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<div class="header">
<div class="headertitle">
<div class="title">SAX </div> </div>
</div><!--header-->
<div class="contents">
<div class="toc"><h3>目录</h3>
<ul><li class="level1"><a href="#Reader">Reader</a><ul><li class="level2"><a href="#Handler">处理器</a></li>
<li class="level2"><a href="#GenericReader">GenericReader</a></li>
</ul>
</li>
<li class="level1"><a href="#Writer">Writer</a><ul><li class="level2"><a href="#PrettyWriter">PrettyWriter</a></li>
<li class="level2"><a href="#CompletenessReset">完整性及重置</a></li>
<li class="level2"><a href="#CustomDataStructure">解析JSON至自定义结构</a></li>
<li class="level2"><a href="#Filtering">过滤JSON</a></li>
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<div class="textblock"><p>"SAX"此术语源于<a href="http://en.wikipedia.org/wiki/Simple_API_for_XML">Simple API for XML</a>。我们借了此术语去套用在JSON的解析及生成。</p>
<p>在RapidJSON中，<code>Reader</code>（<code>GenericReader<...></code>的typedef）是JSON的SAX风格解析器，而<code>Writer</code>（<code>GenericWriter<...></code>的typedef）则是JSON的SAX风格生成器。</p>
<h1><a class="anchor" id="Reader"></a>
Reader</h1>
<p><code>Reader</code>从输入流解析一个JSON。当它从流中读取字符时，它会基于JSON的语法去分析字符，并向处理器发送事件。</p>
<p>例如，以下是一个JSON。</p>
<div class="fragment"><div class="line">{</div>
<div class="line"> <span class="stringliteral">"hello"</span>: <span class="stringliteral">"world"</span>,</div>
<div class="line"> <span class="stringliteral">"t"</span>: true ,</div>
<div class="line"> <span class="stringliteral">"f"</span>: <span class="keyword">false</span>,</div>
<div class="line"> <span class="stringliteral">"n"</span>: null,</div>
<div class="line"> <span class="stringliteral">"i"</span>: 123,</div>
<div class="line"> <span class="stringliteral">"pi"</span>: 3.1416,</div>
<div class="line"> <span class="stringliteral">"a"</span>: [1, 2, 3, 4]</div>
<div class="line">}</div>
</div><!-- fragment --><p>当一个<code>Reader</code>解析此JSON时，它会顺序地向处理器发送以下的事件：</p>
<div class="fragment"><div class="line">StartObject()</div>
<div class="line">Key("hello", 5, true)</div>
<div class="line">String("world", 5, true)</div>
<div class="line">Key("t", 1, true)</div>
<div class="line">Bool(true)</div>
<div class="line">Key("f", 1, true)</div>
<div class="line">Bool(false)</div>
<div class="line">Key("n", 1, true)</div>
<div class="line">Null()</div>
<div class="line">Key("i")</div>
<div class="line">UInt(123)</div>
<div class="line">Key("pi")</div>
<div class="line">Double(3.1416)</div>
<div class="line">Key("a")</div>
<div class="line">StartArray()</div>
<div class="line">Uint(1)</div>
<div class="line">Uint(2)</div>
<div class="line">Uint(3)</div>
<div class="line">Uint(4)</div>
<div class="line">EndArray(4)</div>
<div class="line">EndObject(7)</div>
</div><!-- fragment --><p>除了一些事件参数需要再作解释，这些事件可以轻松地与JSON对上。我们可以看看<code>simplereader</code>例子怎样产生和以上完全相同的结果：</p>
<div class="fragment"><div class="line"><span class="preprocessor">#include "<a class="code" href="reader_8h.html">rapidjson/reader.h</a>"</span></div>
<div class="line"><span class="preprocessor">#include <iostream></span></div>
<div class="line"></div>
<div class="line"><span class="keyword">using namespace </span>rapidjson;</div>
<div class="line"><span class="keyword">using namespace </span>std;</div>
<div class="line"></div>
<div class="line"><span class="keyword">struct </span>MyHandler {</div>
<div class="line"> <span class="keywordtype">bool</span> Null() { cout << <span class="stringliteral">"Null()"</span> << endl; <span class="keywordflow">return</span> <span class="keyword">true</span>; }</div>
<div class="line"> <span class="keywordtype">bool</span> Bool(<span class="keywordtype">bool</span> b) { cout << <span class="stringliteral">"Bool("</span> << boolalpha << b << <span class="stringliteral">")"</span> << endl; <span class="keywordflow">return</span> <span class="keyword">true</span>; }</div>
<div class="line"> <span class="keywordtype">bool</span> Int(<span class="keywordtype">int</span> i) { cout << <span class="stringliteral">"Int("</span> << i << <span class="stringliteral">")"</span> << endl; <span class="keywordflow">return</span> <span class="keyword">true</span>; }</div>
<div class="line"> <span class="keywordtype">bool</span> Uint(<span class="keywordtype">unsigned</span> u) { cout << <span class="stringliteral">"Uint("</span> << u << <span class="stringliteral">")"</span> << endl; <span class="keywordflow">return</span> <span class="keyword">true</span>; }</div>
<div class="line"> <span class="keywordtype">bool</span> Int64(int64_t i) { cout << <span class="stringliteral">"Int64("</span> << i << <span class="stringliteral">")"</span> << endl; <span class="keywordflow">return</span> <span class="keyword">true</span>; }</div>
