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<div class="title">编码 </div> </div>
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</div><!--header-->
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<div class="contents">
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<div class="toc"><h3>目录</h3>
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<ul><li class="level1"><a href="#Unicode">Unicode</a><ul><li class="level2"><a href="#UTF">Unicode 转换格式</a></li>
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<li class="level2"><a href="#CharacterType">字符类型</a></li>
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<li class="level2"><a href="#AutoUTF">AutoUTF</a></li>
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<li class="level2"><a href="#ASCII">ASCII</a></li>
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</ul>
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</li>
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<li class="level1"><a href="#ValidationTranscoding">校验及转码</a><ul><li class="level2"><a href="#Transcoder">转码器</a></li>
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</ul>
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</li>
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</ul>
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</div>
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<div class="textblock"><p>根据 <a href="http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf">ECMA-404</a>：</p>
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<blockquote class="doxtable">
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<p>(in Introduction) JSON text is a sequence of Unicode code points.</p>
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<p>翻译：JSON 文本是 Unicode 码点的序列。 </p>
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</blockquote>
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<p>较早的 <a href="http://www.ietf.org/rfc/rfc4627.txt">RFC4627</a> 申明：</p>
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<blockquote class="doxtable">
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<p>(in §3) JSON text SHALL be encoded in Unicode. The default encoding is UTF-8.</p>
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<p>翻译：JSON 文本应该以 Unicode 编码。缺省的编码为 UTF-8。 </p>
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</blockquote>
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<blockquote class="doxtable">
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<p>(in §6) JSON may be represented using UTF-8, UTF-16, or UTF-32. When JSON is written in UTF-8, JSON is 8bit compatible. When JSON is written in UTF-16 or UTF-32, the binary content-transfer-encoding must be used.</p>
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<p>翻译：JSON 可使用 UTF-8、UTF-16 或 UTF-32 表示。当 JSON 以 UTF-8 写入，该 JSON 是 8 位兼容的。当 JSON 以 UTF-16 或 UTF-32 写入，就必须使用二进制的内容传送编码。 </p>
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</blockquote>
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<p>RapidJSON 支持多种编码。它也能检查 JSON 的编码，以及在不同编码中进行转码。所有这些功能都是在内部实现，无需使用外部的程序库（如 <a href="http://site.icu-project.org/">ICU</a>）。</p>
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<h1><a class="anchor" id="Unicode"></a>
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Unicode</h1>
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<p>根据 <a href="http://www.unicode.org/standard/translations/t-chinese.html">Unicode 的官方网站</a>： >Unicode 给每个字符提供了一个唯一的数字， 不论是什么平台、 不论是什么程序、 不论是什么语言。</p>
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<p>这些唯一数字称为码点（code point），其范围介乎 <code>0x0</code> 至 <code>0x10FFFF</code> 之间。</p>
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<h2><a class="anchor" id="UTF"></a>
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Unicode 转换格式</h2>
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<p>存储 Unicode 码点有多种编码方式。这些称为 Unicode 转换格式（Unicode Transformation Format, UTF）。RapidJSON 支持最常用的 UTF，包括：</p>
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<ul>
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<li>UTF-8：8 位可变长度编码。它把一个码点映射至 1 至 4 个字节。</li>
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<li>UTF-16：16 位可变长度编码。它把一个码点映射至 1 至 2 个 16 位编码单元（即 2 至 4 个字节）。</li>
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<li>UTF-32：32 位固定长度编码。它直接把码点映射至单个 32 位编码单元（即 4 字节）。</li>
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</ul>
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<p>对于 UTF-16 及 UTF-32 来说，字节序（endianness）是有影响的。在内存中，它们通常都是以该计算机的字节序来存储。然而，当要储存在文件中或在网上传输，我们需要指明字节序列的字节序，是小端（little endian, LE）还是大端（big-endian, BE）。</p>
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<p>RapidJSON 通过 <code><a class="el" href="encodings_8h_source.html">rapidjson/encodings.h</a></code> 中的 struct 去提供各种编码：</p>
