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<div class="title">Pointer </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="#JsonPointer">JSON Pointer</a></li>
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<li class="level1"><a href="#BasicUsage">基本使用方法</a></li>
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<li class="level1"><a href="#HelperFunctions">辅助函数</a></li>
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<li class="level1"><a href="#ResolvingPointer">解析 Pointer</a><ul><li class="level2"><a href="#autotoc_md47">解析负号 token</a></li>
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<li class="level2"><a href="#autotoc_md48">解析 Document 及 Value</a></li>
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</ul>
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</li>
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<li class="level1"><a href="#ErrorHandling">错误处理</a></li>
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<li class="level1"><a href="#URIFragment">URI 片段表示方式</a></li>
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<li class="level1"><a href="#autotoc_md49">字符串化</a></li>
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<li class="level1"><a href="#UserSuppliedTokens">使用者提供的 tokens</a></li>
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</ul>
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</div>
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<div class="textblock"><p>（本功能于 v1.1.0 发布）</p>
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<p>JSON Pointer 是一个标准化（<a href="https://tools.ietf.org/html/rfc6901">RFC6901</a>）的方式去选取一个 JSON Document（DOM）中的值。这类似于 XML 的 XPath。然而，JSON Pointer 简单得多，而且每个 JSON Pointer 仅指向单个值。</p>
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<p>使用 RapidJSON 的 JSON Pointer 实现能简化一些 DOM 的操作。</p>
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<h1><a class="anchor" id="JsonPointer"></a>
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JSON Pointer</h1>
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<p>一个 JSON Pointer 由一串（零至多个）token 所组成，每个 token 都有 <code>/</code> 前缀。每个 token 可以是一个字符串或数字。例如，给定一个 JSON： </p><div class="fragment"><div class="line">{</div>
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<div class="line"> "foo" : ["bar", "baz"],</div>
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<div class="line"> "pi" : 3.1416</div>
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<div class="line">}</div>
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</div><!-- fragment --><p>以下的 JSON Pointer 解析为：</p>
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<ol type="1">
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<li><code>"/foo"</code> → <code>[ "bar", "baz" ]</code></li>
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<li><code>"/foo/0"</code> → <code>"bar"</code></li>
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<li><code>"/foo/1"</code> → <code>"baz"</code></li>
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<li><code>"/pi"</code> → <code>3.1416</code></li>
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</ol>
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<p>要注意，一个空 JSON Pointer <code>""</code> （零个 token）解析为整个 JSON。</p>
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<h1><a class="anchor" id="BasicUsage"></a>
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基本使用方法</h1>
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<p>以下的代码范例不解自明。</p>
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<div class="fragment"><div class="line"><span class="preprocessor">#include "rapidjson/pointer.h"</span></div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// ...</span></div>
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<div class="line"><a class="code" href="namespacerapidjson.html#ace11b5b575baf1cccd5ba5f8586dcdc8">Document</a> d;</div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// 使用 Set() 创建 DOM</span></div>
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<div class="line"><a class="code" href="namespacerapidjson.html#a080910f74d2f5046e6724280159cf374">Pointer</a>(<span class="stringliteral">"/project"</span>).Set(d, <span class="stringliteral">"RapidJSON"</span>);</div>
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<div class="line"><a class="code" href="namespacerapidjson.html#a080910f74d2f5046e6724280159cf374">Pointer</a>(<span class="stringliteral">"/stars"</span>).Set(d, 10);</div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// { "project" : "RapidJSON", "stars" : 10 }</span></div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// 使用 Get() 访问 DOM。若该值不存在则返回 nullptr。</span></div>
