129 lines
12 KiB
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129 lines
12 KiB
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<title>std::ios_base::xalloc</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_ios_base_xalloc skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ios_base::</span>xalloc</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-io.html">io</a>‎ | <a href="cpp-io-ios_base.html">ios base</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span> <span class="kw4">int</span> xalloc<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<p>返回（程序范围内）唯一的值，它能用于通过调用 <span class="t-lc"><a href="cpp-io-ios_base-iword.html">iword()</a></span> 和 <span class="t-lc"><a href="cpp-io-ios_base-pword.html">pword()</a></span> 访问 <code>std::ios_base</code> 的私有存储中一个 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span></span></span> 和一个 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="sy2">*</span></span></span> 元素。到 <code>xalloc</code> 的调用不分配内存。</p>
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<p><span class="t-rev-inl t-since-cxx14"><span>此函数线程安全；从多个线程共时访问不导致数据竞争。</span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span></p>
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<p>等效地自增 <code>std::ios_base</code> 的私有静态数据成员，如同以执行 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> index<span class="sy2">++</span><span class="sy4">;</span></span></span> ，若 <code>index</code> 是该静态成员的名称<span class="t-rev-inl t-since-cxx14"><span>（它可以为 <span class="t-lc"><a href="cpp-atomic-atomic.html">std::atomic</a></span> 或以其他方式以支持多线程共时访问）</span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span>。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<p>（无）</p>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>用作 pword/iword 下标的独有整数。</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-template-editlink" style=""></div>
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<div class="t-example">
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<p>将基类 pword 存储用于导出的流对象的运行时类型鉴别。</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 <iostream></span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> charT, <span class="kw1">class</span> traits <span class="sy1">=</span> <a href="cpp-string-char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1"><</span>charT<span class="sy1">></span> <span class="sy1">></span>
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<span class="kw1">class</span> mystream <span class="sy4">:</span> <span class="kw1">public</span> <a href="cpp-io-basic_ostream.html"><span class="kw1715">std::<span class="me2">basic_ostream</span></span></a><span class="sy1"><</span>charT, traits<span class="sy1">></span>
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<span class="br0">{</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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<span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> xindex<span class="sy4">;</span>
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mystream<span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1715">std::<span class="me2">basic_ostream</span></span></a><span class="sy1"><</span>charT, traits<span class="sy1">></span><span class="sy3">&</span> ostr<span class="br0">)</span> <span class="sy4">:</span>
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<a href="cpp-io-basic_ostream.html"><span class="kw1715">std::<span class="me2">basic_ostream</span></span></a><span class="sy1"><</span>charT, traits<span class="sy1">></span><span class="br0">(</span>ostr.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>
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<span class="br0">{</span>
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this<span class="sy2">-</span><span class="sy1">></span>pword<span class="br0">(</span>xindex<span class="br0">)</span> <span class="sy1">=</span> this<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> myfn<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="sy2">*</span>this <span class="sy1"><<</span> <span class="st0">"[special handling for mystream]"</span><span class="sy4">;</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="co1">// 每个 mystream 特化从 xalloc() 获得独有的下标</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> charT, <span class="kw1">class</span> traits<span class="sy1">></span>
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<span class="kw4">const</span> <span class="kw4">int</span> mystream<span class="sy1"><</span>charT, traits<span class="sy1">></span><span class="sy4">::</span><span class="me2">xindex</span> <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">xalloc</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 此 I/O 操纵符将能用于辨识身为 mystream 的 ostream</span>
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<span class="co1">// 通过查找存储于 pword 的指针</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> charT, <span class="kw1">class</span> traits<span class="sy1">></span>
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<a href="cpp-io-basic_ostream.html"><span class="kw1715">std::<span class="me2">basic_ostream</span></span></a><span class="sy1"><</span>charT,traits<span class="sy1">></span><span class="sy3">&</span> mymanip<span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1715">std::<span class="me2">basic_ostream</span></span></a><span class="sy1"><</span>charT,traits<span class="sy1">></span><span class="sy3">&</span> os<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="br0">(</span>os.<span class="me1">pword</span><span class="br0">(</span>mystream<span class="sy1"><</span>charT,traits<span class="sy1">></span><span class="sy4">::</span><span class="me2">xindex</span><span class="br0">)</span> <span class="sy1">==</span> <span class="sy3">&</span>os<span class="br0">)</span>
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<span class="kw1">static_cast</span><span class="sy1"><</span>mystream<span class="sy1"><</span>charT,traits<span class="sy1">></span><span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span>os<span class="br0">)</span>.<span class="me1">myfn</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> os<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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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"cout, narrow-character test "</span> <span class="sy1"><<</span> mymanip <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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mystream<span class="sy1"><</span><span class="kw4">char</span><span class="sy1">></span> myout<span class="br0">(</span><a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a><span class="br0">)</span><span class="sy4">;</span>
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myout <span class="sy1"><<</span> <span class="st0">"myout, narrow-character test "</span> <span class="sy1"><<</span> mymanip <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1759">std::<span class="me2">wcout</span></span></a> <span class="sy1"><<</span> <span class="st0">"wcout, wide-character test "</span> <span class="sy1"><<</span> mymanip <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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mystream<span class="sy1"><</span><span class="kw4">wchar_t</span><span class="sy1">></span> mywout<span class="br0">(</span><a href="cpp-io-cout.html"><span class="kw1759">std::<span class="me2">wcout</span></span></a><span class="br0">)</span><span class="sy4">;</span>
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mywout <span class="sy1"><<</span> <span class="st0">"mywout, wide-character test "</span> <span class="sy1"><<</span> mymanip <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</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">cout, narrow-character test
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myout, narrow-character test [special handling for mystream]
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wcout, wide-character test
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mywout, wide-character test [special handling for mystream]</pre></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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<div class="t-dsc-member-div">
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<div><a href="cpp-io-ios_base-pword.html"><span class="t-lines"><span>pword</span></span></a></div>
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</div>
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</td>
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<td>若需要则重置私有存储的大小，并访问位于指定下标的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="sy2">*</span></span></span> 元素<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-io-ios_base-iword.html"><span class="t-lines"><span>iword</span></span></a></div>
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</div>
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</td>
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<td>如果有必要的话，调整私有存储的大小，并且访问位于提供的下标的<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span></span></span>元素<br />
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<span class="t-mark">(公开成员函数)</span></td>
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