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<div class="fragment"><div class="line"><span class="keyword">namespace </span><a class="code" href="namespacerapidjson.html">rapidjson</a> {</div>
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<div class="line"> </div>
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<div class="line"><span class="keyword">template</span><<span class="keyword">typename</span> CharType = <span class="keywordtype">char</span>></div>
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<div class="line"><span class="keyword">struct </span>UTF8;</div>
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<div class="line"> </div>
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<div class="line"><span class="keyword">template</span><<span class="keyword">typename</span> CharType = <span class="keywordtype">wchar_t</span>></div>
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<div class="line"><span class="keyword">struct </span>UTF16;</div>
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<div class="line"> </div>
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<div class="line"><span class="keyword">template</span><<span class="keyword">typename</span> CharType = <span class="keywordtype">wchar_t</span>></div>
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<div class="line"><span class="keyword">struct </span>UTF16LE;</div>
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<div class="line"> </div>
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<div class="line"><span class="keyword">template</span><<span class="keyword">typename</span> CharType = <span class="keywordtype">wchar_t</span>></div>
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<div class="line"><span class="keyword">struct </span>UTF16BE;</div>
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<div class="line"> </div>
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<div class="line"><span class="keyword">template</span><<span class="keyword">typename</span> CharType = <span class="keywordtype">unsigned</span>></div>
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<div class="line"><span class="keyword">struct </span>UTF32;</div>
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<div class="line"> </div>
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<div class="line"><span class="keyword">template</span><<span class="keyword">typename</span> CharType = <span class="keywordtype">unsigned</span>></div>
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<div class="line"><span class="keyword">struct </span>UTF32LE;</div>
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<div class="line"> </div>
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<div class="line"><span class="keyword">template</span><<span class="keyword">typename</span> CharType = <span class="keywordtype">unsigned</span>></div>
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<div class="line"><span class="keyword">struct </span>UTF32BE;</div>
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<div class="line"> </div>
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<div class="line">} <span class="comment">// namespace rapidjson</span></div>
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</div><!-- fragment --><p>对于在内存中的文本，我们正常会使用 <code>UTF8</code>、<code>UTF16</code> 或 <code>UTF32</code>。对于处理经过 I/O 的文本，我们可使用 <code>UTF8</code>、<code>UTF16LE</code>、<code>UTF16BE</code>、<code>UTF32LE</code> 或 <code>UTF32BE</code>。</p>
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<p>当使用 DOM 风格的 API，<code>GenericValue<Encoding></code> 及 <code>GenericDocument<Encoding></code> 里的 <code>Encoding</code> 模板参数是用于指明内存中存储的 JSON 字符串使用哪种编码。因此通常我们会在此参数中使用 <code>UTF8</code>、<code>UTF16</code> 或 <code>UTF32</code>。如何选择，视乎应用软件所使用的操作系统及其他程序库。例如，Windows API 使用 UTF-16 表示 Unicode 字符，而多数的 Linux 发行版本及应用软件则更喜欢 UTF-8。</p>
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<p>使用 UTF-16 的 DOM 声明例子：</p>
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<div class="fragment"><div class="line"><span class="keyword">typedef</span> GenericDocument<UTF16<> > WDocument;</div>
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<div class="line"><span class="keyword">typedef</span> GenericValue<UTF16<> > WValue;</div>
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</div><!-- fragment --><p>可以在 <a href="doc/stream.zh-cn.md">DOM's Encoding</a> 一节看到更详细的使用例子。</p>
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<h2><a class="anchor" id="CharacterType"></a>
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字符类型</h2>
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<p>从之前的声明中可以看到，每个编码都有一个 <code>CharType</code> 模板参数。这可能比较容易混淆，实际上，每个 <code>CharType</code> 存储一个编码单元，而不是一个字符（码点）。如之前所谈及，在 UTF-8 中一个码点可能会编码成 1 至 4 个编码单元。</p>