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<div class="line"><span class="keywordflow">if</span> (<a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a>* stars = <a class="code" href="namespacerapidjson.html#a080910f74d2f5046e6724280159cf374">Pointer</a>(<span class="stringliteral">"/stars"</span>).Get(d))</div>
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<div class="line"> stars->SetInt(stars->GetInt() + 1);</div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// { "project" : "RapidJSON", "stars" : 11 }</span></div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// Set() 和 Create() 自动生成父值（如果它们不存在）。</span></div>
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<div class="line"><a class="code" href="namespacerapidjson.html#a080910f74d2f5046e6724280159cf374">Pointer</a>(<span class="stringliteral">"/a/b/0"</span>).Create(d);</div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// { "project" : "RapidJSON", "stars" : 11, "a" : { "b" : [ null ] } }</span></div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// GetWithDefault() 返回引用。若该值不存在则会深拷贝缺省值。</span></div>
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<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a>& hello = <a class="code" href="namespacerapidjson.html#a080910f74d2f5046e6724280159cf374">Pointer</a>(<span class="stringliteral">"/hello"</span>).GetWithDefault(d, <span class="stringliteral">"world"</span>);</div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// { "project" : "RapidJSON", "stars" : 11, "a" : { "b" : [ null ] }, "hello" : "world" }</span></div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// Swap() 和 Set() 相似</span></div>
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<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> x(<span class="stringliteral">"C++"</span>);</div>
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<div class="line"><a class="code" href="namespacerapidjson.html#a080910f74d2f5046e6724280159cf374">Pointer</a>(<span class="stringliteral">"/hello"</span>).Swap(d, x);</div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// { "project" : "RapidJSON", "stars" : 11, "a" : { "b" : [ null ] }, "hello" : "C++" }</span></div>
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<div class="line"><span class="comment">// x 变成 "world"</span></div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// 删去一个成员或元素，若值存在返回 true</span></div>
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<div class="line"><span class="keywordtype">bool</span> success = <a class="code" href="namespacerapidjson.html#a080910f74d2f5046e6724280159cf374">Pointer</a>(<span class="stringliteral">"/a"</span>).Erase(d);</div>
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<div class="line">assert(success);</div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// { "project" : "RapidJSON", "stars" : 10 }</span></div>
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</div><!-- fragment --><h1><a class="anchor" id="HelperFunctions"></a>
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辅助函数</h1>
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<p>由于面向对象的调用习惯可能不符直觉，RapidJSON 也提供了一些辅助函数，它们把成员函数包装成自由函数。</p>
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<p>以下的例子与上面例子所做的事情完全相同。</p>
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<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#ace11b5b575baf1cccd5ba5f8586dcdc8">Document</a> d;</div>
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<div class="line"> </div>
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<div class="line">SetValueByPointer(d, <span class="stringliteral">"/project"</span>, <span class="stringliteral">"RapidJSON"</span>);</div>
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<div class="line">SetValueByPointer(d, <span class="stringliteral">"/stars"</span>, 10);</div>
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<div class="line"> </div>
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<div class="line"><span class="keywordflow">if</span> (<a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a>* stars = GetValueByPointer(d, <span class="stringliteral">"/stars"</span>))</div>
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<div class="line"> stars->SetInt(stars->GetInt() + 1);</div>
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<div class="line"> </div>
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<div class="line">CreateValueByPointer(d, <span class="stringliteral">"/a/b/0"</span>);</div>
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<div class="line"> </div>
