167 lines
16 KiB
HTML
167 lines
16 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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<title>this 指针</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_this skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading">this 指针</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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<td class="t-sdsc-nopad"><code><b>this</b></code></td>
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<td class="t-sdsc-nopad"></td>
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<td class="t-sdsc-nopad"></td>
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<p>关键词 <code>this</code> 是一个<a href="cpp-language-value_category.html">纯右值</a><a href="cpp-language-expressions.html">表达式</a>，它的值是<a href="cpp-language-overload_resolution.html">隐式对象形参</a>（在其上调用非静态成员函数的对象）的地址。它能出现于下列语境：</p>
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<div class="t-li1"><span class="t-li">1)</span> 在任意非静态<a href="cpp-language-member_functions.html">成员函数</a>体内，包括<a href="cpp-language-initializer_list.html" class="mw-redirect">成员初始化器列表</a></div>
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<div class="t-li1"><span class="t-li">2)</span> 在非静态成员函数的<a href="cpp-language-function.html">声明</a>中，（可能存在的）cv 限定符序列之后的任何位置，包括
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<table class="t-rev-begin">
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<tr class="t-rev t-until-cxx17">
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<td>
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<ul>
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<li><a href="cpp-language-except_spec.html">动态异常说明</a>、</li>
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</ul>
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</td>
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<td><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
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</table>
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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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<ul>
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<li><a href="cpp-language-noexcept_spec.html">noexcept 说明</a>以及</li>
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<li>尾随返回类型</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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</div>
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<table class="t-rev-begin">
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<div class="t-li1"><span class="t-li">3)</span> 在<a href="cpp-language-data_members.html#.E6.88.90.E5.91.98.E5.88.9D.E5.A7.8B.E5.8C.96">默认成员初始化器</a>中</div>
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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>在类 <code>X</code> 的成员函数中，<code>this</code> 的类型是 <code>X*</code>（指向 X 的指针）。如果成员函数有 <a href="cpp-language-member_functions.html">cv 限定</a>，那么 <code>this</code> 的类型是 <code>cv X*</code>（指向有相同 cv 限定的 X 的指针）。因为构造函数与析构函数不能有 cv 限定，所以它们中 <code>this</code> 的类型始终是 <code>X*</code>，即使在构造或销毁 const 对象时也是如此。</p>
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<p>当在任何允许 <code>this</code> 关键词的语境（非静态成员函数体、成员初始化器列表、默认成员初始化器）中使用非静态数据成员时，它的名字前会自动添加隐含的 <code><b>this-></b></code>，产生一个成员访问表达式（如果该成员是虚成员函数，那么产生虚函数调用）。</p>
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<p>在类模板中，<code>this</code> 是一个<a href="cpp-language-dependent_name.html">待决表达式</a>，而且显式的 <code>this-></code> 可以用于强行使另一表达式变为待决的。</p>
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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="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
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<span class="kw1">struct</span> B
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<span class="br0">{</span>
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<span class="kw4">int</span> var<span class="sy4">;</span>
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<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<span class="sy1">></span>
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<span class="kw1">struct</span> D <span class="sy4">:</span> B<span class="sy1"><</span>T<span class="sy1">></span>
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<span class="br0">{</span>
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D<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co1">// var = 1; // 错误： 'var' 未在此作用域中声明</span>
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this<span class="sy2">-</span><span class="sy1">></span>var <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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<p>在对象的<a href="cpp-language-constructor.html">构造期间</a>，如果通过并非直接或间接从构造函数的 <code>this</code> 指针获得的泛左值访问对象或其任何子对象的值，那么如此获得的对象或子对象的值是未指明的。换言之，构造函数中 this 指针不能被别名化：</p>
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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="kw4">extern</span> <span class="kw1">struct</span> D d<span class="sy4">;</span>
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<span class="kw1">struct</span> D
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<span class="br0">{</span>
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D<span class="br0">(</span><span class="kw4">int</span> a<span class="br0">)</span> <span class="sy4">:</span> a<span class="br0">(</span>a<span class="br0">)</span>, b<span class="br0">(</span>d.<span class="me1">a</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// b(a) 或 b(this->a) 是正确的</span>
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<span class="kw4">int</span> a, b<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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D d <span class="sy1">=</span> D<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 因为 b(d.a) 未通过 this 访问，所以 d.b 现在是未指明的</span></pre></div>
