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<div class="title">教程 </div> </div>
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<div class="toc"><h3>目录</h3>
<ul><li class="level1"><a href="#ValueDocument">Value 及 Document</a></li>
<li class="level1"><a href="#QueryValue">查询Value</a><ul><li class="level2"><a href="#QueryArray">查询Array</a></li>
<li class="level2"><a href="#QueryObject">查询Object</a></li>
<li class="level2"><a href="#QueryNumber">查询Number</a></li>
<li class="level2"><a href="#QueryString">查询String</a></li>
</ul>
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<li class="level1"><a href="#CreateModifyValues">创建／修改值</a><ul><li class="level2"><a href="#ChangeValueType">改变Value类型</a></li>
<li class="level2"><a href="#MoveSemantics">转移语意（Move Semantics）</a><ul><li class="level3"><a href="#TemporaryValues">转移语意及临时值</a></li>
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<li class="level2"><a href="#CreateString">创建String</a></li>
<li class="level2"><a href="#ModifyArray">修改Array</a></li>
<li class="level2"><a href="#ModifyObject">修改Object</a></li>
<li class="level2"><a href="#DeepCopyValue">深复制Value</a></li>
<li class="level2"><a href="#SwapValues">交换Value</a></li>
</ul>
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<li class="level1"><a href="#WhatsNext">下一部分</a></li>
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<div class="textblock"><p>本教程简介文件对象模型（Document Object Model, DOM）API。</p>
<p>如<a href="../readme.zh-cn.md#用法一览">用法一览</a>中所示，可以解析一个JSON至DOM，然后就可以轻松查询及修改DOM，并最终转换回JSON。</p>
<h1><a class="anchor" id="ValueDocument"></a>
Value 及 Document</h1>
<p>每个JSON值都储存为<code>Value</code>类，而<code>Document</code>类则表示整个DOM，它存储了一个DOM树的根<code>Value</code>。RapidJSON的所有公开类型及函数都在<code>rapidjson</code>命名空间中。</p>
<h1><a class="anchor" id="QueryValue"></a>
查询Value</h1>
<p>在本节中，我们会使用到<code>example/tutorial/tutorial.cpp</code>中的代码片段。</p>
<p>假设我们用C语言的字符串储存一个JSON（<code>const char* json</code>）： </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>把它解析至一个<code>Document</code>： </p><div class="fragment"><div class="line"><span class="preprocessor">#include "<a class="code" href="document_8h.html">rapidjson/document.h</a>"</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="comment">// ...</span></div>
<div class="line"><a class="code" href="classrapidjson_1_1_generic_document.html">Document</a> document;</div>
<div class="line">document.<a class="code" href="classrapidjson_1_1_generic_document.html#aea842b533a858c9a3861451ad9e8642c">Parse</a>(json);</div>
</div><!-- fragment --><p>那么现在该JSON就会被解析至<code>document</code>中，成为一棵*DOM树*:</p>
<div class="image">
<img src="tutorial.png" alt="tutorial.png"/>
<div class="caption">
教程中的DOM</div></div>
<p> 自从RFC 7159作出更新，合法JSON文件的根可以是任何类型的JSON值。而在较早的RFC 4627中，根值只允许是Object或Array。而在上述例子中，根是一个Object。 </p><div class="fragment"><div class="line">assert(document.IsObject());</div>
</div><!-- fragment --><p>让我们查询一下根Object中有没有<code>"hello"</code>成员。由于一个<code>Value</code>可包含不同类型的值，我们可能需要验证它的类型，并使用合适的API去获取其值。在此例中，<code>"hello"</code>成员关联到一个JSON String。 </p><div class="fragment"><div class="line">assert(document.HasMember(<span class="stringliteral">"hello"</span>));</div>
<div class="line">assert(document[<span class="stringliteral">"hello"</span>].IsString());</div>
<div class="line">printf(<span class="stringliteral">"hello = %s\n"</span>, document[<span class="stringliteral">"hello"</span>].GetString());</div>
</div><!-- fragment --><div class="fragment"><div class="line">world</div>
</div><!-- fragment --><p>JSON True/False值是以<code>bool</code>表示的。 </p><div class="fragment"><div class="line">assert(document[<span class="stringliteral">"t"</span>].IsBool());</div>