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<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a>& hello = GetValueByPointerWithDefault(d, <span class="stringliteral">"/hello"</span>, <span class="stringliteral">"world"</span>);</div>
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<div class="line"> </div>
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<div class="line"><a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a> x(<span class="stringliteral">"C++"</span>);</div>
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<div class="line">SwapValueByPointer(d, <span class="stringliteral">"/hello"</span>, x);</div>
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<div class="line"> </div>
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<div class="line"><span class="keywordtype">bool</span> success = EraseValueByPointer(d, <span class="stringliteral">"/a"</span>);</div>
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<div class="line">assert(success);</div>
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</div><!-- fragment --><p>以下对比 3 种调用方式：</p>
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<ol type="1">
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<li><code>Pointer(source).<Method>(root, ...)</code></li>
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<li><code><Method>ValueByPointer(root, Pointer(source), ...)</code></li>
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<li><code><Method>ValueByPointer(root, source, ...)</code></li>
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</ol>
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<h1><a class="anchor" id="ResolvingPointer"></a>
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解析 Pointer</h1>
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<p><code>Pointer::Get()</code> 或 <code>GetValueByPointer()</code> 函数并不修改 DOM。若那些 token 不能匹配 DOM 里的值，这些函数便返回 <code>nullptr</code>。使用者可利用这个方法来检查一个值是否存在。</p>
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<p>注意，数值 token 可表示数组索引或成员名字。解析过程中会按值的类型来匹配。</p>
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<div class="fragment"><div class="line">{</div>
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<div class="line"> "0" : 123,</div>
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<div class="line"> "1" : [456]</div>
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<div class="line">}</div>
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</div><!-- fragment --><ol type="1">
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<li><code>"/0"</code> → <code>123</code></li>
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<li><code>"/1/0"</code> → <code>456</code></li>
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</ol>
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<p>Token <code>"0"</code> 在第一个 pointer 中被当作成员名字。它在第二个 pointer 中被当作成数组索引。</p>
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<p>其他函数会改变 DOM，包括 <code>Create()</code>、<code>GetWithDefault()</code>、<code>Set()</code>、<code>Swap()</code>。这些函数总是成功的。若一些父值不存在，就会创建它们。若父值类型不匹配 token，也会强行改变其类型。改变类型也意味着完全移除其 DOM 子树的内容。</p>
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<p>例如，把上面的 JSON 解译至 <code>d</code> 之后，</p>
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<div class="fragment"><div class="line">SetValueByPointer(d, <span class="stringliteral">"1/a"</span>, 789); <span class="comment">// { "0" : 123, "1" : { "a" : 789 } }</span></div>
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</div><!-- fragment --><h2><a class="anchor" id="autotoc_md47"></a>
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解析负号 token</h2>
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<p>另外，<a href="https://tools.ietf.org/html/rfc6901">RFC6901</a> 定义了一个特殊 token <code>-</code> （单个负号），用于表示数组最后元素的下一个元素。 <code>Get()</code> 只会把此 token 当作成员名字 '"-"'。而其他函数则会以此解析数组，等同于对数组调用 <code>Value::PushBack()</code> 。</p>
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<div class="fragment"><div class="line"><a class="code" href="namespacerapidjson.html#ace11b5b575baf1cccd5ba5f8586dcdc8">Document</a> d;</div>
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<div class="line">d.Parse(<span class="stringliteral">"{\"foo\":[123]}"</span>);</div>
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<div class="line">SetValueByPointer(d, <span class="stringliteral">"/foo/-"</span>, 456); <span class="comment">// { "foo" : [123, 456] }</span></div>
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<div class="line">SetValueByPointer(d, <span class="stringliteral">"/-"</span>, 789); <span class="comment">// { "foo" : [123, 456], "-" : 789 }</span></div>