<div class="line"> <span class="keywordtype">bool</span> Uint64(uint64_t u) { cout << <span class="stringliteral">"Uint64("</span> << u << <span class="stringliteral">")"</span> << endl; <span class="keywordflow">return</span> <span class="keyword">true</span>; }</div>
<div class="line"> <span class="keywordtype">bool</span> Double(<span class="keywordtype">double</span> d) { cout << <span class="stringliteral">"Double("</span> << d << <span class="stringliteral">")"</span> << endl; <span class="keywordflow">return</span> <span class="keyword">true</span>; }</div>
<div class="line"> <span class="keywordtype">bool</span> String(<span class="keyword">const</span> <span class="keywordtype">char</span>* str, <a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> length, <span class="keywordtype">bool</span> copy) { </div>
<div class="line"> cout << <span class="stringliteral">"String("</span> << str << <span class="stringliteral">", "</span> << length << <span class="stringliteral">", "</span> << boolalpha << copy << <span class="stringliteral">")"</span> << endl;</div>
<div class="line"> <span class="keywordflow">return</span> <span class="keyword">true</span>;</div>
<div class="line"> }</div>
<div class="line"> <span class="keywordtype">bool</span> StartObject() { cout << <span class="stringliteral">"StartObject()"</span> << endl; <span class="keywordflow">return</span> <span class="keyword">true</span>; }</div>
<div class="line"> <span class="keywordtype">bool</span> Key(<span class="keyword">const</span> <span class="keywordtype">char</span>* str, <a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> length, <span class="keywordtype">bool</span> copy) { </div>
<div class="line"> cout << <span class="stringliteral">"Key("</span> << str << <span class="stringliteral">", "</span> << length << <span class="stringliteral">", "</span> << boolalpha << copy << <span class="stringliteral">")"</span> << endl;</div>
<div class="line"> <span class="keywordflow">return</span> <span class="keyword">true</span>;</div>
<div class="line"> }</div>
<div class="line"> <span class="keywordtype">bool</span> EndObject(<a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> memberCount) { cout << <span class="stringliteral">"EndObject("</span> << memberCount << <span class="stringliteral">")"</span> << endl; <span class="keywordflow">return</span> <span class="keyword">true</span>; }</div>
<div class="line"> <span class="keywordtype">bool</span> StartArray() { cout << <span class="stringliteral">"StartArray()"</span> << endl; <span class="keywordflow">return</span> <span class="keyword">true</span>; }</div>
<div class="line"> <span class="keywordtype">bool</span> EndArray(<a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> elementCount) { cout << <span class="stringliteral">"EndArray("</span> << elementCount << <span class="stringliteral">")"</span> << endl; <span class="keywordflow">return</span> <span class="keyword">true</span>; }</div>
<div class="line">};</div>
<div class="line"></div>
<div class="line"><span class="keywordtype">void</span> main() {</div>
<div class="line"> <span class="keyword">const</span> <span class="keywordtype">char</span> json[] = <span class="stringliteral">" { \"hello\" : \"world\", \"t\" : true , \"f\" : false, \"n\": null, \"i\":123, \"pi\": 3.1416, \"a\":[1, 2, 3, 4] } "</span>;</div>
<div class="line"></div>
<div class="line"> MyHandler handler;</div>
<div class="line"> <a class="code" href="classrapidjson_1_1_generic_reader.html">Reader</a> reader;</div>
<div class="line"> <a class="code" href="structrapidjson_1_1_generic_string_stream.html">StringStream</a> ss(json);</div>
<div class="line"> reader.<a class="code" href="classrapidjson_1_1_generic_reader.html#ac9c540b77de19661f6f45e04b9b0937b">Parse</a>(ss, handler);</div>
<div class="line">}</div>
</div><!-- fragment --><p>注意RapidJSON使用模板去静态挷定<code>Reader</code>类型及处理器的类形，而不是使用含虚函数的类。这个范式可以通过把函数内联而改善性能。</p>
<h2><a class="anchor" id="Handler"></a>
处理器</h2>
<p>如前例所示，使用者需要实现一个处理器（handler），用于处理来自<code>Reader</code>的事件（函数调用）。处理器必须包含以下的成员函数。</p>
<div class="fragment"><div class="line"><span class="keyword">class </span>Handler {</div>
<div class="line"> <span class="keywordtype">bool</span> Null();</div>
<div class="line"> <span class="keywordtype">bool</span> Bool(<span class="keywordtype">bool</span> b);</div>
<div class="line"> <span class="keywordtype">bool</span> Int(<span class="keywordtype">int</span> i);</div>
<div class="line"> <span class="keywordtype">bool</span> Uint(<span class="keywordtype">unsigned</span> i);</div>