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</div>
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<p>如果程序能保证对象由 <span class="t-c"><span class="mw-geshi cpp source-cpp">new</span></span> 分配，那么可以执行 <span class="t-c"><span class="mw-geshi cpp source-cpp">delete this<span class="sy4">;</span></span></span>，然而这会令指向被解分配对象的所有指针失效，包括 <code>this</code> 指针自身：在 <span class="t-c"><span class="mw-geshi cpp source-cpp">delete this<span class="sy4">;</span></span></span> 返回后，这种成员函数不能引用类的成员（因为这牵涉到 <code>this</code> 的隐式解引用），且不可再调用其他成员函数。</p>
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<table class="t-rev-begin">
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<p>例如，这被用于 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 的控制块的负责减少引用数的成员函数中，在最后一个到被管理对象的引用离开作用域时。</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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<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="kw1">class</span> ref
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<span class="br0">{</span>
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<span class="co1">// ...</span>
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<span class="kw4">void</span> incRef<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="sy2">++</span>mnRef<span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">void</span> decRef<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">if</span> <span class="br0">(</span><span class="sy2">--</span>mnRef <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span> delete this<span class="sy4">;</span> <span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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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 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="kw1">class</span> T
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<span class="br0">{</span>
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<span class="kw4">int</span> x<span class="sy4">;</span>
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<span class="kw4">void</span> foo<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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x <span class="sy1">=</span> <span class="nu0">6</span><span class="sy4">;</span> <span class="co1">// 等同于 this->x = 6;</span>
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this<span class="sy2">-</span><span class="sy1">></span>x <span class="sy1">=</span> <span class="nu0">5</span><span class="sy4">;</span> <span class="co1">// 显式使用 this-></span>
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<span class="br0">}</span>
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<span class="kw4">void</span> foo<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span>
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<span class="br0">{</span>
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<span class="co1">// x = 7; // 错误：*this 是常量</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> foo<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="co1">// 形参 x 遮蔽拥有相同名字的成员</span>
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<span class="br0">{</span>
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this<span class="sy2">-</span><span class="sy1">></span>x <span class="sy1">=</span> x<span class="sy4">;</span> <span class="co1">// 无限定的 x 代表形参</span>
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<span class="co1">// 需要用‘this->’消歧义</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> y<span class="sy4">;</span>
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T<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="sy4">:</span> x<span class="br0">(</span>x<span class="br0">)</span>, <span class="co1">// 用形参 x 初始化成员 x</span>
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y<span class="br0">(</span>this<span class="sy2">-</span><span class="sy1">></span>x<span class="br0">)</span> <span class="co1">// 用成员 x 初始化成员 y</span>
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<span class="br0">{</span><span class="br0">}</span>
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T<span class="sy3">&</span> operator<span class="sy1">=</span> <span class="br0">(</span> <span class="kw4">const</span> T<span class="sy3">&</span> b <span class="br0">)</span>
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<span class="br0">{</span>
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x <span class="sy1">=</span> b.<span class="me1">x</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span> <span class="co1">// 许多重载运算符都返回 *this</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">class</span> Outer <span class="br0">{</span>
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<span class="kw4">int</span> a<span class="br0">[</span>sizeof<span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 错误：不在成员函数中</span>
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<span class="kw4">unsigned</span> <span class="kw4">int</span> sz <span class="sy1">=</span> sizeof<span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：在默认成员初始化器中</span>
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<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">int</span> b<span class="br0">[</span>sizeof<span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="kw1">struct</span> Inner <span class="br0">{</span>
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<span class="kw4">int</span> c<span class="br0">[</span>sizeof<span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 错误：不在 Inner 的成员函数中</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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