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features/parsing/parser_callbacks/index.html
+44 -44
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@@ -2,13 +2,13 @@
<span class=k>using</span><span class=w> </span><span class=n>parser_callback_t</span><span class=w> </span><span class=o>=</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>function</span><span class=o>&lt;</span><span class=kt>bool</span><span class=p>(</span><span class=kt>int</span><span class=w> </span><span class=n>depth</span><span class=p>,</span><span class=w> </span><span class=n>parse_event_t</span><span class=w> </span><span class=n>event</span><span class=p>,</span><span class=w> </span><span class=n>BasicJsonType</span><span class=o>&amp;</span><span class=w> </span><span class=n>parsed</span><span class=p>)</span><span class=o>&gt;</span><span class=p>;</span>
</code></pre></div> <h2 id=callback-event-types>Callback event types<a class=headerlink href=#callback-event-types title="Permanent link">&para;</a></h2> <p>We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following table describes the values of the parameters <code>depth</code>, <code>event</code>, and <code>parsed</code>.</p> <table> <thead> <tr> <th>parameter <code>event</code></th> <th>description</th> <th>parameter <code>depth</code></th> <th>parameter <code>parsed</code></th> </tr> </thead> <tbody> <tr> <td><code>parse_event_t::object_start</code></td> <td>the parser read <code>{</code> and started to process a <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> value with type discarded</td> </tr> <tr> <td><code>parse_event_t::key</code></td> <td>the parser read a key of a value in an object</td> <td>depth of the currently parsed <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> string containing the key</td> </tr> <tr> <td><code>parse_event_t::object_end</code></td> <td>the parser read <code>}</code> and finished processing a <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> object</td> </tr> <tr> <td><code>parse_event_t::array_start</code></td> <td>the parser read <code>[</code> and started to process a <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> value with type discarded</td> </tr> <tr> <td><code>parse_event_t::array_end</code></td> <td>the parser read <code>]</code> and finished processing a <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> array</td> </tr> <tr> <td><code>parse_event_t::value</code></td> <td>the parser finished reading a <abbr title="JavaScript Object Notation">JSON</abbr> value</td> <td>depth of the value</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> value</td> </tr> </tbody> </table> <details class=example> <summary>Example</summary> <p>When parsing the following <abbr title="JavaScript Object Notation">JSON</abbr> text,</p> <div class=highlight><pre><span></span><code><span class=p>{</span>
<span class=w> </span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Berlin&quot;</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;location&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Berlin"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"location"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=mf>52.519444</span><span class=p>,</span>
<span class=w> </span><span class=mf>13.406667</span>
<span class=w> </span><span class=p>]</span>
<span class=p>}</span>
</code></pre></div> <p>these calls are made to the callback function:</p> <table> <thead> <tr> <th>event</th> <th>depth</th> <th>parsed</th> </tr> </thead> <tbody> <tr> <td><code>object_start</code></td> <td>0</td> <td><em>discarded</em></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;name&quot;</span></code></td> </tr> <tr> <td><code>value</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;Berlin&quot;</span></code></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;location&quot;</span></code></td> </tr> <tr> <td><code>array_start</code></td> <td>1</td> <td><em>discarded</em></td> </tr> <tr> <td><code>value</code></td> <td>2</td> <td><code class=highlight><span class=mf>52.519444</span></code></td> </tr> <tr> <td><code>value</code></td> <td>2</td> <td><code class=highlight><span class=mf>13.406667</span></code></td> </tr> <tr> <td><code>array_end</code></td> <td>1</td> <td><code class=highlight><span class=p>[</span><span class=mf>52.519444</span><span class=p>,</span><span class=mf>13.406667</span><span class=p>]</span></code></td> </tr> <tr> <td><code>object_end</code></td> <td>0</td> <td><code class=highlight><span class=p>{</span><span class=nt>&quot;location&quot;</span><span class=p>:[</span><span class=mf>52.519444</span><span class=p>,</span><span class=mf>13.406667</span><span class=p>],</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=s2>&quot;Berlin&quot;</span><span class=p>}</span></code></td> </tr> </tbody> </table> </details> <div class="admonition note"> <p class=admonition-title>No built-in nesting depth limit</p> <p>The library has no built-in limit on recursion/nesting depth while parsing. A parser callback can only <em>discard</em> content it has already parsed (by returning <code class=highlight><span class=nb>false</span></code>); it cannot make parsing fail once a nesting limit is exceeded partway through reading a deeply nested value. If you need to reject over-deep untrusted input outright, track <code>depth</code> in a callback and <code>throw</code> from it once your limit is exceeded (a thrown exception propagates out of <code>parse()</code> as usual).