140 lines
7.1 KiB
HTML
140 lines
7.1 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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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>nullptr ，指针字面量</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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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_nullptr 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">nullptr ，指针字面量</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-language.html">language</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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<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<table class="t-sdsc-begin">
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td><code><b>nullptr</b></code></td>
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<td class="t-sdsc-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p>关键词 <code>nullptr</code> 代表指针字面量。它是 <span class="t-lc"><a href="cpp-types-nullptr_t.html">std::nullptr_t</a></span> 类型的<a href="cpp-language-value_category.html">纯右值</a>。存在从 <code>nullptr</code> 到任何指针类型及任何成员指针类型的<a href="cpp-language-implicit_conversion.html">隐式转换</a>。同样的转换对于任何空指针常量也存在，空指针常量包括 <span class="t-lc"><a href="cpp-types-nullptr_t.html">std::nullptr_t</a></span> 的值，以及宏 <span class="t-lc"><a href="cpp-types-NULL.html">NULL</a></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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<p>演示 <code>nullptr</code> 保持空指针常量的含义，即使它不再是字面量。</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 <cstddef></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">constexpr</span> T clone<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> t<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> t<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span>
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<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">"函数 g 已调用<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</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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g<span class="br0">(</span>nullptr<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 良好</span>
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g<span class="br0">(</span><a href="cpp-types-NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 良好</span>
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g<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 良好</span>
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g<span class="br0">(</span>clone<span class="br0">(</span>nullptr<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 良好</span>
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<span class="co1">// g(clone(NULL)); // 错误：非字面量的零不能为空指针常量</span>
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<span class="co1">// g(clone(0)); // 错误：非字面量的零不能为空指针常量</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">函数 g 已调用
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函数 g 已调用
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函数 g 已调用
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函数 g 已调用</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
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<p><a href="cpp-keyword-nullptr.html"><tt>nullptr</tt></a></p>
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<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
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<div class="t-ref-std-c++17">
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<ul>
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<li>C++17 标准（ISO/IEC 14882:2017）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.11 Pointer conversions [conv.ptr]</li>
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</ul>
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</dd>
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</dl>
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</div>
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<div class="t-ref-std-c++14">
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<ul>
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<li>C++14 标准（ISO/IEC 14882:2014）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>4.10 Pointer conversions [conv.ptr]</li>
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</ul>
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</dd>
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</dl>
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</div>
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<div class="t-ref-std-c++11">
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<ul>
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<li>C++11 标准（ISO/IEC 14882:2011）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>4.10 Pointer conversions [conv.ptr]</li>
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</ul>
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</dd>
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</dl>
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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-types-NULL.html"><span class="t-lines"><span>NULL</span></span></a></div>
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</div>
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</td>
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<td>实现定义的空指针常量<br />
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<span class="t-mark">(宏常量)</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-types-nullptr_t.html"><span class="t-lines"><span>nullptr_t</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>空指针字面量 <strong class="selflink"><tt>nullptr</tt></strong> 的类型<br />
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<span class="t-mark">(typedef)</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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</div>
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</body>
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</html>
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