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web/cppreference-zh/chmhelp/cpp-io-ios_base-iword.html
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<!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>std::ios_base::iword</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_io_ios_base_iword 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"><span style="font-size:0.7em; line-height:130%">std::ios_base::</span>iword</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-io.html">io</a>‎ | <a href="cpp-io-ios_base.html">ios base</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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<table class="t-dcl-begin">
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<td class="t-dcl-nopad">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span><span class="sy3">&</span> iword<span class="br0">(</span> <span class="kw4">int</span> index <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td class="t-dcl-nopad"></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<p>首先，充分地分配或重置私有存储（ <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span></span></span> 的动态数组或另一可索引数据结构）以确保 <code>index</code> 是合法下标，然后返回到带下标 <code>index</code> 的私有存储 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span></span></span> 元素。</p>
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<p>引用可能被此 <code>ios_base</code> 对象上任何其他操作非法化，包含另一对 <code>iword()</code> 的调用，但维持返回值，使得以相同下标从 <span class="t-c"><span class="mw-geshi cpp source-cpp">iword<span class="br0">(</span>index<span class="br0">)</span></span></span> 读取将产生相同值（直至下次到 <span class="t-c"><span class="mw-geshi cpp source-cpp">copyfmt<span class="br0">(</span><span class="br0">)</span></span></span> 的调用）。值能用于任何目的。必须通过先前的 <span class="t-lc"><a href="cpp-io-ios_base-xalloc.html">xalloc()</a></span> 调用获得元素下标，否则行为未定义。新元素初始化为 <span class="t-lc">0</span> 。</p>
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<p>若分配失败，则调用可能抛出 <span class="t-lc"><a href="cpp-io-ios_base-failure.html">std::ios_base::failure</a></span> 的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-io-basic_ios.html"><span class="kw1712">std::<span class="me2">basic_ios</span></span></a><span class="sy1"><></span><span class="sy4">::</span><span class="me2">setstate</span><span class="br0">(</span>badbit<span class="br0">)</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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<table class="t-par-begin">
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<tr class="t-par">
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<td>index</td>
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<td>-</td>
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<td>元素的下标值</td>
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</tr>
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</table>
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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>到该元素的引用。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>设置 badbit 时可能抛出 <span class="t-lc"><a href="cpp-io-ios_base-failure.html">std::ios_base::failure</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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<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="co2">#include <string></span>
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<span class="kw1">struct</span> Foo <span class="br0">{</span>
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<span class="kw4">static</span> <span class="kw4">int</span> foo_xalloc<span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> data<span class="sy4">;</span>
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Foo<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy3">&</span> s<span class="br0">)</span> <span class="sy4">:</span> data<span class="br0">(</span>s<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// 分配 Foo 对象所用的 iword 存储</span>
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<span class="kw4">int</span> Foo<span class="sy4">::</span><span class="me2">foo_xalloc</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">// 若 iword 保有 1 则此用户定义 operator<< 打印字符串</span>
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<a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os, Foo<span class="sy3">&</span> f<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">iword</span><span class="br0">(</span>Foo<span class="sy4">::</span><span class="me2">foo_xalloc</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">1</span><span class="br0">)</span>
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<span class="kw1">return</span> os <span class="sy1"><<</span> <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="br0">(</span>f.<span class="me1">data</span>.<span class="me1">rbegin</span><span class="br0">(</span><span class="br0">)</span>, f.<span class="me1">data</span>.<span class="me1">rend</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">else</span>
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<span class="kw1">return</span> os <span class="sy1"><<</span> f.<span class="me1">data</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="co1">// 此 I/O 操纵符在 0 与 1 间翻转存储于 iword 的数</span>
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<a href="cpp-io-ios_base.html"><span class="kw1669">std::<span class="me2">ios_base</span></span></a><span class="sy3">&</span> rev<span class="br0">(</span><a href="cpp-io-ios_base.html"><span class="kw1669">std::<span class="me2">ios_base</span></span></a><span class="sy3">&</span> os<span class="br0">)</span>
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<span class="br0">{</span>
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os.<span class="me1">iword</span><span class="br0">(</span>Foo<span class="sy4">::</span><span class="me2">foo_xalloc</span><span class="br0">)</span> <span class="sy1">=</span> <span class="sy3">!</span>os.<span class="me1">iword</span><span class="br0">(</span>Foo<span class="sy4">::</span><span class="me2">foo_xalloc</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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Foo f<span class="br0">(</span><span class="st0">"example"</span><span class="br0">)</span><span class="sy4">;</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> f <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span> <span class="sy1"><<</span> rev <span class="sy1"><<</span> f <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span> <span class="sy1"><<</span> rev <span class="sy1"><<</span> f <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">example
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elpmaxe
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example</pre></div>
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</div>
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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-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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-io-ios_base-xalloc.html"><span class="t-lines"><span>xalloc</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</span></span></span></div>
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</div>
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</td>
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<td>返回能安全用作 pword() 和 iword() 下标的程序范围内独有的整数<br />
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<span class="t-mark">(公开静态成员函数)</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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</div>
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</body>
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</html>
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