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</div><!-- fragment --><h2><a class="anchor" id="autotoc_md48"></a>
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解析 Document 及 Value</h2>
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<p>当使用 <code>p.Get(root)</code> 或 <code>GetValueByPointer(root, p)</code>，<code>root</code> 是一个（常数） <code>Value&</code>。这意味着，它也可以是 DOM 里的一个子树。</p>
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<p>其他函数有两组签名。一组使用 <code>Document& document</code> 作为参数，另一组使用 <code>Value& root</code>。第一组使用 <code>document.GetAllocator()</code> 去创建值，而第二组则需要使用者提供一个 allocator，如同 DOM 里的函数。</p>
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<p>以上例子都不需要 allocator 参数，因为它的第一个参数是 <code>Document&</code>。但如果你需要对一个子树进行解析，就需要如下面的例子般提供 allocator：</p>
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<div class="fragment"><div class="line"><span class="keyword">class </span>Person {</div>
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<div class="line"><span class="keyword">public</span>:</div>
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<div class="line"> Person() {</div>
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<div class="line"> document_ = <span class="keyword">new</span> <a class="code" href="namespacerapidjson.html#ace11b5b575baf1cccd5ba5f8586dcdc8">Document</a>();</div>
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<div class="line"> <span class="comment">// CreateValueByPointer() here no need allocator</span></div>
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<div class="line"> SetLocation(CreateValueByPointer(*document_, <span class="stringliteral">"/residence"</span>), ...);</div>
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<div class="line"> SetLocation(CreateValueByPointer(*document_, <span class="stringliteral">"/office"</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="keyword">private</span>:</div>
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<div class="line"> <span class="keywordtype">void</span> SetLocation(<a class="code" href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">Value</a>& location, <span class="keyword">const</span> <span class="keywordtype">char</span>* country, <span class="keyword">const</span> <span class="keywordtype">char</span>* addresses[2]) {</div>
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<div class="line"> Value::Allocator& a = document_->GetAllocator();</div>
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<div class="line"> <span class="comment">// SetValueByPointer() here need allocator</span></div>
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<div class="line"> SetValueByPointer(location, <span class="stringliteral">"/country"</span>, country, a);</div>
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<div class="line"> SetValueByPointer(location, <span class="stringliteral">"/address/0"</span>, address[0], a);</div>
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<div class="line"> SetValueByPointer(location, <span class="stringliteral">"/address/1"</span>, address[1], a);</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="comment">// ...</span></div>
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<div class="line"> </div>
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<div class="line"> <a class="code" href="namespacerapidjson.html#ace11b5b575baf1cccd5ba5f8586dcdc8">Document</a>* document_;</div>
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<div class="line">};</div>
|
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</div><!-- fragment --><p><code>Erase()</code> 或 <code>EraseValueByPointer()</code> 不需要 allocator。而且它们成功删除值之后会返回 <code>true</code>。</p>
|
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<h1><a class="anchor" id="ErrorHandling"></a>
|
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错误处理</h1>
|
||
<p><code>Pointer</code> 在其建构函数里会解译源字符串。若有解析错误，<code>Pointer::IsValid()</code> 返回 <code>false</code>。你可使用 <code>Pointer::GetParseErrorCode()</code> 和 <code>GetParseErrorOffset()</code> 去获取错信息。</p>
|
||
<p>要注意的是，所有解析函数都假设 pointer 是合法的。对一个非法 pointer 解析会造成断言失败。</p>
|
||
<h1><a class="anchor" id="URIFragment"></a>
|
||
URI 片段表示方式</h1>
|
||
<p>除了我们一直在使用的字符串方式表示 JSON pointer，<a href="https://tools.ietf.org/html/rfc6901">RFC6901</a> 也定义了一个 JSON Pointer 的 URI 片段（fragment）表示方式。URI 片段是定义于 <a href="https://tools.ietf.org/html/rfc3986">RFC3986</a> "Uniform Resource Identifier (URI): Generic Syntax"。</p>
|
||
<p>URI 片段的主要分别是必然以 <code>#</code> （pound sign）开头，而一些字符也会以百分比编码成 UTF-8 序列。例如，以下的表展示了不同表示法下的 C/C++ 字符串常数。</p>
|
||
<table class="markdownTable">
