131 lines
14 KiB
HTML
131 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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<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>类</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_language_classes 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">类</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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<p>类是一种用户定义类型。</p>
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<p>类的类型可以由类说明符定义，它在<a href="cpp-language-declarations.html">声明</a>语法的 <span class="t-spar">声明说明符序列</span> 中出现。类说明符拥有下列语法：</p>
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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><span class="t-spar">类关键词</span> <span class="t-spar">属性</span> <span class="t-spar">类头名</span> <code><b>{</b></code> <span class="t-spar">成员说明</span> <code><b>}</b></code></td>
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<td>(1)</td>
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<td class="t-sdsc-nopad"></td>
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</tr>
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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><span class="t-spar">类关键词</span> <span class="t-spar">属性</span> <span class="t-spar">类头名</span> <code><b>:</b></code> <span class="t-spar">基类说明符序列</span> <code><b>{</b></code> <span class="t-spar">成员说明</span> <code><b>}</b></code></td>
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<td>(2)</td>
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<td class="t-sdsc-nopad"></td>
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</tr>
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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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</table>
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<table class="t-par-begin">
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<tr class="t-par">
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<td><span class="t-spar">类关键词</span></td>
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<td>-</td>
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<td><a href="cpp-keyword-class.html"><tt>class</tt></a>、<a href="cpp-keyword-struct.html"><tt>struct</tt></a>、<a href="cpp-keyword-union.html"><tt>union</tt></a> 之一</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">属性</span></td>
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<td>-</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span> 可选的任意数量的<a href="cpp-language-attributes.html">属性</a>序列</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">类头名</span></td>
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<td>-</td>
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<td>被定义的类名。可以前接 <span class="t-spar">嵌套名说明符</span>（名字和作用域解析运算符的序列，以作用域解析运算符结尾），可以后随关键词 <a href="cpp-keyword-final.html"><tt>final</tt></a>。可以忽略该名字，此时类是无名的（注意无名类不可以是 final 的）</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">基类说明符序列</span></td>
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<td>-</td>
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<td>一个或多个父类及其各自所用的继承模型的列表（见<a href="cpp-language-derived_class.html">派生类</a>）</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">成员说明</span></td>
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<td>-</td>
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<td>访问说明符、成员对象和成员函数的声明和定义的列表（见<a href="cpp-language-class.html">类定义</a>）</td>
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</tr>
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</table>
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<p>类关键词 <code>struct</code> 和 <code>class</code> 在 C++ 中不可分，除了默认<a href="cpp-language-access.html">访问模式</a>和默认<a href="cpp-language-derived_class.html">继承模式</a>，当类定义使用 <code>struct</code> <span class="t-spar">类关键词</span> 则为 <code>public</code>，若类定义使用 <code>class</code> <span class="t-spar">类关键词</span> 则为 <code>private</code>。<code>class</code> 和 <code>struct</code> 均可用于<a href="cpp-language-class.html">类定义</a>。</p>
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<p>使用类关键词 <code>union</code> 所得的是<a href="cpp-language-union.html">联合体定义</a>，它定义在同一时刻只保有其一个数据成员的类。</p>
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<p>类可以拥有下列种类的成员：</p>
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<div class="t-li1"><span class="t-li">1)</span> 数据成员：</div>
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<div class="t-li2"><span class="t-li">a)</span> <a href="cpp-language-data_members.html">非静态数据成员</a>，包括<a href="cpp-language-bit_field.html">位域</a>。</div>
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<div class="t-li2"><span class="t-li">b)</span> <a href="cpp-language-static.html#.E9.9D.99.E6.80.81.E6.95.B0.E6.8D.AE.E6.88.90.E5.91.98">静态数据成员</a></div>
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<div class="t-li1"><span class="t-li">2)</span> 成员函数：</div>
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<div class="t-li2"><span class="t-li">a)</span> <a href="cpp-language-member_functions.html">非静态成员函数</a></div>
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<div class="t-li2"><span class="t-li">b)</span> <a href="cpp-language-static.html#.E9.9D.99.E6.80.81.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">静态成员函数</a></div>
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<div class="t-li1"><span class="t-li">3)</span> 嵌套类型：</div>