<div class="line">printf(<span class="stringliteral">"t = %s\n"</span>, document[<span class="stringliteral">"t"</span>].GetBool() ? <span class="stringliteral">"true"</span> : <span class="stringliteral">"false"</span>);</div>
</div><!-- fragment --><div class="fragment"><div class="line"><span class="keyword">true</span></div>
</div><!-- fragment --><p>JSON Null值可用<code>IsNull()</code>查询。 </p><div class="fragment"><div class="line">printf(<span class="stringliteral">"n = %s\n"</span>, document[<span class="stringliteral">"n"</span>].IsNull() ? <span class="stringliteral">"null"</span> : <span class="stringliteral">"?"</span>);</div>
</div><!-- fragment --><div class="fragment"><div class="line">null</div>
</div><!-- fragment --><p>JSON Number类型表示所有数值。然而，C++需要使用更专门的类型。</p>
<div class="fragment"><div class="line">assert(document[<span class="stringliteral">"i"</span>].IsNumber());</div>
<div class="line"></div>
<div class="line"><span class="comment">// 在此情况下，IsUint()/IsInt64()/IsUInt64()也会返回 true</span></div>
<div class="line">assert(document[<span class="stringliteral">"i"</span>].IsInt()); </div>
<div class="line">printf(<span class="stringliteral">"i = %d\n"</span>, document[<span class="stringliteral">"i"</span>].GetInt());</div>
<div class="line"><span class="comment">// 另一种用法： (int)document["i"]</span></div>
<div class="line"></div>
<div class="line">assert(document[<span class="stringliteral">"pi"</span>].IsNumber());</div>
<div class="line">assert(document[<span class="stringliteral">"pi"</span>].IsDouble());</div>
<div class="line">printf(<span class="stringliteral">"pi = %g\n"</span>, document[<span class="stringliteral">"pi"</span>].GetDouble());</div>
</div><!-- fragment --><div class="fragment"><div class="line">i = 123</div>
<div class="line">pi = 3.1416</div>
</div><!-- fragment --><p>JSON Array包含一些元素。 </p><div class="fragment"><div class="line"><span class="comment">// 使用引用来连续访问，方便之余还更高效。</span></div>
<div class="line"><span class="keyword">const</span> <a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a>& a = document[<span class="stringliteral">"a"</span>];</div>
<div class="line">assert(a.IsArray());</div>
<div class="line"><span class="keywordflow">for</span> (<a class="code" href="namespacerapidjson.html#a44eb33eaa523e36d466b1ced64b85c84">SizeType</a> i = 0; i < a.Size(); i++) <span class="comment">// 使用 SizeType 而不是 size_t</span></div>
<div class="line"> printf(<span class="stringliteral">"a[%d] = %d\n"</span>, i, a[i].GetInt());</div>
</div><!-- fragment --><div class="fragment"><div class="line">a[0] = 1</div>
<div class="line">a[1] = 2</div>
<div class="line">a[2] = 3</div>
<div class="line">a[3] = 4</div>
</div><!-- fragment --><p>注意，RapidJSON并不自动转换各种JSON类型。例如，对一个String的Value调用<code>GetInt()</code>是非法的。在调试模式下，它会被断言失败。在发布模式下，其行为是未定义的。</p>
<p>以下将会讨论有关查询各类型的细节。</p>
<h2><a class="anchor" id="QueryArray"></a>
查询Array</h2>
<p>缺省情况下，<code>SizeType</code>是<code>unsigned</code>的typedef。在多数系统中，Array最多能存储2^32-1个元素。</p>
<p>你可以用整数字面量访问元素，如<code>a[0]</code>、<code>a[1]</code>、<code>a[2]</code>。</p>
<p>Array与<code>std::vector</code>相似，除了使用索引，也可使用迭代器来访问所有元素。 </p><div class="fragment"><div class="line"><span class="keywordflow">for</span> (Value::ConstValueIterator itr = a.Begin(); itr != a.End(); ++itr)</div>
<div class="line"> printf(<span class="stringliteral">"%d "</span>, itr->GetInt());</div>
</div><!-- fragment --><p>还有一些熟悉的查询函数：</p><ul>
<li><code>SizeType Capacity() const</code></li>
<li><code>bool Empty() const</code></li>
</ul>
<h2><a class="anchor" id="QueryObject"></a>
查询Object</h2>
<p>和Array相似，我们可以用迭代器去访问所有Object成员：</p>
<div class="fragment"><div class="line"><span class="keyword">static</span> <span class="keyword">const</span> <span class="keywordtype">char</span>* kTypeNames[] = </div>