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<p>对于 <code>UTF16(LE|BE)</code> 及 <code>UTF32(LE|BE)</code> 来说，<code>CharType</code> 必须分别是一个至少 2 及 4 字节的整数类型。</p>
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<p>注意 C++11 新添了 <code>char16_t</code> 及 <code>char32_t</code> 类型，也可分别用于 <code>UTF16</code> 及 <code>UTF32</code>。</p>
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<h2><a class="anchor" id="AutoUTF"></a>
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AutoUTF</h2>
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<p>上述所介绍的编码都是在编译期静态挷定的。换句话说，使用者必须知道内存或流之中使用了哪种编码。然而，有时候我们可能需要读写不同编码的文件，而且这些编码需要在运行时才能决定。</p>
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<p><code>AutoUTF</code> 是为此而设计的编码。它根据输入或输出流来选择使用哪种编码。目前它应该与 <code>EncodedInputStream</code> 及 <code>EncodedOutputStream</code> 结合使用。</p>
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<h2><a class="anchor" id="ASCII"></a>
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ASCII</h2>
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<p>虽然 JSON 标准并未提及 <a href="http://en.wikipedia.org/wiki/ASCII">ASCII</a>，有时候我们希望写入 7 位的 ASCII JSON，以供未能处理 UTF-8 的应用程序使用。由于任 JSON 都可以把 Unicode 字符表示为 <code>\uXXXX</code> 转义序列，JSON 总是可用 ASCII 来编码。</p>
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<p>以下的例子把 UTF-8 的 DOM 写成 ASCII 的 JSON：</p>
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<div class="fragment"><div class="line"><span class="keyword">using namespace </span><a class="code" href="namespacerapidjson.html">rapidjson</a>;</div>
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<div class="line"><a class="code" href="classrapidjson_1_1_generic_document.html">Document</a> d; <span class="comment">// UTF8<></span></div>
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<div class="line"><span class="comment">// ...</span></div>
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<div class="line"><a class="code" href="classrapidjson_1_1_generic_string_buffer.html">StringBuffer</a> buffer;</div>
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<div class="line"><a class="code" href="classrapidjson_1_1_writer.html">Writer<StringBuffer, Document::EncodingType, ASCII<></a> > writer(buffer);</div>
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<div class="line">d.Accept(writer);</div>
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<div class="line">std::cout << buffer.GetString();</div>
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</div><!-- fragment --><p>ASCII 可用于输入流。当输入流包含大于 127 的字节，就会导致 <code>kParseErrorStringInvalidEncoding</code> 错误。</p>
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<p>ASCII * 不能 * 用于内存（<code>Document</code> 的编码，或 <code>Reader</code> 的目标编码)，因为它不能表示 Unicode 码点。</p>
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<h1><a class="anchor" id="ValidationTranscoding"></a>
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校验及转码</h1>
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<p>当 RapidJSON 解析一个 JSON 时，它能校验输入 JSON，判断它是否所标明编码的合法序列。要开启此选项，请把 <code>kParseValidateEncodingFlag</code> 加入 <code>parseFlags</code> 模板参数。</p>
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<p>若输入编码和输出编码并不相同，<code>Reader</code> 及 <code>Writer</code> 会算把文本转码。在这种情况下，并不需要 <code>kParseValidateEncodingFlag</code>，因为它必须解码输入序列。若序列不能被解码，它必然是不合法的。</p>
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<h2><a class="anchor" id="Transcoder"></a>
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转码器</h2>
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<p>虽然 RapidJSON 的编码功能是为 JSON 解析／生成而设计，使用者也可以“滥用”它们来为非 JSON 字符串转码。</p>
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<p>以下的例子把 UTF-8 字符串转码成 UTF-16：</p>
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<div class="fragment"><div class="line"><span class="preprocessor">#include "rapidjson/encodings.h"</span></div>
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<div class="line"> </div>
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<div class="line"><span class="keyword">using namespace </span><a class="code" href="namespacerapidjson.html">rapidjson</a>;</div>
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<div class="line"> </div>
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<div class="line"><span class="keyword">const</span> <span class="keywordtype">char</span>* s = <span class="stringliteral">"..."</span>; <span class="comment">// UTF-8 string</span></div>
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<div class="line"><a class="code" href="structrapidjson_1_1_generic_string_stream.html">StringStream</a> source(s);</div>