|
||
<tr class="markdownTableHead">
|
||
<th class="markdownTableHeadNone">字符串表示方式 </th><th class="markdownTableHeadNone">URI 片段表示方式 </th><th class="markdownTableHeadNone">Pointer Tokens （UTF-8） </th></tr>
|
||
<tr class="markdownTableRowOdd">
|
||
<td class="markdownTableBodyNone"><code>"/foo/0"</code> </td><td class="markdownTableBodyNone"><code>"#/foo/0"</code> </td><td class="markdownTableBodyNone"><code>{"foo", 0}</code> </td></tr>
|
||
<tr class="markdownTableRowEven">
|
||
<td class="markdownTableBodyNone"><code>"/a~1b"</code> </td><td class="markdownTableBodyNone"><code>"#/a~1b"</code> </td><td class="markdownTableBodyNone"><code>{"a/b"}</code> </td></tr>
|
||
<tr class="markdownTableRowOdd">
|
||
<td class="markdownTableBodyNone"><code>"/m~0n"</code> </td><td class="markdownTableBodyNone"><code>"#/m~0n"</code> </td><td class="markdownTableBodyNone"><code>{"m~n"}</code> </td></tr>
|
||
<tr class="markdownTableRowEven">
|
||
<td class="markdownTableBodyNone"><code>"/ "</code> </td><td class="markdownTableBodyNone"><code>"#/%20"</code> </td><td class="markdownTableBodyNone"><code>{" "}</code> </td></tr>
|
||
<tr class="markdownTableRowOdd">
|
||
<td class="markdownTableBodyNone"><code>"/\0"</code> </td><td class="markdownTableBodyNone"><code>"#/%00"</code> </td><td class="markdownTableBodyNone"><code>{"\0"}</code> </td></tr>
|
||
<tr class="markdownTableRowEven">
|
||
<td class="markdownTableBodyNone"><code>"/€"</code> </td><td class="markdownTableBodyNone"><code>"#/%E2%82%AC"</code> </td><td class="markdownTableBodyNone"><code>{"€"}</code> </td></tr>
|
||
</table>
|
||
<p>RapidJSON 完全支持 URI 片段表示方式。它在解译时会自动检测 <code>#</code> 号。</p>
|
||
<h1><a class="anchor" id="autotoc_md49"></a>
|
||
字符串化</h1>
|
||
<p>你也可以把一个 <code>Pointer</code> 字符串化，储存于字符串或其他输出流。例如：</p>
|
||
<div class="fragment"><div class="line">Pointer p(...);</div>
|
||
<div class="line">StringBuffer sb;</div>
|
||
<div class="line">p.Stringify(sb);</div>
|
||
<div class="line">std::cout << sb.GetString() << std::endl;</div>
|
||
</div><!-- fragment --><p>使用 <code>StringifyUriFragment()</code> 可以把 pointer 字符串化为 URI 片段表示法。</p>
|
||
<h1><a class="anchor" id="UserSuppliedTokens"></a>
|
||
使用者提供的 tokens</h1>
|
||
<p>若一个 pointer 会用于多次解析，它应该只被创建一次，然后再施于不同的 DOM ，或在不同时间做解析。这样可以避免多次创键 <code>Pointer</code>，节省时间和内存分配。</p>
|
||
<p>我们甚至可以再更进一步，完全消去解析过程及动态内存分配。我们可以直接生成 token 数组：</p>
|
||
<div class="fragment"><div class="line"><span class="preprocessor">#define NAME(s) { s, sizeof(s) / sizeof(s[0]) - 1, kPointerInvalidIndex }</span></div>
|
||
<div class="line"><span class="preprocessor">#define INDEX(i) { #i, sizeof(#i) - 1, i }</span></div>
|
||
<div class="line"> </div>
|
||
<div class="line"><span class="keyword">static</span> <span class="keyword">const</span> Pointer::Token kTokens[] = { NAME(<span class="stringliteral">"foo"</span>), INDEX(123) };</div>
|
||
<div class="line"><span class="keyword">static</span> <span class="keyword">const</span> <a class="code" href="namespacerapidjson.html#a080910f74d2f5046e6724280159cf374">Pointer</a> p(kTokens, <span class="keyword">sizeof</span>(kTokens) / <span class="keyword">sizeof</span>(kTokens[0]));</div>
|
||
<div class="line"><span class="comment">// Equivalent to static const Pointer p("/foo/123");</span></div>
|
||
</div><!-- fragment --><p>这种做法可能适合内存受限的系统。 </p>
|
||
</div></div><!-- contents -->
|
||
</div><!-- PageDoc -->
|
||
</div><!-- doc-content -->
|
||
<div class="ttc" id="anamespacerapidjson_html_a080910f74d2f5046e6724280159cf374"><div class="ttname"><a href="namespacerapidjson.html#a080910f74d2f5046e6724280159cf374">rapidjson::Pointer</a></div><div class="ttdeci">GenericPointer< Value, CrtAllocator > Pointer</div><div class="ttdoc">GenericPointer for Value (UTF-8, default allocator).</div><div class="ttdef"><b>Definition:</b> fwd.h:126</div></div>
|
||
<div class="ttc" id="anamespacerapidjson_html_aa65fc9fb381b2cbc54f98673eadd6505"><div class="ttname"><a href="namespacerapidjson.html#aa65fc9fb381b2cbc54f98673eadd6505">rapidjson::Value</a></div><div class="ttdeci">GenericValue< UTF8<> > Value</div><div class="ttdoc">GenericValue with UTF8 encoding</div><div class="ttdef"><b>Definition:</b> document.h:2470</div></div>
|
||
<div class="ttc" id="anamespacerapidjson_html_ace11b5b575baf1cccd5ba5f8586dcdc8"><div class="ttname"><a href="namespacerapidjson.html#ace11b5b575baf1cccd5ba5f8586dcdc8">rapidjson::Document</a></div><div class="ttdeci">GenericDocument< UTF8<> > Document</div><div class="ttdoc">GenericDocument with UTF8 encoding</div><div class="ttdef"><b>Definition:</b> document.h:2873</div></div>
|
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