<div class="line"> { <span class="stringliteral">"Null"</span>, <span class="stringliteral">"False"</span>, <span class="stringliteral">"True"</span>, <span class="stringliteral">"Object"</span>, <span class="stringliteral">"Array"</span>, <span class="stringliteral">"String"</span>, <span class="stringliteral">"Number"</span> };</div>
<div class="line"></div>
<div class="line"><span class="keywordflow">for</span> (Value::ConstMemberIterator itr = document.MemberBegin();</div>
<div class="line"> itr != document.MemberEnd(); ++itr)</div>
<div class="line">{</div>
<div class="line"> printf(<span class="stringliteral">"Type of member %s is %s\n"</span>,</div>
<div class="line"> itr->name.GetString(), kTypeNames[itr->value.GetType()]);</div>
<div class="line">}</div>
</div><!-- fragment --><div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4">Type</a> of member hello is String</div>
<div class="line"><a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4">Type</a> of member t is True</div>
<div class="line"><a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4">Type</a> of member f is False</div>
<div class="line"><a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4">Type</a> of member n is Null</div>
<div class="line"><a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4">Type</a> of member i is Number</div>
<div class="line"><a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4">Type</a> of member pi is Number</div>
<div class="line"><a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4">Type</a> of member a is Array</div>
</div><!-- fragment --><p>注意，当<code>operator[](const char*)</code>找不到成员，它会断言失败。</p>
<p>若我们不确定一个成员是否存在，便需要在调用<code>operator[](const char*)</code>前先调用<code>HasMember()</code>。然而，这会导致两次查找。更好的做法是调用<code>FindMember()</code>，它能同时检查成员是否存在并返回它的Value：</p>
<div class="fragment"><div class="line">Value::ConstMemberIterator itr = document.FindMember(<span class="stringliteral">"hello"</span>);</div>
<div class="line"><span class="keywordflow">if</span> (itr != document.MemberEnd())</div>
<div class="line"> printf(<span class="stringliteral">"%s %s\n"</span>, itr->value.GetString());</div>
</div><!-- fragment --><h2><a class="anchor" id="QueryNumber"></a>
查询Number</h2>
<p>JSON只提供一种数值类型──Number。数字可以是整数或实数。RFC 4627规定数字的范围由解析器指定。</p>
<p>由于C++提供多种整数及浮点数类型，DOM尝试尽量提供最广的范围及良好性能。</p>
<p>当解析一个Number时, 它会被存储在DOM之中，成为下列其中一个类型：</p>
<table class="doxtable">
<tr>
<th>类型 </th><th>描述 </th></tr>
<tr>
<td><code>unsigned</code> </td><td>32位无号整数 </td></tr>
<tr>
<td><code>int</code> </td><td>32位有号整数 </td></tr>
<tr>
<td><code>uint64_t</code> </td><td>64位无号整数 </td></tr>
<tr>
<td><code>int64_t</code> </td><td>64位有号整数 </td></tr>
<tr>
<td><code>double</code> </td><td>64位双精度浮点数 </td></tr>
</table>
<p>当查询一个Number时, 你可以检查该数字是否能以目标类型来提取：</p>
<table class="doxtable">
<tr>
<th>查检 </th><th>提取 </th></tr>
<tr>
<td><code>bool IsNumber()</code> </td><td>不适用 </td></tr>
<tr>
<td><code>bool IsUint()</code> </td><td><code>unsigned GetUint()</code> </td></tr>
<tr>
<td><code>bool IsInt()</code> </td><td><code>int GetInt()</code> </td></tr>
<tr>
<td><code>bool IsUint64()</code> </td><td><code>uint64_t GetUint64()</code> </td></tr>
<tr>
<td><code>bool IsInt64()</code> </td><td><code>int64_t GetInt64()</code> </td></tr>
<tr>
<td><code>bool IsDouble()</code> </td><td><code>double GetDouble()</code> </td></tr>
</table>
<p>注意，一个整数可能用几种类型来提取，而无需转换。例如，一个名为<code>x</code>的Value包含123，那么<code>x.IsInt() == x.IsUint() == x.IsInt64() == x.IsUint64() == true</code>。但如果一个名为<code>y</code>的Value包含-3000000000，那么仅会令<code>x.IsInt64() == true</code>。</p>
<p>当要提取Number类型，<code>GetDouble()</code>是会把内部整数的表示转换成<code>double</code>。注意<code>int</code> 和<code>unsigned</code>可以安全地转换至<code>double</code>，但<code>int64_t</code>及<code>uint64_t</code>可能会丧失精度（因为<code>double</code>的尾数只有52位）。</p>
<h2><a class="anchor" id="QueryString"></a>
查询String</h2>