<div class="line"> <span class="keywordtype">bool</span> Int64(int64_t i);</div>
<div class="line"> <span class="keywordtype">bool</span> Uint64(uint64_t i);</div>
<div class="line"> <span class="keywordtype">bool</span> Double(<span class="keywordtype">double</span> d);</div>
<div class="line"> <span class="keywordtype">bool</span> String(<span class="keyword">const</span> Ch* str, <a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> length, <span class="keywordtype">bool</span> copy);</div>
<div class="line"> <span class="keywordtype">bool</span> StartObject();</div>
<div class="line"> <span class="keywordtype">bool</span> Key(<span class="keyword">const</span> Ch* str, <a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> length, <span class="keywordtype">bool</span> copy);</div>
<div class="line"> <span class="keywordtype">bool</span> EndObject(<a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> memberCount);</div>
<div class="line"> <span class="keywordtype">bool</span> StartArray();</div>
<div class="line"> <span class="keywordtype">bool</span> EndArray(<a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> elementCount);</div>
<div class="line">};</div>
</div><!-- fragment --><p>当<code>Reader</code>遇到JSON null值时会调用<code>Null()</code>。</p>
<p>当<code>Reader</code>遇到JSON true或false值时会调用<code>Bool(bool)</code>。</p>
<p>当<code>Reader</code>遇到JSON number，它会选择一个合适的C++类型映射，然后调用<code>Int(int)</code>、<code>Uint(unsigned)</code>、<code>Int64(int64_t)</code>、<code>Uint64(uint64_t)</code>及<code>Double(double)</code>的*其中之一个*。</p>
<p>当<code>Reader</code>遇到JSON string，它会调用<code>String(const char* str, SizeType length, bool copy)</code>。第一个参数是字符串的指针。第二个参数是字符串的长度（不包含空终止符号）。注意RapidJSON支持字串中含有空字符`'\0'<code>。若出现这种情况，便会有</code>strlen(str) < length<code>。最后的</code>copy<code>参数表示处理器是否需要复制该字符串。在正常解析时，</code>copy = true<code>。仅当使用原位解析时，</code>copy = false`。此外，还要注意字符的类型与目标编码相关，我们稍后会再谈这一点。</p>
<p>当<code>Reader</code>遇到JSON object的开始之时，它会调用<code>StartObject()</code>。JSON的object是一个键值对（成员）的集合。若object包含成员，它会先为成员的名字调用<code>Key()</code>，然后再按值的类型调用函数。它不断调用这些键值对，直至最终调用<code>EndObject(SizeType memberCount)</code>。注意<code>memberCount</code>参数对处理器来说只是协助性质，使用者可能不需要此参数。</p>
<p>JSON array与object相似，但更简单。在array开始时，<code>Reader</code>会调用<code>BeginArary()</code>。若array含有元素，它会按元素的类型来读用函数。相似地，最后它会调用<code>EndArray(SizeType elementCount)</code>，其中<code>elementCount</code>参数对处理器来说只是协助性质。</p>
<p>每个处理器函数都返回一个<code>bool</code>。正常它们应返回<code>true</code>。若处理器遇到错误，它可以返回<code>false</code>去通知事件发送方停止继续处理。</p>
<p>例如，当我们用<code>Reader</code>解析一个JSON时，处理器检测到该JSON并不符合所需的schema，那么处理器可以返回<code>false</code>，令<code>Reader</code>停止之后的解析工作。而<code>Reader</code>会进入一个错误状态，并以<code>kParseErrorTermination</code>错误码标识。</p>
<h2><a class="anchor" id="GenericReader"></a>
GenericReader</h2>
<p>前面提及，<code>Reader</code>是<code>GenericReader</code>模板类的typedef：</p>
<div class="fragment"><div class="line"><span class="keyword">namespace </span>rapidjson {</div>
<div class="line"></div>
<div class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> SourceEncoding, <span class="keyword">typename</span> TargetEncoding, <span class="keyword">typename</span> Allocator = MemoryPoolAllocator<> ></div>
<div class="line"><span class="keyword">class </span>GenericReader {</div>
<div class="line"> <span class="comment">// ...</span></div>
<div class="line">};</div>
<div class="line"></div>
<div class="line"><span class="keyword">typedef</span> GenericReader<UTF8<>, UTF8<> > <a class="code" href="namespacerapidjson.html#a4eaef42a208413d1f2c8d4655ecec52d">Reader</a>;</div>
<div class="line"></div>
<div class="line">} <span class="comment">// namespace rapidjson</span></div>
</div><!-- fragment --><p><code>Reader</code>使用UTF-8作为来源及目标编码。来源编码是指JSON流的编码。目标编码是指<code>String()</code>的<code>str</code>参数所用的编码。例如，要解析一个UTF-8流并输出至UTF-16 string事件，你需要这么定义一个reader：</p>
<div class="fragment"><div class="line">GenericReader<UTF8<>, UTF16<> > reader;</div>
</div><!-- fragment --><p>注意到<code>UTF16</code>的缺省类型是<code>wchar_t</code>。因此这个<code>reader</code>需要调用处理器的<code>String(const wchar_t*, SizeType, bool)</code>。</p>
<p>第三个模板参数<code>Allocator</code>是内部数据结构（实际上是一个堆栈）的分配器类型。</p>
<h2><a class="anchor" id="Parsing"></a>
解析</h2>
<p><code>Reader</code>的唯一功能就是解析JSON。</p>
<div class="fragment"><div class="line"><span class="keyword">template</span> <<span class="keywordtype">unsigned</span> parseFlags, <span class="keyword">typename</span> InputStream, <span class="keyword">typename</span> Handler></div>
<div class="line"><span class="keywordtype">bool</span> Parse(InputStream& is, Handler& handler);</div>
<div class="line"></div>