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<div class="t-li2"><span class="t-li">a)</span> 在类定义之中定义的<a href="cpp-language-nested_types.html">嵌套类</a>及<a href="cpp-language-enum.html">枚举</a></div>
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<div class="t-li2"><span class="t-li">b)</span> 既存类型的别名，以 <a href="cpp-language-typedef.html"><code>typedef</code></a> <span class="t-rev-inl t-since-cxx11"><span>或<a href="cpp-language-type_alias.html">类型别名</a></span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>声明定义</div>
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<div class="t-li2"><span class="t-li">c)</span> 就<a href="cpp-language-unqualified_lookup.html#.E6.B3.A8.E5.85.A5.E7.B1.BB.E5.90.8D">查找</a>而言（除非在用作<a href="cpp-language-initializer_list.html" class="mw-redirect">构造函数</a>名时），类的名字在该类自身的定义中，表现为一个作为其自身的类型别名的公开成员：这被称作<i><a href="cpp-language-injected-class-name.html">注入类名</a></i></div>
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<div class="t-li1"><span class="t-li">4)</span> 来自定义于类中的所有无作用域枚举<span class="t-rev-inl t-since-cxx20"><span>，或由 <a href="cpp-language-using_declaration.html">using 声明</a>或 <a href="cpp-language-enum.html#using_enum_.E5.A3.B0.E6.98.8E">using enum 声明</a>引入</span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>的<a href="cpp-language-enum.html">枚举项</a></div>
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<div class="t-li1"><span class="t-li">5)</span> <a href="cpp-language-member_template.html">成员模板</a>（<span class="t-rev-inl t-since-cxx14"><span>变量模板、</span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span>类模板或函数模板）可出现于任何非局部 class/struct/union 定义体内。</div>
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<p>所有成员都在类定义中一次性定义，不能向已定义的类添加成员（与命名空间成员不同）</p>
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<p>类 <code>T</code> 的成员，如果它是静态数据成员、成员函数、成员类型、成员模板、无作用域枚举的枚举项、匿名成员联合体的成员，那么不能使用 <code>T</code> 作为其名字。然而，非静态数据成员可使用名称 <code>T</code> ，只要没有用户声明的构造函数。</p>
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<p>声明或继承了至少一个<a href="cpp-language-virtual.html">虚</a>成员函数的类是<i>多态</i>的。此类型的对象是<a href="cpp-language-object.html#.E5.A4.9A.E6.80.81.E5.AF.B9.E8.B1.A1">多态对象</a>，并拥有作为其对象表示一部分存储的运行时类型信息，它可以通过 <a href="cpp-language-dynamic_cast.html"><code>dynamic_cast</code></a> 和 <a href="cpp-language-typeid.html"><code>typeid</code></a> 查询到。虚成员函数参与动态绑定。</p>
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<p>声明或继承了至少一个纯虚函数的类是<a href="cpp-language-abstract_class.html">抽象类</a>。不能创建此类型的对象。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx11">
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<td>
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<p>拥有 <a href="cpp-language-constexpr.html">constexpr</a> 构造函数的类是<a href="cpp-named_req-LiteralType.html"><span style="font-style:italic">字面类型</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LiteralType)</span></a> ：此类型的对象能用 <a href="cpp-language-constexpr.html">constexpr</a> 函数在编译时进行操作。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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</table>
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<p>一些成员函数是特殊的：某些条件下，即使用户不定义，编译器也会定义它们。它们是：</p>
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<ul>
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<li><a href="cpp-language-default_constructor.html">默认构造函数</a></li>
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<li><a href="cpp-language-copy_constructor.html">复制构造函数</a></li>
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</ul>
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<table class="t-rev-begin t-rev-begin-noborder">
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<tr class="t-rev t-since-cxx11">
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<td>
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<ul>
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<li><a href="cpp-language-move_constructor.html">移动构造函数</a></li>
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</ul>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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</table>
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<ul>
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<li><a href="cpp-language-copy_assignment.html">复制赋值运算符</a></li>
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</ul>
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<table class="t-rev-begin t-rev-begin-noborder">
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<tr class="t-rev t-since-cxx11">
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<td>
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<ul>
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<li><a href="cpp-language-move_assignment.html">移动赋值运算符</a></li>
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</ul>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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</table>
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<ul>
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<li><a href="cpp-language-destructor.html">析构函数</a></li>
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</ul>
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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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</div>
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</body>
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</html>
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