<p>除了<code>GetString()</code>，<code>Value</code>类也有一个<code>GetStringLength()</code>。这里会解释个中原因。</p>
<p>根据RFC 4627，JSON String可包含Unicode字符<code>U+0000</code>，在JSON中会表示为<code>"\\u0000"</code>。问题是，C/C++通常使用空字符结尾字符串（null-terminated string），这种字符串把`<code>\0'</code>作为结束符号。</p>
<p>为了符合RFC 4627，RapidJSON支持包含<code>U+0000</code>的String。若你需要处理这些String，便可使用<code>GetStringLength()</code>去获得正确的字符串长度。</p>
<p>例如，当解析以下的JSON至<code>Document d</code>之后：</p>
<div class="fragment"><div class="line">{ <span class="stringliteral">"s"</span> : <span class="stringliteral">"a\u0000b"</span> }</div>
</div><!-- fragment --><p> <code>"a\\u0000b"</code>值的正确长度应该是3。但<code>strlen()</code>会返回1。</p>
<p><code>GetStringLength()</code>也可以提高性能，因为用户可能需要调用<code>strlen()</code>去分配缓冲。</p>
<p>此外，<code>std::string</code>也支持这个构造函数：</p>
<div class="fragment"><div class="line">string(<span class="keyword">const</span> <span class="keywordtype">char</span>* s, <span class="keywordtype">size_t</span> count);</div>
</div><!-- fragment --><p>此构造函数接受字符串长度作为参数。它支持在字符串中存储空字符，也应该会有更好的性能。</p>
<h2>比较两个Value</h2>
<p>你可使用<code>==</code>及<code>!=</code>去比较两个Value。当且仅当两个Value的类型及内容相同，它们才当作相等。你也可以比较Value和它的原生类型值。以下是一个例子。</p>
<div class="fragment"><div class="line"><span class="keywordflow">if</span> (document[<span class="stringliteral">"hello"</span>] == document[<span class="stringliteral">"n"</span>]) <span class="comment">/*...*/</span>; <span class="comment">// 比较两个值</span></div>
<div class="line"><span class="keywordflow">if</span> (document[<span class="stringliteral">"hello"</span>] == <span class="stringliteral">"world"</span>) <span class="comment">/*...*/</span>; <span class="comment">// 与字符串家面量作比较</span></div>
<div class="line"><span class="keywordflow">if</span> (document[<span class="stringliteral">"i"</span>] != 123) <span class="comment">/*...*/</span>; <span class="comment">// 与整数作比较</span></div>
<div class="line"><span class="keywordflow">if</span> (document[<span class="stringliteral">"pi"</span>] != 3.14) <span class="comment">/*...*/</span>; <span class="comment">// 与double作比较</span></div>
</div><!-- fragment --><p>Array／Object顺序以它们的元素／成员作比较。当且仅当它们的整个子树相等，它们才当作相等。</p>
<p>注意，现时若一个Object含有重复命名的成员，它与任何Object作比较都总会返回<code>false</code>。</p>
<h1><a class="anchor" id="CreateModifyValues"></a>
创建／修改值</h1>
<p>有多种方法去创建值。 当一个DOM树被创建或修改后，可使用<code>Writer</code>再次存储为JSON。</p>
<h2><a class="anchor" id="ChangeValueType"></a>
改变Value类型</h2>
<p>当使用默认构造函数创建一个Value或Document，它的类型便会是Null。要改变其类型，需调用<code>SetXXX()</code>或赋值操作，例如：</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#ace11b5b575baf1cccd5ba5f8586dcdc8">Document</a> d; <span class="comment">// Null</span></div>
<div class="line">d.SetObject();</div>
<div class="line"></div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> v; <span class="comment">// Null</span></div>
<div class="line">v.SetInt(10);</div>
<div class="line">v = 10; <span class="comment">// 简写，和上面的相同</span></div>
</div><!-- fragment --><h3>构造函数的各个重载</h3>
<p>几个类型也有重载构造函数：</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> b(<span class="keyword">true</span>); <span class="comment">// 调用Value(bool)</span></div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> i(-123); <span class="comment">// 调用 Value(int)</span></div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> u(123u); <span class="comment">// 调用Value(unsigned)</span></div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> d(1.5); <span class="comment">// 调用Value(double)</span></div>
</div><!-- fragment --><p>要重建空Object或Array，可在默认构造函数后使用 <code>SetObject()</code>/<code>SetArray()</code>，或一次性使用<code>Value(Type)</code>：</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> o(<a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4acf030b422a32c3647c7c5973bd4dd0a9">kObjectType</a>);</div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> a(<a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4a058c622e1e7d59419ae58b895cbce468">kArrayType</a>);</div>