</p> </div> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">&para;</a></h2> <p>Discarding a value (i.e., returning <code class=highlight><span class=nb>false</span></code>) has different effects depending on the context in which the function was called:</p> <ul> <li>Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read.</li> <li>In case a value outside a structured type is skipped, it is replaced with <code class=highlight><span class=kc>null</span></code>. This case happens if the top-level element is skipped.</li> </ul> <details class=example> <summary>Example</summary> <p>The example below demonstrates the <code>parse()</code> function with and without callback function.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
</code></pre></div> <p>these calls are made to the callback function:</p> <table> <thead> <tr> <th>event</th> <th>depth</th> <th>parsed</th> </tr> </thead> <tbody> <tr> <td><code>object_start</code></td> <td>0</td> <td><em>discarded</em></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>"name"</span></code></td> </tr> <tr> <td><code>value</code></td> <td>1</td> <td><code class=highlight><span class=s2>"Berlin"</span></code></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>"location"</span></code></td> </tr> <tr> <td><code>array_start</code></td> <td>1</td> <td><em>discarded</em></td> </tr> <tr> <td><code>value</code></td> <td>2</td> <td><code class=highlight><span class=mf>52.519444</span></code></td> </tr> <tr> <td><code>value</code></td> <td>2</td> <td><code class=highlight><span class=mf>13.406667</span></code></td> </tr> <tr> <td><code>array_end</code></td> <td>1</td> <td><code class=highlight><span class=p>[</span><span class=mf>52.519444</span><span class=p>,</span><span class=mf>13.406667</span><span class=p>]</span></code></td> </tr> <tr> <td><code>object_end</code></td> <td>0</td> <td><code class=highlight><span class=p>{</span><span class=nt>"location"</span><span class=p>:[</span><span class=mf>52.519444</span><span class=p>,</span><span class=mf>13.406667</span><span class=p>],</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"Berlin"</span><span class=p>}</span></code></td> </tr> </tbody> </table> </details> <div class="admonition note"> <p class=admonition-title>No built-in nesting depth limit</p> <p>The library has no built-in limit on recursion/nesting depth while parsing. A parser callback can only <em>discard</em> content it has already parsed (by returning <code class=highlight><span class=nb>false</span></code>); it cannot make parsing fail once a nesting limit is exceeded partway through reading a deeply nested value. If you need to reject over-deep untrusted input outright, track <code>depth</code> in a callback and <code>throw</code> from it once your limit is exceeded (a thrown exception propagates out of <code>parse()</code> as usual).</p> </div> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">&para;</a></h2> <p>Discarding a value (i.e., returning <code class=highlight><span class=nb>false</span></code>) has different effects depending on the context in which the function was called:</p> <ul> <li>Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read.</li> <li>In case a value outside a structured type is skipped, it is replaced with <code class=highlight><span class=kc>null</span></code>. This case happens if the top-level element is skipped.</li> </ul> <details class=example> <summary>Example</summary> <p>The example below demonstrates the <code>parse()</code> function with and without callback function.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
@@ -17,32 +17,32 @@
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</span>
<span class=w> </span><span class=c1>// a JSON text</span>
<span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>text</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span>
<span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>text</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span>
<span class=s> {</span>
<span class=s> &quot;Image&quot;: {</span>
<span class=s> &quot;Width&quot;: 800,</span>
<span class=s> &quot;Height&quot;: 600,</span>
<span class=s> &quot;Title&quot;: &quot;View from 15th Floor&quot;,</span>
<span class=s> &quot;Thumbnail&quot;: {</span>
<span class=s> &quot;Url&quot;: &quot;http://www.example.com/image/481989943&quot;,</span>
<span class=s> &quot;Height&quot;: 125,</span>
<span class=s> &quot;Width&quot;: 100</span>
<span class=s> "Image": {</span>
<span class=s> "Width": 800,</span>
<span class=s> "Height": 600,</span>
<span class=s> "Title": "View from 15th Floor",</span>
<span class=s> "Thumbnail": {</span>
<span class=s> "Url": "http://www.example.com/image/481989943",</span>
<span class=s> "Height": 125,</span>
<span class=s> "Width": 100</span>
<span class=s> },</span>
<span class=s> &quot;Animated&quot; : false,</span>
<span class=s> &quot;IDs&quot;: [116, 943, 234, 38793]</span>
<span class=s> "Animated" : false,</span>
<span class=s> "IDs": [116, 943, 234, 38793]</span>
<span class=s> }</span>
<span class=s> }</span>