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<div class="line"><a class="code" href="classrapidjson_1_1_generic_string_buffer.html">GenericStringBuffer<UTF16<></a> > target;</div>
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<div class="line"> </div>
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<div class="line"><span class="keywordtype">bool</span> hasError = <span class="keyword">false</span>;</div>
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<div class="line"><span class="keywordflow">while</span> (source.Peek() != <span class="charliteral">'\0'</span>)</div>
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<div class="line"> <span class="keywordflow">if</span> (!<a class="code" href="structrapidjson_1_1_transcoder.html">Transcoder</a><<a class="code" href="structrapidjson_1_1_u_t_f8.html">UTF8<></a>, <a class="code" href="structrapidjson_1_1_u_t_f16.html">UTF16<></a> >::Transcode(source, target)) {</div>
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<div class="line"> hasError = <span class="keyword">true</span>;</div>
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<div class="line"> <span class="keywordflow">break</span>;</div>
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<div class="line"> }</div>
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<div class="line"> </div>
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<div class="line"><span class="keywordflow">if</span> (!hasError) {</div>
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<div class="line"> <span class="keyword">const</span> <span class="keywordtype">wchar_t</span>* t = target.GetString();</div>
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<div class="line"> <span class="comment">// ...</span></div>
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<div class="line">}</div>
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</div><!-- fragment --><p>你也可以用 <code>AutoUTF</code> 及对应的流来在运行时设置内源／目的之编码。 </p>
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</div></div><!-- contents -->
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</div><!-- PageDoc -->
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</div><!-- doc-content -->
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<div class="ttc" id="astructrapidjson_1_1_u_t_f16_html"><div class="ttname"><a href="structrapidjson_1_1_u_t_f16.html">rapidjson::UTF16</a></div><div class="ttdoc">UTF-16 encoding.</div><div class="ttdef"><b>Definition:</b> encodings.h:269</div></div>
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<div class="ttc" id="aclassrapidjson_1_1_generic_string_buffer_html"><div class="ttname"><a href="classrapidjson_1_1_generic_string_buffer.html">rapidjson::GenericStringBuffer</a></div><div class="ttdoc">Represents an in-memory output stream.</div><div class="ttdef"><b>Definition:</b> fwd.h:59</div></div>
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<div class="ttc" id="astructrapidjson_1_1_generic_string_stream_html"><div class="ttname"><a href="structrapidjson_1_1_generic_string_stream.html">rapidjson::GenericStringStream</a></div><div class="ttdoc">Read-only string stream.</div><div class="ttdef"><b>Definition:</b> fwd.h:47</div></div>
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<div class="ttc" id="astructrapidjson_1_1_transcoder_html"><div class="ttname"><a href="structrapidjson_1_1_transcoder.html">rapidjson::Transcoder</a></div><div class="ttdoc">Encoding conversion.</div><div class="ttdef"><b>Definition:</b> encodings.h:658</div></div>
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<div class="ttc" id="aclassrapidjson_1_1_generic_document_html"><div class="ttname"><a href="classrapidjson_1_1_generic_document.html">rapidjson::GenericDocument</a></div><div class="ttdoc">A document for parsing JSON text as DOM.</div><div class="ttdef"><b>Definition:</b> document.h:69</div></div>
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<div class="ttc" id="aclassrapidjson_1_1_writer_html"><div class="ttname"><a href="classrapidjson_1_1_writer.html">rapidjson::Writer</a></div><div class="ttdoc">JSON writer</div><div class="ttdef"><b>Definition:</b> fwd.h:95</div></div>
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<div class="ttc" id="astructrapidjson_1_1_u_t_f8_html"><div class="ttname"><a href="structrapidjson_1_1_u_t_f8.html">rapidjson::UTF8</a></div><div class="ttdoc">UTF-8 encoding.</div><div class="ttdef"><b>Definition:</b> encodings.h:96</div></div>
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<div class="ttc" id="anamespacerapidjson_html"><div class="ttname"><a href="namespacerapidjson.html">rapidjson</a></div><div class="ttdoc">main RapidJSON namespace</div><div class="ttdef"><b>Definition:</b> rapidjson.h:409</div></div>
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