</div><!-- fragment --><h2><a class="anchor" id="MoveSemantics"></a>
转移语意（Move Semantics）</h2>
<p>在设计RapidJSON时有一个非常特别的决定，就是Value赋值并不是把来源Value复制至目的Value，而是把把来源Value转移（move）至目的Value。例如：</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> a(123);</div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> b(456);</div>
<div class="line">b = a; <span class="comment">// a变成Null，b变成数字123。</span></div>
</div><!-- fragment --><div class="image">
<img src="move1.png" alt="move1.png"/>
<div class="caption">
使用移动语意赋值。</div></div>
<p> 为什么？此语意有何优点？</p>
<p>最简单的答案就是性能。对于固定大小的JSON类型（Number、True、False、Null），复制它们是简单快捷。然而，对于可变大小的JSON类型（String、Array、Object），复制它们会产生大量开销，而且这些开销常常不被察觉。尤其是当我们需要创建临时Object，把它复制至另一变量，然后再析构它。</p>
<p>例如，若使用正常*复制*语意：</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> o(<a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4acf030b422a32c3647c7c5973bd4dd0a9">kObjectType</a>);</div>
<div class="line">{</div>
<div class="line"> <a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> contacts(<a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4a058c622e1e7d59419ae58b895cbce468">kArrayType</a>);</div>
<div class="line"> <span class="comment">// 把元素加进contacts数组。</span></div>
<div class="line"> <span class="comment">// ...</span></div>
<div class="line"> o.AddMember(<span class="stringliteral">"contacts"</span>, contacts, d.GetAllocator()); <span class="comment">// 深度复制contacts （可能有大量内存分配）</span></div>
<div class="line"> <span class="comment">// 析构contacts。</span></div>
<div class="line">}</div>
</div><!-- fragment --><div class="image">
<img src="move2.png" alt="move2.png"/>
<div class="caption">
复制语意产生大量的复制操作。</div></div>
<p> 那个<code>o</code> Object需要分配一个和contacts相同大小的缓冲区，对conacts做深度复制，并最终要析构contacts。这样会产生大量无必要的内存分配／释放，以及内存复制。</p>
<p>有一些方案可避免实质地复制这些数据，例如引用计数（reference counting）、垃圾回收（garbage collection, GC）。</p>
<p>为了使RapidJSON简单及快速，我们选择了对赋值采用*转移*语意。这方法与<code>std::auto_ptr</code>相似，都是在赋值时转移拥有权。转移快得多简单得多，只需要析构原来的Value，把来源<code>memcpy()</code>至目标，最后把来源设置为Null类型。</p>
<p>因此，使用转移语意后，上面的例子变成：</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> o(<a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4acf030b422a32c3647c7c5973bd4dd0a9">kObjectType</a>);</div>
<div class="line">{</div>
<div class="line"> <a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> contacts(<a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4a058c622e1e7d59419ae58b895cbce468">kArrayType</a>);</div>
<div class="line"> <span class="comment">// adding elements to contacts array.</span></div>
<div class="line"> o.AddMember(<span class="stringliteral">"contacts"</span>, contacts, d.GetAllocator()); <span class="comment">// 只需 memcpy() contacts本身至新成员的Value（16字节）</span></div>
<div class="line"> <span class="comment">// contacts在这里变成Null。它的析构是平凡的。</span></div>
<div class="line">}</div>
</div><!-- fragment --><div class="image">
<img src="move3.png" alt="move3.png"/>
<div class="caption">
转移语意不需复制。</div></div>
<p> 在C++11中这称为转移赋值操作（move assignment operator）。由于RapidJSON 支持C++03，它在赋值操作采用转移语意，其它修改形函数如<code>AddMember()</code>, <code>PushBack()</code>也采用转移语意。</p>
<h3><a class="anchor" id="TemporaryValues"></a>
转移语意及临时值</h3>
<p>有时候，我们想直接构造一个Value并传递给一个“转移”函数（如<code>PushBack()</code>、<code>AddMember()</code>）。由于临时对象是不能转换为正常的Value引用，我们加入了一个方便的<code>Move()</code>函数：</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> a(<a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4a058c622e1e7d59419ae58b895cbce468">kArrayType</a>);</div>
<div class="line">Document::AllocatorType& allocator = document.GetAllocator();</div>
<div class="line"><span class="comment">// a.PushBack(Value(42), allocator); // 不能通过编译</span></div>