<span class=s> </span><span class=dl>)</span><span class=s>&quot;</span><span class=p>;</span>
<span class=s> </span><span class=dl>)</span><span class=s>"</span><span class=p>;</span>
<span class=w> </span><span class=c1>// parse and serialize JSON</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_complete</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>text</span><span class=p>);</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j_complete</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;</span><span class=se>\n\n</span><span class=s>&quot;</span><span class=p>;</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j_complete</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</span><span class=p>;</span>
<span class=w> </span><span class=c1>// define parser callback</span>
<span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parser_callback_t</span><span class=w> </span><span class=n>cb</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>[](</span><span class=kt>int</span><span class=w> </span><span class=n>depth</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse_event_t</span><span class=w> </span><span class=n>event</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>&amp;</span><span class=w> </span><span class=n>parsed</span><span class=p>)</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=c1>// skip object elements with key &quot;Thumbnail&quot;</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>event</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse_event_t</span><span class=o>::</span><span class=n>key</span><span class=w> </span><span class=k>and</span><span class=w> </span><span class=n>parsed</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>json</span><span class=p>(</span><span class=s>&quot;Thumbnail&quot;</span><span class=p>))</span>
<span class=w> </span><span class=c1>// skip object elements with key "Thumbnail"</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>event</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse_event_t</span><span class=o>::</span><span class=n>key</span><span class=w> </span><span class=k>and</span><span class=w> </span><span class=n>parsed</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>json</span><span class=p>(</span><span class=s>"Thumbnail"</span><span class=p>))</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=nb>false</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
@@ -54,40 +54,40 @@
<span class=w> </span><span class=c1>// parse (with callback) and serialize JSON</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_filtered</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>text</span><span class=p>,</span><span class=w> </span><span class=n>cb</span><span class=p>);</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j_filtered</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j_filtered</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span>
<span class=w> </span><span class=nt>&quot;Image&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>&quot;Animated&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>false</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;Height&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>600</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;IDs&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=nt>"Image"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>"Animated"</span><span class=p>:</span><span class=w> </span><span class=kc>false</span><span class=p>,</span>
<span class=w> </span><span class=nt>"Height"</span><span class=p>:</span><span class=w> </span><span class=mi>600</span><span class=p>,</span>
<span class=w> </span><span class=nt>"IDs"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=mi>116</span><span class=p>,</span>
<span class=w> </span><span class=mi>943</span><span class=p>,</span>
<span class=w> </span><span class=mi>234</span><span class=p>,</span>
<span class=w> </span><span class=mi>38793</span>
<span class=w> </span><span class=p>],</span>
<span class=w> </span><span class=nt>&quot;Thumbnail&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>&quot;Height&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>125</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;Url&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;http://www.example.com/image/481989943&quot;</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>100</span>
<span class=w> </span><span class=nt>"Thumbnail"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>"Height"</span><span class=p>:</span><span class=w> </span><span class=mi>125</span><span class=p>,</span>
<span class=w> </span><span class=nt>"Url"</span><span class=p>:</span><span class=w> </span><span class=s2>"http://www.example.com/image/481989943"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"Width"</span><span class=p>:</span><span class=w> </span><span class=mi>100</span>
<span class=w> </span><span class=p>},</span>
<span class=w> </span><span class=nt>&quot;Title&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;View from 15th Floor&quot;</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
<span class=w> </span><span class=nt>"Title"</span><span class=p>:</span><span class=w> </span><span class=s2>"View from 15th Floor"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"Width"</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
