144 lines
14 KiB
HTML
144 lines
14 KiB
HTML
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<title>std::mblen</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_string_multibyte_mblen 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::</span>mblen</h1>
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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-multibyte.html">multibyte</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-cstdlib.html"><cstdlib></a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> mblen<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> s, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n <span class="br0">)</span><span class="sy4">;</span></span></div>
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<p>确定多字节字符的字节大小，其首字节为 <code>s</code> 所指向。</p>
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<p>若 <code>s</code> 是空指针，则重置全局转换状态并确定是否使用迁移序列。</p>
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<p>此函数等价于调用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-string-multibyte-mbtowc.html"><span class="kw1162">std::<span class="me2">mbtowc</span></span></a><span class="br0">(</span><span class="br0">(</span><span class="kw4">wchar_t</span><span class="sy2">*</span><span class="br0">)</span><span class="nu0">0</span>, s, n<span class="br0">)</span></span></span> ，除了不影响 <span class="t-lc"><a href="cpp-string-multibyte-mbtowc.html">std::mbtowc</a></span> 的转换状态。</p>
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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>每次对 <code>mblen</code> 的调用更新内部全局转换状态（ <span class="t-lc"><a href="cpp-string-multibyte-mbstate_t.html">std::mbstate_t</a></span> 类型的静态对象，只为此函数所知）。若多字节编码使用迁移状态，则必须留意以避免回撤或多次扫描。任何情况下，多线程不应无同步地调用 <code>mblen</code> ：可用 <span class="t-lc"><a href="cpp-string-multibyte-mbrlen.html">std::mbrlen</a></span> 代替。</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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<td>s</td>
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<td>-</td>
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<td>指向多字节字符的指针</td>
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<td>n</td>
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<td>-</td>
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<td><code>s</code>中能被检验的字节数限制</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>若 <code>s</code> 不是空指针，则返回多字节字符所含的字节数，或若 <code>s</code> 所指的首字节不组成合法多字节字符则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="nu0">1</span></span></span> ，或若 <code>s</code> 指向空字符 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">'<span class="es5">\0</span>'</span></span></span> 则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> 。</p>
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<p>若 <code>s</code> 是空指针，则重置内部转换状态为初始迁移状态，并若当前多字节编码非状态依赖（不使用迁移序列）则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> ，或者若当前多字节编码为状态依赖（使用迁移序列）则返回非零。</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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <clocale></span>
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<span class="co2">#include <cstdlib></span>
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<span class="co2">#include <iomanip></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <stdexcept></span>
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<span class="co2">#include <string_view></span>
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<span class="co1">// 多字节字符串中的字符数是 mblen() 的和</span>
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<span class="co1">// 注意：更简单的手段是 std::mbstowcs(NULL, s.c_str(), s.size())</span>
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<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> strlen_mb<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-string-basic_string_view.html"><span class="kw1247">std::<span class="me2">string_view</span></span></a> s<span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> result <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> ptr <span class="sy1">=</span> s.<span class="me1">data</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> end <span class="sy1">=</span> ptr <span class="sy2">+</span> s.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">mblen</span><span class="br0">(</span><a href="cpp-types-NULL.html"><span class="kw103">NULL</span></a>, <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 重置转换状态</span>
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<span class="kw1">while</span> <span class="br0">(</span>ptr <span class="sy1"><</span> end<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">int</span> next <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">mblen</span><span class="br0">(</span>ptr, end<span class="sy2">-</span>ptr<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>next <span class="sy1">==</span> <span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">throw</span> <a href="cpp-error-runtime_error.html"><span class="kw770">std::<span class="me2">runtime_error</span></span></a><span class="br0">(</span><span class="st0">"strlen_mb(): conversion error"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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ptr <span class="sy2">+</span><span class="sy1">=</span> next<span class="sy4">;</span>
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<span class="sy2">++</span>result<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> result<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> dump_bytes<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-string-basic_string_view.html"><span class="kw1247">std::<span class="me2">string_view</span></span></a> str<span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-hex.html"><span class="kw1781">std::<span class="me2">hex</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-uppercase.html"><span class="kw1773">std::<span class="me2">uppercase</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-setfill.html"><span class="kw1794">std::<span class="me2">setfill</span></span></a><span class="br0">(</span><span class="st0">'0'</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">unsigned</span> <span class="kw4">char</span> c <span class="sy4">:</span> str<span class="br0">)</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-setw.html"><span class="kw1796">std::<span class="me2">setw</span></span></a><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span>c<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-hex.html"><span class="kw1780">std::<span class="me2">dec</span></span></a> <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>
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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="co1">// 允许 mblen() 以 UTF-8 多字节编码工作</span>
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<a href="cpp-locale-setlocale.html"><span class="kw1989">std::<span class="me2">setlocale</span></span></a><span class="br0">(</span><a href="cpp-locale-LC_categories.html"><span class="kw1992">LC_ALL</span></a>, <span class="st0">"en_US.utf8"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// UTF-8 窄多字节编码</span>
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<span class="kw4">const</span> <a href="cpp-string-basic_string_view.html"><span class="kw1247">std::<span class="me2">string_view</span></span></a> str <span class="sy1">=</span> <span class="st0">"z<span class="es3">\u00df</span><span class="es3">\u6c34</span><span class="es4">\U0001f34c</span>"</span><span class="sy4">;</span> <span class="co1">// 或 u8"zß水🍌"</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>str<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" is "</span> <span class="sy1"><<</span> strlen_mb<span class="br0">(</span>str<span class="br0">)</span>
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<span class="sy1"><<</span> <span class="st0">" characters, but as much as "</span> <span class="sy1"><<</span> str.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" bytes: "</span><span class="sy4">;</span>
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dump_bytes<span class="br0">(</span>str<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>可能的输出：</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">zß水🍌 is 10 bytes, but only 4 characters</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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<div class="t-dsc-member-div">
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<div><a href="cpp-string-multibyte-mbtowc.html"><span class="t-lines"><span>mbtowc</span></span></a></div>
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</td>
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<td>将下一个多字节字符转换成宽字符<br />
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<span class="t-mark">(函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-string-multibyte-mbrlen.html"><span class="t-lines"><span>mbrlen</span></span></a></div>
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<td>给定状态，返回下一个多字节字符中的字节数<br />
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<span class="t-mark">(函数)</span></td>
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