<div class="line">a.PushBack(<a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a>().SetInt(42), allocator); <span class="comment">// fluent API</span></div>
<div class="line">a.PushBack(<a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a>(42).Move(), allocator); <span class="comment">// 和上一行相同</span></div>
</div><!-- fragment --><h2><a class="anchor" id="CreateString"></a>
创建String</h2>
<p>RapidJSON提供两个String的存储策略。</p>
<ol type="1">
<li>copy-string: 分配缓冲区，然后把来源数据复制至它。</li>
<li>const-string: 简单地储存字符串的指针。</li>
</ol>
<p>Copy-string总是安全的，因为它拥有数据的克隆。Const-string可用于存储字符串字面量，以及用于在DOM一节中将会提到的in-situ解析中。</p>
<p>为了让用户自定义内存分配方式，当一个操作可能需要内存分配时，RapidJSON要求用户传递一个allocator实例作为API参数。此设计避免了在每个Value存储allocator（或document）的指针。</p>
<p>因此，当我们把一个copy-string赋值时, 调用含有allocator的<code>SetString()</code>重载函数：</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#ace11b5b575baf1cccd5ba5f8586dcdc8">Document</a> document;</div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> author;</div>
<div class="line"><span class="keywordtype">char</span> buffer[10];</div>
<div class="line"><span class="keywordtype">int</span> len = sprintf(buffer, <span class="stringliteral">"%s %s"</span>, <span class="stringliteral">"Milo"</span>, <span class="stringliteral">"Yip"</span>); <span class="comment">// 动态创建的字符串。</span></div>
<div class="line">author.SetString(buffer, len, document.GetAllocator());</div>
<div class="line">memset(buffer, 0, <span class="keyword">sizeof</span>(buffer));</div>
<div class="line"><span class="comment">// 清空buffer后author.GetString() 仍然包含 "Milo Yip"</span></div>
</div><!-- fragment --><p>在此例子中，我们使用<code>Document</code>实例的allocator。这是使用RapidJSON时常用的惯用法。但你也可以用其他allocator实例。</p>
<p>另外，上面的<code>SetString()</code>需要长度参数。这个API能处理含有空字符的字符串。另一个<code>SetString()</code>重载函数没有长度参数，它假设输入是空字符结尾的，并会调用类似<code>strlen()</code>的函数去获取长度。</p>
<p>最后，对于字符串字面量或有安全生命周期的字符串，可以使用const-string版本的<code>SetString()</code>，它没有allocator参数。对于字符串家面量（或字符数组常量），只需简单地传递字面量，又安全又高效：</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> s;</div>
<div class="line">s.SetString(<span class="stringliteral">"rapidjson"</span>); <span class="comment">// 可包含空字符，长度在编译萁推导</span></div>
<div class="line">s = <span class="stringliteral">"rapidjson"</span>; <span class="comment">// 上行的缩写</span></div>
</div><!-- fragment --><p>对于字符指针，RapidJSON需要作一个标记，代表它不复制也是安全的。可以使用<code>StringRef</code>函数：</p>
<div class="fragment"><div class="line"><span class="keyword">const</span> <span class="keywordtype">char</span> * cstr = getenv(<span class="stringliteral">"USER"</span>);</div>
<div class="line"><span class="keywordtype">size_t</span> cstr_len = ...; <span class="comment">// 如果有长度</span></div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> s;</div>
<div class="line"><span class="comment">// s.SetString(cstr); // 这不能通过编译</span></div>
<div class="line">s.SetString(<a class="code" href="structrapidjson_1_1_generic_string_ref.html#aa6b9fd9f6aa49405a574c362ba9af6b5">StringRef</a>(cstr)); <span class="comment">// 可以，假设它的生命周期案全，并且是以空字符结尾的</span></div>
<div class="line">s = <a class="code" href="structrapidjson_1_1_generic_string_ref.html#aa6b9fd9f6aa49405a574c362ba9af6b5">StringRef</a>(cstr); <span class="comment">// 上行的缩写</span></div>
<div class="line">s.SetString(<a class="code" href="structrapidjson_1_1_generic_string_ref.html#aa6b9fd9f6aa49405a574c362ba9af6b5">StringRef</a>(cstr, cstr_len));<span class="comment">// 更快，可处理空字符</span></div>
<div class="line">s = <a class="code" href="structrapidjson_1_1_generic_string_ref.html#aa6b9fd9f6aa49405a574c362ba9af6b5">StringRef</a>(cstr, cstr_len); <span class="comment">// 上行的缩写</span></div>
</div><!-- fragment --><h2><a class="anchor" id="ModifyArray"></a>
修改Array</h2>
<p>Array类型的Value提供与<code>std::vector</code>相似的API。</p>
<ul>
<li><code>Clear()</code></li>
