</span> my <span class="sy1">=</span> MyRange<span class="sy1"><</span><span class="kw4">int</span>, <span class="nu0">4</span><span class="sy1">></span><span class="br0">{</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">42</span>, <span class="nu0">3</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> valid_iter <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">max_element</span><span class="br0">(</span>my<span class="br0">)</span><span class="sy4">;</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="sy2">*</span>valid_iter <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="co1">// OK: 42</span>
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<span class="kw4">auto</span> getMyBorrowedRangeByValue <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span> <span class="br0">{</span>
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<span class="kw4">static</span> <span class="kw4">int</span> sa<span class="br0">[</span><span class="nu0">4</span><span class="br0">]</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">42</span>, <span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">return</span> MyBorrowedRange<span class="sy1"><</span><span class="kw4">int</span>, <a href="cpp-iterator-size.html"><span class="kw684">std::<span class="me2">size</span></span></a><span class="br0">(</span>sa<span class="br0">)</span><span class="sy1">></span><span class="br0">{</span>sa<span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> valid_iter2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">max_element</span><span class="br0">(</span>getMyBorrowedRangeByValue<span class="br0">(</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="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="sy2">*</span>valid_iter2 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// OK ： 42</span>
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<span class="br0">}</span></pre></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">42 42</pre></div>
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