<div class="line"><span class="comment">// 使用 parseFlags = kDefaultParseFlags</span></div>
<div class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> InputStream, <span class="keyword">typename</span> Handler></div>
<div class="line"><span class="keywordtype">bool</span> Parse(InputStream& is, Handler& handler);</div>
</div><!-- fragment --><p>若在解析中出现错误，它会返回<code>false</code>。使用者可调用<code>bool HasParseEror()</code>, <code>ParseErrorCode GetParseErrorCode()</code>及<code>size_t GetErrorOffset()</code>获取错误状态。实际上<code>Document</code>使用这些<code>Reader</code>函数去获取解析错误。请参考DOM去了解有关解析错误的细节。</p>
<h1><a class="anchor" id="Writer"></a>
Writer</h1>
<p><code>Reader</code>把JSON转换（解析）成为事件。<code>Writer</code>做完全相反的事情。它把事件转换成JSON。</p>
<p><code>Writer</code>是非常容易使用的。若你的应用程序只需把一些数据转换成JSON，可能直接使用<code>Writer</code>，会比建立一个<code>Document</code>然后用<code>Writer</code>把它转换成JSON更加方便。</p>
<p>在<code>simplewriter</code>例子里，我们做<code>simplereader</code>完全相反的事情。</p>
<div class="fragment"><div class="line"><span class="preprocessor">#include "rapidjson/writer.h"</span></div>
<div class="line"><span class="preprocessor">#include "rapidjson/stringbuffer.h"</span></div>
<div class="line"><span class="preprocessor">#include <iostream></span></div>
<div class="line"></div>
<div class="line"><span class="keyword">using namespace </span>rapidjson;</div>
<div class="line"><span class="keyword">using namespace </span>std;</div>
<div class="line"></div>
<div class="line"><span class="keywordtype">void</span> main() {</div>
<div class="line"> <a class="code" href="classrapidjson_1_1_generic_string_buffer.html">StringBuffer</a> s;</div>
<div class="line"> <a class="code" href="classrapidjson_1_1_writer.html">Writer<StringBuffer></a> writer(s);</div>
<div class="line"></div>
<div class="line"> writer.StartObject();</div>
<div class="line"> writer.Key(<span class="stringliteral">"hello"</span>);</div>
<div class="line"> writer.String(<span class="stringliteral">"world"</span>);</div>
<div class="line"> writer.Key(<span class="stringliteral">"t"</span>);</div>
<div class="line"> writer.Bool(<span class="keyword">true</span>);</div>
<div class="line"> writer.Key(<span class="stringliteral">"f"</span>);</div>
<div class="line"> writer.Bool(<span class="keyword">false</span>);</div>
<div class="line"> writer.Key(<span class="stringliteral">"n"</span>);</div>
<div class="line"> writer.Null();</div>
<div class="line"> writer.Key(<span class="stringliteral">"i"</span>);</div>
<div class="line"> writer.Uint(123);</div>
<div class="line"> writer.Key(<span class="stringliteral">"pi"</span>);</div>
<div class="line"> writer.<a class="code" href="classrapidjson_1_1_writer.html#ad5f042d9c1a8ce5be2d52e18255b4390">Double</a>(3.1416);</div>
<div class="line"> writer.Key(<span class="stringliteral">"a"</span>);</div>
<div class="line"> writer.StartArray();</div>
<div class="line"> <span class="keywordflow">for</span> (<span class="keywordtype">unsigned</span> i = 0; i < 4; i++)</div>
<div class="line"> writer.Uint(i);</div>
<div class="line"> writer.EndArray();</div>
<div class="line"> writer.EndObject();</div>
<div class="line"></div>
<div class="line"> cout << s.GetString() << endl;</div>
<div class="line">}</div>
</div><!-- fragment --><div class="fragment"><div class="line">{<span class="stringliteral">"hello"</span>:<span class="stringliteral">"world"</span>,<span class="stringliteral">"t"</span>:<span class="keyword">true</span>,<span class="stringliteral">"f"</span>:<span class="keyword">false</span>,<span class="stringliteral">"n"</span>:null,<span class="stringliteral">"i"</span>:123,<span class="stringliteral">"pi"</span>:3.1416,<span class="stringliteral">"a"</span>:[0,1,2,3]}</div>
</div><!-- fragment --><p><code>String()</code>及<code>Key()</code>各有两个重载。一个是如处理器concept般，有3个参数。它能处理含空字符的字符串。另一个是如上中使用的较简单版本。</p>
<p>注意到，例子代码中的<code>EndArray()</code>及<code>EndObject()</code>并没有参数。可以传递一个<code>SizeType</code>的参数，但它会被<code>Writer</code>忽略。</p>
<p>你可能会怀疑，为什么不使用<code>sprintf()</code>或<code>std::stringstream</code>去建立一个JSON？</p>
<p>这有几个原因：</p><ol type="1">
<li><code>Writer</code>必然会输出一个结构良好（well-formed）的JSON。若然有错误的事件次序（如<code>Int()</code>紧随<code>StartObject()</code>出现），它会在调试模式中产生断言失败。</li>
<li><code>Writer::String()</code>可处理字符串转义（如把码点<code>U+000A</code>转换成<code>\n</code>）及进行Unicode转码。</li>
<li><code>Writer</code>一致地处理number的输出。</li>
<li><code>Writer</code>实现了事件处理器concept。可用于处理来自<code>Reader</code>、<code>Document</code>或其他事件发生器。</li>
<li><code>Writer</code>可对不同平台进行优化。</li>
</ol>
<p>无论如何，使用<code>Writer</code> API去生成JSON甚至乎比这些临时方法更简单。</p>
<h2><a class="anchor" id=""></a>
</h2>