<li><code>Reserve(SizeType, Allocator&)</code></li>
<li><code>Value& PushBack(Value&, Allocator&)</code></li>
<li><code>template <typename T> GenericValue& PushBack(T, Allocator&)</code></li>
<li><code>Value& PopBack()</code></li>
<li><code>ValueIterator Erase(ConstValueIterator pos)</code></li>
<li><code>ValueIterator Erase(ConstValueIterator first, ConstValueIterator last)</code></li>
</ul>
<p>注意，<code>Reserve(...)</code>及<code>PushBack(...)</code>可能会为数组元素分配内存，所以需要一个allocator。</p>
<p>以下是<code>PushBack()</code>的例子：</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> a(<a class="code" href="namespacerapidjson.html#ae79a4751c1c460ff0de5ecc07874f3e4a058c622e1e7d59419ae58b895cbce468">kArrayType</a>);</div>
<div class="line">Document::AllocatorType& allocator = document.GetAllocator();</div>
<div class="line"></div>
<div class="line"><span class="keywordflow">for</span> (<span class="keywordtype">int</span> i = 5; i <= 10; i++)</div>
<div class="line"> a.PushBack(i, allocator); <span class="comment">// 可能需要调用realloc()所以需要allocator</span></div>
<div class="line"></div>
<div class="line"><span class="comment">// 流畅接口（Fluent interface）</span></div>
<div class="line">a.PushBack(<span class="stringliteral">"Lua"</span>, allocator).PushBack(<span class="stringliteral">"Mio"</span>, allocator);</div>
</div><!-- fragment --><p>与STL不一样的是，<code>PushBack()</code>/<code>PopBack()</code>返回Array本身的引用。这称为流畅接口（_fluent interface_）。</p>
<p>如果你想在Array中加入一个非常量字符串，或是一个没有足够生命周期的字符串（见<a href="#CreateString">Create String</a>），你需要使用copy-string API去创建一个String。为了避免加入中间变量，可以就地使用一个<a href="#TemporaryValues">临时值</a>：</p>
<div class="fragment"><div class="line"><span class="comment">// 就地Value参数</span></div>
<div class="line">contact.PushBack(<a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a>(<span class="stringliteral">"copy"</span>, document.GetAllocator()).Move(), <span class="comment">// copy string</span></div>
<div class="line"> document.GetAllocator());</div>
<div class="line"></div>
<div class="line"><span class="comment">// 显式Value参数</span></div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> val(<span class="stringliteral">"key"</span>, document.GetAllocator()); <span class="comment">// copy string</span></div>
<div class="line">contact.PushBack(val, document.GetAllocator());</div>
</div><!-- fragment --><h2><a class="anchor" id="ModifyObject"></a>
修改Object</h2>
<p>Object是键值对的集合。每个键必须为String。要修改Object，方法是增加或移除成员。以下的API用来增加城员：</p>
<ul>
<li><code>Value& AddMember(Value&, Value&, Allocator& allocator)</code></li>
<li><code>Value& AddMember(StringRefType, Value&, Allocator&)</code></li>
<li><code>template <typename T> Value& AddMember(StringRefType, T value, Allocator&)</code></li>
</ul>
<p>以下是一个例子。</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> contact(kObject);</div>
<div class="line">contact.AddMember(<span class="stringliteral">"name"</span>, <span class="stringliteral">"Milo"</span>, document.GetAllocator());</div>
<div class="line">contact.AddMember(<span class="stringliteral">"married"</span>, <span class="keyword">true</span>, document.GetAllocator());</div>
</div><!-- fragment --><p>使用<code>StringRefType</code>作为name参数的重载版本与字符串的<code>SetString</code>的接口相似。 这些重载是为了避免复制<code>name</code>字符串，因为JSON object中经常会使用常数键名。</p>
<p>如果你需要从非常数字符串或生命周期不足的字符串创建键名（见<a href="#CreateString">创建String</a>），你需要使用copy-string API。为了避免中间变量，可以就地使用<a href="#TemporaryValues">临时值</a>：</p>
<div class="fragment"><div class="line"><span class="comment">// 就地Value参数</span></div>
<div class="line">contact.AddMember(<a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a>(<span class="stringliteral">"copy"</span>, document.GetAllocator()).Move(), <span class="comment">// copy string</span></div>
