156 lines
12 KiB
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156 lines
12 KiB
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<title>std::codecvt<InternT,ExternT,State>::max_length, do_max_length</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::codecvt<InternT,ExternT,State>::</span>max_length, do_max_length</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-locale.html">locale</a>‎ | <a href="cpp-locale-codecvt.html">codecvt</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-locale.html"><locale></a></code></div>
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<td rowspan="142">(1)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">public</span><span class="sy4">:</span><br />
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<span class="kw4">int</span> max_length<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">throw</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">public</span><span class="sy4">:</span><br />
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<span class="kw4">int</span> max_length<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<td rowspan="142">(2)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">protected</span><span class="sy4">:</span><br />
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<span class="kw1">virtual</span> <span class="kw4">int</span> do_max_length<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">throw</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">protected</span><span class="sy4">:</span><br />
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<span class="kw1">virtual</span> <span class="kw4">int</span> do_max_length<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 公开成员函数，调用最终派生类的成员函数 <code>do_max_length</code> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 返回对于任何合法范围 <code>[from, from_end)</code> 和任何 <code>state</code> 合法 ， <span class="t-c"><span class="mw-geshi cpp source-cpp">do_length<span class="br0">(</span>state, from, from_end, <span class="nu0">1</span><span class="br0">)</span></span></span> 所能返回的最大值。</div>
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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>若以 <span class="t-lc"><a href="cpp-locale-codecvt-in.html">in()</a></span> 转换生成一个 <code>InternT</code> 字符，能消耗的最大 <code>ExternT</code> 字符数。</p>
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<p>不转换特化 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-locale-codecvt.html"><span class="kw1939">std::<span class="me2">codecvt</span></span></a><span class="sy1"><</span><span class="kw4">char</span>, <span class="kw4">char</span>, <a href="cpp-string-multibyte-mbstate_t.html"><span class="kw1179">std::<span class="me2">mbstate_t</span></span></a><span class="sy1">></span></span></span> 返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span></span></span> 。</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>若编码依赖状态（ <span class="t-c"><span class="mw-geshi cpp source-cpp">encoding<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="sy2">-</span><span class="nu0">1</span></span></span> ），则可能消耗多于 <code>max_length()</code> 个外部字符生成一个内部字符。</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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <locale></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <codecvt></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">"In codecvt_utf8, the longest multibyte character is "</span>
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<span class="sy1"><<</span> <a href="cpp-locale-codecvt_utf8.html"><span class="kw1983">std::<span class="me2">codecvt_utf8</span></span></a><span class="sy1"><</span><span class="kw4">wchar_t</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>.<span class="me1">max_length</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" bytes long<span class="es1">\n</span>"</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 header-consuming codecvt_utf8, the longest multibyte character is "</span>
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<span class="sy1"><<</span> <a href="cpp-locale-codecvt_utf8.html"><span class="kw1983">std::<span class="me2">codecvt_utf8</span></span></a><span class="sy1"><</span><span class="kw4">wchar_t</span>,
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<span class="nu12">0x10ffff</span>,
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<a href="cpp-locale-codecvt_mode.html"><span class="kw1987">std::<span class="me2">consume_header</span></span></a><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>.<span class="me1">max_length</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" bytes long<span class="es1">\n</span>"</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 this system's en_US.utf8, the longest multibyte character is "</span>
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<span class="sy1"><<</span> <a href="cpp-locale-use_facet.html"><span class="kw1897">std::<span class="me2">use_facet</span></span></a><span class="sy1"><</span><a href="cpp-locale-codecvt.html"><span class="kw1939">std::<span class="me2">codecvt</span></span></a><span class="sy1"><</span><span class="kw4">wchar_t</span>, <span class="kw4">char</span>, <a href="cpp-string-multibyte-mbstate_t.html"><span class="kw1179">std::<span class="me2">mbstate_t</span></span></a><span class="sy1">>></span><span class="br0">(</span>
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<a href="cpp-locale-locale.html"><span class="kw1894">std::<span class="me2">locale</span></span></a><span class="br0">(</span><span class="st0">"en_US.utf8"</span><span class="br0">)</span>
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<span class="br0">)</span>.<span class="me1">max_length</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" bytes long<span class="es1">\n</span>"</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 this system's zh_CN.gb18030, the longest multibyte character is "</span>
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<span class="sy1"><<</span> <a href="cpp-locale-use_facet.html"><span class="kw1897">std::<span class="me2">use_facet</span></span></a><span class="sy1"><</span><a href="cpp-locale-codecvt.html"><span class="kw1939">std::<span class="me2">codecvt</span></span></a><span class="sy1"><</span><span class="kw4">wchar_t</span>, <span class="kw4">char</span>, <a href="cpp-string-multibyte-mbstate_t.html"><span class="kw1179">std::<span class="me2">mbstate_t</span></span></a><span class="sy1">>></span><span class="br0">(</span>
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<a href="cpp-locale-locale.html"><span class="kw1894">std::<span class="me2">locale</span></span></a><span class="br0">(</span><span class="st0">"zh_CN.gb18030"</span><span class="br0">)</span>
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<span class="br0">)</span>.<span class="me1">max_length</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" bytes long<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></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 codecvt_utf8, the longest multibyte character is 4 bytes long
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In header-consuming codecvt_utf8, the longest multibyte character is 7 bytes long
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In this system's en_US.utf8, the longest multibyte character is 6 bytes long
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In this system's zh_CN.gb18030, the longest multibyte character is 4 bytes long</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><span class="t-lines"><span>MB_CUR_MAX</span></span></div>
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<td>当前 C 本地环境中多字节字符的最大字节数<br />
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<span class="t-mark">(宏变量)</span></td>
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<div><a href="cpp-locale-codecvt-encoding.html"><span class="t-lines"><span>do_encoding</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[虚]</span></span></span></div>
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</div>
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</td>
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<td>返回产生一个 internT 字符所需的 externT 字符数，若此值为常数<br />
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<span class="t-mark">(虚受保护成员函数)</span></td>
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