<p><code>Writer</code>与<code>Reader</code>有少许设计区别。<code>Writer</code>是一个模板类，而不是一个typedef。 并没有<code>GenericWriter</code>。以下是<code>Writer</code>的声明。</p>
<div class="fragment"><div class="line"><span class="keyword">namespace </span>rapidjson {</div>
<div class="line"></div>
<div class="line"><span class="keyword">template</span><<span class="keyword">typename</span> OutputStream, <span class="keyword">typename</span> SourceEncoding = UTF8<>, <span class="keyword">typename</span> TargetEncoding = UTF8<>, <span class="keyword">typename</span> Allocator = CrtAllocator<> ></div>
<div class="line"><span class="keyword">class </span>Writer {</div>
<div class="line"><span class="keyword">public</span>:</div>
<div class="line"> <a class="code" href="classrapidjson_1_1_writer.html#ac592904fad01d9d0f29bb0585fb02aa7">Writer</a>(OutputStream& os, Allocator* allocator = 0, <span class="keywordtype">size_t</span> levelDepth = kDefaultLevelDepth)</div>
<div class="line"><span class="comment">// ...</span></div>
<div class="line">};</div>
<div class="line"></div>
<div class="line">} <span class="comment">// namespace rapidjson</span></div>
</div><!-- fragment --><p><code>OutputStream</code>模板参数是输出流的类型。它的类型不可以被自动推断，必须由使用者提供。</p>
<p><code>SourceEncoding</code>模板参数指定了<code>String(const Ch*, ...)</code>的编码。</p>
<p><code>TargetEncoding</code>模板参数指定输出流的编码。</p>
<p>最后一个<code>Allocator</code>是分配器的类型，用于分配内部数据结构（一个堆栈）。</p>
<p>此外，<code>Writer</code>的构造函数有一<code>levelDepth</code>参数。存储每层阶信息的初始内存分配量受此参数影响。</p>
<h2><a class="anchor" id="PrettyWriter"></a>
PrettyWriter</h2>
<p><code>Writer</code>所输出的是没有空格字符的最紧凑JSON，适合网络传输或储存，但不适合人类阅读。</p>
<p>因此，RapidJSON提供了一个<code>PrettyWriter</code>，它在输出中加入缩进及换行。</p>
<p><code>PrettyWriter</code>的用法与<code>Writer</code>几乎一样，不同之处是<code>PrettyWriter</code>提供了一个<code>SetIndent(Ch indentChar, unsigned indentCharCount)</code>函数。缺省的缩进是4个空格。</p>
<h2><a class="anchor" id="CompletenessReset"></a>
完整性及重置</h2>
<p>一个<code>Writer</code>只可输出单个JSON，其根节点可以是任何JSON类型。当处理完单个根节点事件（如<code>String()</code>），或匹配的最后<code>EndObject()</code>或<code>EndArray()</code>事件，输出的JSON是结构完整（well-formed）及完整的。使用者可调用<code>Writer::IsComplete()</code>去检测完整性。</p>
<p>当JSON完整时，<code>Writer</code>不能再接受新的事件。不然其输出便会是不合法的（例如有超过一个根节点）。为了重新利用<code>Writer</code>对象，使用者可调用<code>Writer::Reset(OutputStream& os)</code>去重置其所有内部状态及设置新的输出流。</p>
<h1><a class="anchor" id="Techniques"></a>
技巧</h1>
<h2><a class="anchor" id="CustomDataStructure"></a>
解析JSON至自定义结构</h2>
<p><code>Document</code>的解析功能完全依靠<code>Reader</code>。实际上<code>Document</code>是一个处理器，在解析JSON时接收事件去建立一个DOM。</p>
<p>使用者可以直接使用<code>Reader</code>去建立其他数据结构。这消除了建立DOM的步骤，从而减少了内存开销并改善性能。</p>
<p>在以下的<code>messagereader</code>例子中，<code>ParseMessages()</code>解析一个JSON，该JSON应该是一个含键值对的object。</p>
<div class="fragment"><div class="line"><span class="preprocessor">#include "<a class="code" href="reader_8h.html">rapidjson/reader.h</a>"</span></div>
<div class="line"><span class="preprocessor">#include "rapidjson/error/en.h"</span></div>
<div class="line"><span class="preprocessor">#include <iostream></span></div>
<div class="line"><span class="preprocessor">#include <string></span></div>
<div class="line"><span class="preprocessor">#include <map></span></div>
<div class="line"></div>
<div class="line"><span class="keyword">using namespace </span>std;</div>
<div class="line"><span class="keyword">using namespace </span>rapidjson;</div>
<div class="line"></div>
<div class="line"><span class="keyword">typedef</span> map<string, string> MessageMap;</div>
<div class="line"></div>
<div class="line"><span class="keyword">struct </span>MessageHandler</div>
<div class="line"> : <span class="keyword">public</span> <a class="code" href="structrapidjson_1_1_base_reader_handler.html">BaseReaderHandler</a><UTF8<>, MessageHandler> {</div>
<div class="line"> MessageHandler() : state_(kExpectObjectStart) {</div>
<div class="line"> }</div>
<div class="line"></div>
<div class="line"> <span class="keywordtype">bool</span> StartObject() {</div>
<div class="line"> <span class="keywordflow">switch</span> (state_) {</div>
<div class="line"> <span class="keywordflow">case</span> kExpectObjectStart:</div>
<div class="line"> state_ = kExpectNameOrObjectEnd;</div>
<div class="line"> <span class="keywordflow">return</span> <span class="keyword">true</span>;</div>
<div class="line"> <span class="keywordflow">default</span>:</div>
<div class="line"> <span class="keywordflow">return</span> <span class="keyword">false</span>;</div>
<div class="line"> }</div>
<div class="line"> }</div>
<div class="line"></div>
<div class="line"> <span class="keywordtype">bool</span> String(<span class="keyword">const</span> <span class="keywordtype">char</span>* str, <a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> length, <span class="keywordtype">bool</span>) {</div>