<span class=p>{</span>
<span class=w> </span><span class=nt>&quot;Image&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>&quot;Animated&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>false</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;Height&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>600</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;IDs&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=nt>"Image"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>"Animated"</span><span class=p>:</span><span class=w> </span><span class=kc>false</span><span class=p>,</span>
<span class=w> </span><span class=nt>"Height"</span><span class=p>:</span><span class=w> </span><span class=mi>600</span><span class=p>,</span>
<span class=w> </span><span class=nt>"IDs"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=mi>116</span><span class=p>,</span>
<span class=w> </span><span class=mi>943</span><span class=p>,</span>
<span class=w> </span><span class=mi>234</span><span class=p>,</span>
<span class=w> </span><span class=mi>38793</span>
<span class=w> </span><span class=p>],</span>
<span class=w> </span><span class=nt>&quot;Title&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;View from 15th Floor&quot;</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
<span class=w> </span><span class=nt>"Title"</span><span class=p>:</span><span class=w> </span><span class=s2>"View from 15th Floor"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"Width"</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=recipe-rejecting-duplicate-object-keys>Recipe: rejecting duplicate object keys<a class=headerlink href=#recipe-rejecting-duplicate-object-keys title="Permanent link">&para;</a></h2> <p>The <abbr title="JavaScript Object Notation">JSON</abbr> specification leaves the handling of objects with repeated keys up to the implementation. As described in <a href=../../../api/basic_json/object_t/#behavior><code>object_t</code></a>, it is unspecified which value for a repeated key ends up in the resulting <code class=highlight><span class=n>json</span></code> value -- once parsing has produced that value, the duplicate is already gone, because object storage maps each key to a single value. If duplicate keys should instead be treated as an error, a parser callback can detect them while the object is still being read, before that ambiguity ever applies.</p> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
@@ -124,7 +124,7 @@
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>key</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>parsed</span><span class=p>.</span><span class=n>get_ref</span><span class=o>&lt;</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>&amp;&gt;</span><span class=p>();</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=o>!</span><span class=n>seen</span><span class=p>.</span><span class=n>insert</span><span class=p>(</span><span class=n>key</span><span class=p>).</span><span class=n>second</span><span class=p>)</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>&quot;duplicate JSON object key: &quot;</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=n>key</span><span class=p>);</span>
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>"duplicate JSON object key: "</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=n>key</span><span class=p>);</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=nb>true</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
@@ -138,22 +138,22 @@
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</span>
<span class=w> </span><span class=c1>// parsing succeeds when all keys are unique</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>parse_strict</span><span class=p>(</span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;one&quot;: 1, &quot;two&quot;: 2}</span><span class=dl>)</span><span class=s>&quot;</span><span class=p>);</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>parse_strict</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"one": 1, "two": 2}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=w> </span><span class=c1>// parsing throws when a key is repeated</span>
<span class=w> </span><span class=k>try</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>parse_strict</span><span class=p>(</span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;one&quot;: 1, &quot;one&quot;: 2}</span><span class=dl>)</span><span class=s>&quot;</span><span class=p>);</span>
<span class=w> </span><span class=n>parse_strict</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"one": 1, "one": 2}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=k>catch</span><span class=w> </span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>exception</span><span class=o>&amp;</span><span class=w> </span><span class=n>e</span><span class=p>)</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>e</span><span class=p>.</span><span class=n>what</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>e</span><span class=p>.</span><span class=n>what</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=nt>&quot;one&quot;</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>&quot;two&quot;</span><span class=p>:</span><span class=mi>2</span><span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=nt>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>}</span>
