141 lines
12 KiB
HTML
141 lines
12 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>std::tolower</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_string_byte_tolower skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>tolower</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-string.html">string</a>‎ | <a href="cpp-string-byte.html">byte</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-cctype.html"><cctype></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> tolower<span class="br0">(</span> <span class="kw4">int</span> ch <span class="br0">)</span><span class="sy4">;</span></span></div>
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<p>按照当前安装的 C 本地环境所定义的规则，转换给定字符为小写。</p>
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<p>默认 "C" 本地环境中，分别以小写字母 <code>abcdefghijklmnopqrstuvwxyz</code> 替换大写字母 <code>ABCDEFGHIJKLMNOPQRSTUVWXYZ</code> 。</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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<td>ch</td>
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<td>-</td>
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<td>要转换的字符。若 <code>ch</code> 的值不能表示为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> 且不等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-io-c.html"><span class="kw1882">EOF</span></a></span></span> ，则行为未定义</td>
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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><code>ch</code> 的小写版本，或若无列于当前 C 本地环境的小写版本，则为不修改的 <code>ch</code> 。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>同所有其他来自 <a href="cpp-header-cctype.html"><tt><cctype></tt></a> 的函数，若参数值既不能表示为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> 又不等于 <code>EOF</code> 则 <code>std::tolower</code> 的行为未定义。为了以简单的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char</span></span></span> （或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">signed</span> <span class="kw4">char</span></span></span> ）安全使用此函数，首先要将参数转换为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> ：</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">char</span> my_tolower<span class="br0">(</span><span class="kw4">char</span> ch<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> <span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">char</span><span class="sy1">></span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">tolower</span><span class="br0">(</span><span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">unsigned</span> <span class="kw4">char</span><span class="sy1">></span><span class="br0">(</span>ch<span class="br0">)</span><span class="br0">)</span><span class="br0">)</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>类似地，迭代器的值类型为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">signed</span> <span class="kw4">char</span></span></span> 时，不应直接将它们用于标准算法。而是要首先转换值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> ：</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"><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> str_tolower<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s<span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-algorithm-transform.html"><span class="kw1589">std::<span class="me2">transform</span></span></a><span class="br0">(</span>s.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, s.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, s.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>,
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<span class="co1">// static_cast<int(*)(int)>(std::tolower) // 错误</span>
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<span class="co1">// [](int c){ return std::tolower(c); } // 错误</span>
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<span class="co1">// [](char c){ return std::tolower(c); } // 错误</span>
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<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">unsigned</span> <span class="kw4">char</span> c<span class="br0">)</span><span class="br0">{</span> <span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">tolower</span><span class="br0">(</span>c<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 正确</span>
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<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> s<span class="sy4">;</span>
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<span class="br0">}</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 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 <cctype></span>
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<span class="co2">#include <clocale></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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<span class="kw4">unsigned</span> <span class="kw4">char</span> c <span class="sy1">=</span> <span class="st0">'<span class="es2">\xb4</span>'</span><span class="sy4">;</span> <span class="co1">// ISO-8859-15 中的字符 Ž</span>
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<span class="co1">// 但在 ISO-8859-1 中为 ´ （锐音符）</span>
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<a href="cpp-locale-setlocale.html"><span class="kw1990">std::<span class="me2">setlocale</span></span></a><span class="br0">(</span><a href="cpp-locale-LC_categories.html"><span class="kw1993">LC_ALL</span></a>, <span class="st0">"en_US.iso88591"</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> <a href="cpp-io-manip-hex.html"><span class="kw1782">std::<span class="me2">hex</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-showbase.html"><span class="kw1766">std::<span class="me2">showbase</span></span></a><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> <span class="st0">"in iso8859-1, tolower('0xb4') gives "</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">tolower</span><span class="br0">(</span>c<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<a href="cpp-locale-setlocale.html"><span class="kw1990">std::<span class="me2">setlocale</span></span></a><span class="br0">(</span><a href="cpp-locale-LC_categories.html"><span class="kw1993">LC_ALL</span></a>, <span class="st0">"en_US.iso885915"</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> <span class="st0">"in iso8859-15, tolower('0xb4') gives "</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">tolower</span><span class="br0">(</span>c<span class="br0">)</span> <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">in iso8859-1, tolower('0xb4') gives 0xb4
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in iso8859-15, tolower('0xb4') gives 0xb8</pre></div>
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</div>
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</div>
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<p><br /></p>
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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-string-byte-toupper.html"><span class="t-lines"><span>toupper</span></span></a></div>
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</div>
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</td>
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<td>转换字符为大写<br />
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<span class="t-mark">(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-locale-tolower.html"><span class="t-lines"><span>tolower<span class="t-dsc-small">(std::locale)</span></span></span></a></div>
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</div>
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</td>
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<td>用本地环境的 ctype 刻面将字符转换为小写<br />
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<span class="t-mark">(函数模板)</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-string-wide-towlower.html"><span class="t-lines"><span>towlower</span></span></a></div>
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</div>
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</td>
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<td>转换宽字符为小写<br />
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<span class="t-mark">(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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