<div class="line"> <a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a>().Move(), <span class="comment">// null value</span></div>
<div class="line"> document.GetAllocator());</div>
<div class="line"></div>
<div class="line"><span class="comment">// 显式参数</span></div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> key(<span class="stringliteral">"key"</span>, document.GetAllocator()); <span class="comment">// copy string name</span></div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> val(42); <span class="comment">// 某Value</span></div>
<div class="line">contact.AddMember(key, val, document.GetAllocator());</div>
</div><!-- fragment --><p>移除成员有几个选择：</p>
<ul>
<li><code>bool RemoveMember(const Ch* name)</code>：使用键名来移除成员（线性时间复杂度）。</li>
<li><code>bool RemoveMember(const Value& name)</code>：除了<code>name</code>是一个Value，和上一行相同。</li>
<li><code>MemberIterator RemoveMember(MemberIterator)</code>：使用迭代器移除成员（_常数_时间复杂度）。</li>
<li><code>MemberIterator EraseMember(MemberIterator)</code>：和上行相似但维持成员次序（线性时间复杂度）。</li>
<li><code>MemberIterator EraseMember(MemberIterator first, MemberIterator last)</code>：移除一个范围内的成员，维持次序（线性时间复杂度）。</li>
</ul>
<p><code>MemberIterator RemoveMember(MemberIterator)</code>使用了“转移最后”手法来达成常数时间复杂度。基本上就是析构迭代器位置的成员，然后把最后的成员转移至迭代器位置。因此，成员的次序会被改变。</p>
<h2><a class="anchor" id="DeepCopyValue"></a>
深复制Value</h2>
<p>若我们真的要复制一个DOM树，我们可使用两个APIs作深复制：含allocator的构造函数及<code>CopyFrom()</code>。</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#ace11b5b575baf1cccd5ba5f8586dcdc8">Document</a> d;</div>
<div class="line">Document::AllocatorType& a = d.GetAllocator();</div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> v1(<span class="stringliteral">"foo"</span>);</div>
<div class="line"><span class="comment">// Value v2(v1); // 不容许</span></div>
<div class="line"></div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> v2(v1, a); <span class="comment">// 制造一个克隆</span></div>
<div class="line">assert(v1.IsString()); <span class="comment">// v1不变</span></div>
<div class="line">d.SetArray().PushBack(v1, a).PushBack(v2, a);</div>
<div class="line">assert(v1.IsNull() && v2.IsNull()); <span class="comment">// 两个都转移动d</span></div>
<div class="line"></div>
<div class="line">v2.CopyFrom(d, a); <span class="comment">// 把整个document复制至v2</span></div>
<div class="line">assert(d.IsArray() && d.Size() == 2); <span class="comment">// d不变</span></div>
<div class="line">v1.SetObject().AddMember(<span class="stringliteral">"array"</span>, v2, a);</div>
<div class="line">d.PushBack(v1, a);</div>
</div><!-- fragment --><h2><a class="anchor" id="SwapValues"></a>
交换Value</h2>
<p>RapidJSON也提供<code>Swap()</code>。</p>
<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> a(123);</div>
<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> b(<span class="stringliteral">"Hello"</span>);</div>
<div class="line">a.Swap(b);</div>
<div class="line">assert(a.IsString());</div>
<div class="line">assert(b.IsInt());</div>
</div><!-- fragment --><p>无论两棵DOM树有多复杂，交换是很快的（常数时间）。</p>
<h1><a class="anchor" id="WhatsNext"></a>
下一部分</h1>
<p>本教程展示了如何询查及修改DOM树。RapidJSON还有一个重要概念：</p>
<ol type="1">
<li><a class="el" href="md_doc_stream_8zh-cn.html">流</a> 是读写JSON的通道。流可以是内存字符串、文件流等。用户也可以自定义流。</li>
<li><a class="el" href="md_doc_encoding_8zh-cn.html">编码</a>定义在流或内存中使用的字符编码。RapidJSON也在内部提供Unicode转换及校验功能。</li>
<li><a class="el" href="md_doc_dom_8zh-cn.html">DOM</a>的基本功能已在本教程里介绍。还有更高级的功能，如原位（*in situ*）解析、其他解析选项及高级用法。</li>
<li><a class="el" href="md_doc_sax_8zh-cn.html">SAX</a> 是RapidJSON解析／生成功能的基础。学习使用<code>Reader</code>/<code>Writer</code>去实现更高性能的应用程序。也可以使用<code>PrettyWriter</code>去格式化JSON。</li>
<li><a class="el" href="md_doc_performance_8zh-cn.html">性能</a>展示一些我们做的及第三方的性能测试。</li>
<li>技术内幕讲述一些RapidJSON内部的设计及技术。</li>
</ol>
<p>你也可以参考常见问题、API文档、例子及单元测试。 </p>
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