<div class="line"> <span class="keywordflow">switch</span> (state_) {</div>
<div class="line"> <span class="keywordflow">case</span> kExpectNameOrObjectEnd:</div>
<div class="line"> name_ = string(str, length);</div>
<div class="line"> state_ = kExpectValue;</div>
<div class="line"> <span class="keywordflow">return</span> <span class="keyword">true</span>;</div>
<div class="line"> <span class="keywordflow">case</span> kExpectValue:</div>
<div class="line"> messages_.insert(MessageMap::value_type(name_, <span class="keywordtype">string</span>(str, length)));</div>
<div class="line"> state_ = kExpectNameOrObjectEnd;</div>
<div class="line"> <span class="keywordflow">return</span> <span class="keyword">true</span>;</div>
<div class="line"> <span class="keywordflow">default</span>:</div>
<div class="line"> <span class="keywordflow">return</span> <span class="keyword">false</span>;</div>
<div class="line"> }</div>
<div class="line"> }</div>
<div class="line"></div>
<div class="line"> <span class="keywordtype">bool</span> EndObject(<a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a>) { <span class="keywordflow">return</span> state_ == kExpectNameOrObjectEnd; }</div>
<div class="line"></div>
<div class="line"> <span class="keywordtype">bool</span> Default() { <span class="keywordflow">return</span> <span class="keyword">false</span>; } <span class="comment">// All other events are invalid.</span></div>
<div class="line"></div>
<div class="line"> MessageMap messages_;</div>
<div class="line"> <span class="keyword">enum</span> State {</div>
<div class="line"> kExpectObjectStart,</div>
<div class="line"> kExpectNameOrObjectEnd,</div>
<div class="line"> kExpectValue,</div>
<div class="line"> }state_;</div>
<div class="line"> std::string name_;</div>
<div class="line">};</div>
<div class="line"></div>
<div class="line"><span class="keywordtype">void</span> ParseMessages(<span class="keyword">const</span> <span class="keywordtype">char</span>* json, MessageMap& messages) {</div>
<div class="line"> <a class="code" href="classrapidjson_1_1_generic_reader.html">Reader</a> reader;</div>
<div class="line"> MessageHandler handler;</div>
<div class="line"> <a class="code" href="structrapidjson_1_1_generic_string_stream.html">StringStream</a> ss(json);</div>
<div class="line"> <span class="keywordflow">if</span> (reader.<a class="code" href="classrapidjson_1_1_generic_reader.html#ac9c540b77de19661f6f45e04b9b0937b">Parse</a>(ss, handler))</div>
<div class="line"> messages.swap(handler.messages_); <span class="comment">// Only change it if success.</span></div>
<div class="line"> <span class="keywordflow">else</span> {</div>
<div class="line"> <a class="code" href="group___r_a_p_i_d_j_s_o_n___e_r_r_o_r_s.html#ga7d3acf640886b1f2552dc8c4cd6dea60">ParseErrorCode</a> e = reader.<a class="code" href="classrapidjson_1_1_generic_reader.html#a40fd326bad7d7033f096b549cbf159db">GetParseErrorCode</a>();</div>
<div class="line"> <span class="keywordtype">size_t</span> o = reader.<a class="code" href="classrapidjson_1_1_generic_reader.html#ab907a36c410e96154c99797a6a0ce16d">GetErrorOffset</a>();</div>
<div class="line"> cout << <span class="stringliteral">"Error: "</span> << <a class="code" href="group___r_a_p_i_d_j_s_o_n___e_r_r_o_r_s.html#gabdaf1a7a4db30fb0e3d927fdf0fabe79">GetParseError_En</a>(e) << endl;;</div>
<div class="line"> cout << <span class="stringliteral">" at offset "</span> << o << <span class="stringliteral">" near '"</span> << string(json).substr(o, 10) << <span class="stringliteral">"...'"</span> << endl;</div>
<div class="line"> }</div>
<div class="line">}</div>
<div class="line"></div>
<div class="line"><span class="keywordtype">int</span> main() {</div>
<div class="line"> MessageMap messages;</div>
<div class="line"></div>
<div class="line"> <span class="keyword">const</span> <span class="keywordtype">char</span>* json1 = <span class="stringliteral">"{ \"greeting\" : \"Hello!\", \"farewell\" : \"bye-bye!\" }"</span>;</div>
<div class="line"> cout << json1 << endl;</div>
<div class="line"> ParseMessages(json1, messages);</div>
<div class="line"></div>
<div class="line"> <span class="keywordflow">for</span> (MessageMap::const_iterator itr = messages.begin(); itr != messages.end(); ++itr)</div>
<div class="line"> cout << itr->first << <span class="stringliteral">": "</span> << itr->second << endl;</div>
<div class="line"></div>
<div class="line"> cout << endl << <span class="stringliteral">"Parse a JSON with invalid schema."</span> << endl;</div>
