95 lines
11 KiB
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95 lines
11 KiB
HTML
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<title>C++ 具名要求：可空指针 (NullablePointer) (C++11 起)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_named_req_NullablePointer skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">C++ 具名要求：</span><span style="font-style:italic">可空指针</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(NullablePointer)</span> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span></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-named_req.html">named req</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>指定该类型是能与 <span class="t-lc"><a href="cpp-types-nullptr_t.html">std::nullptr_t</a></span> 对象进行比较的指针式类型。</p>
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<h3><span class="mw-headline" id=".E8.A6.81.E6.B1.82">要求</span></h3>
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<p>类型必须满足所有下列要求：</p>
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<ul>
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<li><a href="cpp-named_req-EqualityComparable.html"><span style="font-style:italic">可相等比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EqualityComparable)</span></a></li>
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<li><a href="cpp-named_req-DefaultConstructible.html"><span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span></a></li>
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<li><a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a></li>
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<li><a href="cpp-named_req-CopyAssignable.html"><span style="font-style:italic">可复制赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span></a></li>
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<li><a href="cpp-named_req-Destructible.html"><span style="font-style:italic">可析构</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Destructible)</span></a></li>
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</ul>
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<p>此外，此类型的一个值初始化的对象必须产生该类型的空值（null）。空值必须仅与自身等价。该类型的默认初始化可拥有不确定值。</p>
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<p>此类型必须可按语境转换成 bool。若其值等价于其空值则此转换的效果为 false，否则为 true。</p>
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<p>此类型进行的操作均不可抛异常。</p>
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<p>此类型必须满足下列额外的表达式，给定该类型的两个值 <code>p</code> 与 <code>q</code>，以及 <code>np</code> 是 <span class="t-lc"><a href="cpp-types-nullptr_t.html">std::nullptr_t</a></span> 类型的值（可有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> 限定）：</p>
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<table class="t-dsc-begin">
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<tr class="t-dsc-hitem">
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<td>表达式</td>
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<td>效果</td>
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</tr>
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<tr class="t-dsc">
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">Type p<span class="br0">(</span>np<span class="br0">)</span><span class="sy4">;</span></span></span>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">Type p <span class="sy1">=</span> np<span class="sy4">;</span></span></span></p>
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</td>
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<td>之后 <code>p</code> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">nullptr</span></span>。</td>
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</tr>
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<tr class="t-dsc">
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">Type<span class="br0">(</span>np<span class="br0">)</span></span></span></td>
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<td>等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">nullptr</span></span> 的临时对象。</td>
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</tr>
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<tr class="t-dsc">
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy1">=</span> np</span></span></td>
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<td>必须返回 <code>Type&</code>，而且之后 <code>p</code> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">nullptr</span></span>。</td>
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</tr>
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<tr class="t-dsc">
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy3">!</span><span class="sy1">=</span> q</span></span></td>
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<td>必须返回能按语境转换成 bool 的值。效果为 <code>!(p == q)</code>。</td>
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</tr>
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<tr class="t-dsc">
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy1">==</span> np</span></span>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">np <span class="sy1">==</span> p</span></span></p>
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</td>
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<td>必须返回能按语境转换成 bool 的值。效果为 <code>(p == Type())</code>。</td>
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</tr>
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<tr class="t-dsc">
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy3">!</span><span class="sy1">=</span> np</span></span>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">np <span class="sy3">!</span><span class="sy1">=</span> p</span></span></p>
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</td>
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<td>必须返回能按语境转换成 bool 的值。效果为 <code>!(p == np)</code>。</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>注意，对<strong class="selflink"><span style="font-style:italic">可空指针</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(NullablePointer)</span></strong> 类型不要求解引用（<code>operator*</code> 或 <code>operator-></code>）。满足这些要求的最小化类型是</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">class</span> handle <span class="br0">{</span>
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<span class="kw4">int</span> id <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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handle<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
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handle<span class="br0">(</span><a href="cpp-types-nullptr_t.html"><span class="kw102">std::<span class="me2">nullptr_t</span></span></a><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
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<span class="kw1">explicit</span> operator <span class="kw4">bool</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> id <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw1">friend</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span>handle l, handle r<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> l.<span class="me1">id</span> <span class="sy1">==</span> r.<span class="me1">id</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw1">friend</span> <span class="kw4">bool</span> operator<span class="sy3">!</span><span class="sy1">=</span><span class="br0">(</span>handle l, handle r<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="sy3">!</span><span class="br0">(</span>l <span class="sy1">==</span> r<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="co1">// 或仅一个预置的 operator== (C++20 起) </span>
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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<h3><span class="mw-headline" id=".E6.A0.87.E5.87.86.E5.BA.93">标准库</span></h3>
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<p>下列类型必须满足<strong class="selflink"><span style="font-style:italic">可空指针</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(NullablePointer)</span></strong> ：</p>
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<ul>
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<li>每个<a href="cpp-named_req-Allocator.html"><span style="font-style:italic">分配器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Allocator)</span></a> 类型 <code>X</code> 的成员类型 <code>X::pointer</code>、<code>X::const_pointer</code>、<code>X::void_pointer</code> 及 <code>X::const_void_pointer</code></li>
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<li><span class="t-lc"><a href="cpp-memory-unique_ptr.html">std::unique_ptr</a></span> 的成员类型 <code>X::pointer</code></li>
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<li>类型 <span class="t-lc"><a href="cpp-error-exception_ptr.html">std::exception_ptr</a></span></li>
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</ul>
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