<div class="line"> <span class="keyword">const</span> <span class="keywordtype">char</span>* json2 = <span class="stringliteral">"{ \"greeting\" : \"Hello!\", \"farewell\" : \"bye-bye!\", \"foo\" : {} }"</span>;</div>
<div class="line"> cout << json2 << endl;</div>
<div class="line"> ParseMessages(json2, messages);</div>
<div class="line"></div>
<div class="line"> <span class="keywordflow">return</span> 0;</div>
<div class="line">}</div>
</div><!-- fragment --><div class="fragment"><div class="line">{ <span class="stringliteral">"greeting"</span> : <span class="stringliteral">"Hello!"</span>, <span class="stringliteral">"farewell"</span> : <span class="stringliteral">"bye-bye!"</span> }</div>
<div class="line">farewell: bye-bye!</div>
<div class="line">greeting: Hello!</div>
<div class="line"></div>
<div class="line">Parse a JSON with invalid schema.</div>
<div class="line">{ <span class="stringliteral">"greeting"</span> : <span class="stringliteral">"Hello!"</span>, <span class="stringliteral">"farewell"</span> : <span class="stringliteral">"bye-bye!"</span>, <span class="stringliteral">"foo"</span> : {} }</div>
<div class="line">Error: Terminate parsing due to Handler error.</div>
<div class="line"> at offset 59 near <span class="stringliteral">'} }...'</span></div>
</div><!-- fragment --><p>第一个JSON（<code>json1</code>）被成功地解析至<code>MessageMap</code>。由于<code>MessageMap</code>是一个<code>std::map</code>，打印次序按键值排序。此次序与JSON中的次序不同。</p>
<p>在第二个JSON（<code>json2</code>）中，<code>foo</code>的值是一个空object。由于它是一个object，<code>MessageHandler::StartObject()</code>会被调用。然而，在<code>state_ = kExpectValue</code>的情况下，该函数会返回<code>false</code>，并导致解析过程终止。错误代码是<code>kParseErrorTermination</code>。</p>
<h2><a class="anchor" id="Filtering"></a>
过滤JSON</h2>
<p>如前面提及过，<code>Writer</code>可处理<code>Reader</code>发出的事件。<code>example/condense/condense.cpp</code>例子简单地设置<code>Writer</code>作为一个<code>Reader</code>的处理器，因此它能移除JSON中的所有空白字符。<code>example/pretty/pretty.cpp</code>例子使用同样的关系，只是以<code>PrettyWriter</code>取代<code>Writer</code>。因此<code>pretty</code>能够重新格式化JSON，加入缩进及换行。</p>
<p>实际上，我们可以使用SAX风格API去加入（多个）中间层去过滤JSON的内容。例如<code>capitalize</code>例子可以把所有JSON string改为大写。</p>
<div class="fragment"><div class="line"><span class="preprocessor">#include "<a class="code" href="reader_8h.html">rapidjson/reader.h</a>"</span></div>
<div class="line"><span class="preprocessor">#include "rapidjson/writer.h"</span></div>
<div class="line"><span class="preprocessor">#include "rapidjson/filereadstream.h"</span></div>
<div class="line"><span class="preprocessor">#include "rapidjson/filewritestream.h"</span></div>
<div class="line"><span class="preprocessor">#include "rapidjson/error/en.h"</span></div>
<div class="line"><span class="preprocessor">#include <vector></span></div>
<div class="line"><span class="preprocessor">#include <cctype></span></div>
<div class="line"></div>
<div class="line"><span class="keyword">using namespace </span>rapidjson;</div>
<div class="line"></div>
<div class="line"><span class="keyword">template</span><<span class="keyword">typename</span> OutputHandler></div>
<div class="line"><span class="keyword">struct </span>CapitalizeFilter {</div>
<div class="line"> CapitalizeFilter(OutputHandler& out) : out_(out), buffer_() {</div>
<div class="line"> }</div>
<div class="line"></div>
<div class="line"> <span class="keywordtype">bool</span> Null() { <span class="keywordflow">return</span> out_.Null(); }</div>
<div class="line"> <span class="keywordtype">bool</span> Bool(<span class="keywordtype">bool</span> b) { <span class="keywordflow">return</span> out_.Bool(b); }</div>
<div class="line"> <span class="keywordtype">bool</span> Int(<span class="keywordtype">int</span> i) { <span class="keywordflow">return</span> out_.Int(i); }</div>
<div class="line"> <span class="keywordtype">bool</span> Uint(<span class="keywordtype">unsigned</span> u) { <span class="keywordflow">return</span> out_.Uint(u); }</div>
<div class="line"> <span class="keywordtype">bool</span> Int64(int64_t i) { <span class="keywordflow">return</span> out_.Int64(i); }</div>
<div class="line"> <span class="keywordtype">bool</span> Uint64(uint64_t u) { <span class="keywordflow">return</span> out_.Uint64(u); }</div>
<div class="line"> <span class="keywordtype">bool</span> Double(<span class="keywordtype">double</span> d) { <span class="keywordflow">return</span> out_.Double(d); }</div>
<div class="line"> <span class="keywordtype">bool</span> String(<span class="keyword">const</span> <span class="keywordtype">char</span>* str, <a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> length, <span class="keywordtype">bool</span>) { </div>
<div class="line"> buffer_.clear();</div>
<div class="line"> <span class="keywordflow">for</span> (<a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> i = 0; i < length; i++)</div>
<div class="line"> buffer_.push_back(std::toupper(str[i]));</div>
<div class="line"> <span class="keywordflow">return</span> out_.String(&buffer_.front(), length, <span class="keyword">true</span>); <span class="comment">// true = output handler need to copy the string</span></div>
<div class="line"> }</div>
<div class="line"> <span class="keywordtype">bool</span> StartObject() { <span class="keywordflow">return</span> out_.StartObject(); }</div>
<div class="line"> <span class="keywordtype">bool</span> Key(<span class="keyword">const</span> <span class="keywordtype">char</span>* str, <a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> length, <span class="keywordtype">bool</span> copy) { <span class="keywordflow">return</span> String(str, length, copy); }</div>
<div class="line"> <span class="keywordtype">bool</span> EndObject(<a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> memberCount) { <span class="keywordflow">return</span> out_.EndObject(memberCount); }</div>
<div class="line"> <span class="keywordtype">bool</span> StartArray() { <span class="keywordflow">return</span> out_.StartArray(); }</div>
<div class="line"> <span class="keywordtype">bool</span> EndArray(<a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> elementCount) { <span class="keywordflow">return</span> out_.EndArray(elementCount); }</div>
<div class="line"></div>
<div class="line"> OutputHandler& out_;</div>
<div class="line"> std::vector<char> buffer_;</div>
<div class="line">};</div>
<div class="line"></div>
<div class="line"><span class="keywordtype">int</span> main(<span class="keywordtype">int</span>, <span class="keywordtype">char</span>*[]) {</div>
<div class="line"> <span class="comment">// Prepare JSON reader and input stream.</span></div>
<div class="line"> <a class="code" href="classrapidjson_1_1_generic_reader.html">Reader</a> reader;</div>
<div class="line"> <span class="keywordtype">char</span> readBuffer[65536];</div>
<div class="line"> <a class="code" href="classrapidjson_1_1_file_read_stream.html">FileReadStream</a> is(stdin, readBuffer, <span class="keyword">sizeof</span>(readBuffer));</div>
<div class="line"></div>
<div class="line"> <span class="comment">// Prepare JSON writer and output stream.</span></div>
<div class="line"> <span class="keywordtype">char</span> writeBuffer[65536];</div>
<div class="line"> <a class="code" href="classrapidjson_1_1_file_write_stream.html">FileWriteStream</a> os(stdout, writeBuffer, <span class="keyword">sizeof</span>(writeBuffer));</div>
<div class="line"> <a class="code" href="classrapidjson_1_1_writer.html">Writer<FileWriteStream></a> writer(os);</div>
<div class="line"></div>
<div class="line"> <span class="comment">// JSON reader parse from the input stream and let writer generate the output.</span></div>
<div class="line"> CapitalizeFilter<Writer<FileWriteStream> > filter(writer);</div>
<div class="line"> <span class="keywordflow">if</span> (!reader.<a class="code" href="classrapidjson_1_1_generic_reader.html#ac9c540b77de19661f6f45e04b9b0937b">Parse</a>(is, filter)) {</div>
<div class="line"> fprintf(stderr, <span class="stringliteral">"\nError(%u): %s\n"</span>, (<span class="keywordtype">unsigned</span>)reader.<a class="code" href="classrapidjson_1_1_generic_reader.html#ab907a36c410e96154c99797a6a0ce16d">GetErrorOffset</a>(), <a class="code" href="group___r_a_p_i_d_j_s_o_n___e_r_r_o_r_s.html#gabdaf1a7a4db30fb0e3d927fdf0fabe79">GetParseError_En</a>(reader.<a class="code" href="classrapidjson_1_1_generic_reader.html#a40fd326bad7d7033f096b549cbf159db">GetParseErrorCode</a>()));</div>
<div class="line"> <span class="keywordflow">return</span> 1;</div>
<div class="line"> }</div>
<div class="line"></div>
<div class="line"> <span class="keywordflow">return</span> 0;</div>
<div class="line">}</div>
</div><!-- fragment --><p>注意到，不可简单地把JSON当作字符串去改为大写。例如： </p><div class="fragment"><div class="line">[<span class="stringliteral">"Hello\nWorld"</span>]</div>
</div><!-- fragment --><p>简单地把整个JSON转为大写的话会产生错误的转义符： </p><div class="fragment"><div class="line">[<span class="stringliteral">"HELLO\NWORLD"</span>]</div>
</div><!-- fragment --><p>而<code>capitalize</code>就会产生正确的结果： </p><div class="fragment"><div class="line">[<span class="stringliteral">"HELLO\nWORLD"</span>]</div>
</div><!-- fragment --><p>我们还可以开发更复杂的过滤器。然而，由于SAX风格API在某一时间点只能提供单一事件的信息，使用者需要自行记录一些上下文信息（例如从根节点起的路径、储存其他相关值）。对于处理某些情况，用DOM会比SAX更容易实现。 </p>
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