<span class=err>duplica</span><span class=kc>te</span><span class=w> </span><span class=err>JSON</span><span class=w> </span><span class=err>objec</span><span class=kc>t</span><span class=w> </span><span class=err>key</span><span class=p>:</span><span class=w> </span><span class=err>o</span><span class=kc>ne</span>
</code></pre></div> </details> <p>This approach has two limitations:</p> <ul> <li>The depth-indexed bookkeeping must account for the fact that <code>object_start</code> reports the depth of the <em>parent</em> of the object, while the <code>key</code> events inside that object are reported one depth deeper (see the event table above); it is easy to get this off by one for nested objects.</li> <li>The thrown exception cannot carry a <code>parse_error</code>-style byte offset, because position tracking only exists inside the parser and lexer, not at the callback layer.</li> </ul> <p>For strict validation with precise error positions, implementing a <a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> instead gives access to the parser's position information directly.</p> <h2 id=recipe-streaming-a-large-homogeneous-array>Recipe: streaming a large homogeneous array<a class=headerlink href=#recipe-streaming-a-large-homogeneous-array title="Permanent link">&para;</a></h2> <p>A common use case is a huge top-level array of many similarly-shaped objects, too large to hold entirely in memory as a <code class=highlight><span class=n>json</span></code> value. A parser callback can hand off each completed element to a user function and then discard it, so memory usage stays bounded by a single element (plus the not-yet-parsed tail of the input) rather than the whole document. Since the top-level array's <code>array_start</code>/<code>array_end</code> are reported at <code>depth == 0</code> (its parent is the document root), the object elements it contains are reported at <code>depth == 1</code>:</p> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>input</span><span class=p>(</span><span class=s>&quot;large_array.json&quot;</span><span class=p>);</span>
</code></pre></div> </details> <p>This approach has two limitations:</p> <ul> <li>The depth-indexed bookkeeping must account for the fact that <code>object_start</code> reports the depth of the <em>parent</em> of the object, while the <code>key</code> events inside that object are reported one depth deeper (see the event table above); it is easy to get this off by one for nested objects.</li> <li>The thrown exception cannot carry a <code>parse_error</code>-style byte offset, because position tracking only exists inside the parser and lexer, not at the callback layer.</li> </ul> <p>For strict validation with precise error positions, implementing a <a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> instead gives access to the parser's position information directly.</p> <h2 id=recipe-streaming-a-large-homogeneous-array>Recipe: streaming a large homogeneous array<a class=headerlink href=#recipe-streaming-a-large-homogeneous-array title="Permanent link">&para;</a></h2> <p>A common use case is a huge top-level array of many similarly-shaped objects, too large to hold entirely in memory as a <code class=highlight><span class=n>json</span></code> value. A parser callback can hand off each completed element to a user function and then discard it, so memory usage stays bounded by a single element (plus the not-yet-parsed tail of the input) rather than the whole document. Since the top-level array's <code>array_start</code>/<code>array_end</code> are reported at <code>depth == 0</code> (its parent is the document root), the object elements it contains are reported at <code>depth == 1</code>:</p> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>input</span><span class=p>(</span><span class=s>"large_array.json"</span><span class=p>);</span>
<span class=k>auto</span><span class=w> </span><span class=n>callback</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>[](</span><span class=kt>int</span><span class=w> </span><span class=n>depth</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse_event_t</span><span class=w> </span><span class=n>event</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>parsed</span><span class=p>)</span><span class=w> </span><span class=o>-&gt;</span><span class=w> </span><span class=kt>bool</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>depth</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=mi>1</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=n>event</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse_event_t</span><span class=o>::</span><span class=n>object_end</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
@@ -168,10 +168,10 @@
<span class=k>auto</span><span class=w> </span><span class=n>callback</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>[](</span><span class=kt>int</span><span class=w> </span><span class=n>depth</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse_event_t</span><span class=w> </span><span class=cm>/*event*/</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=cm>/*parsed*/</span><span class=p>)</span><span class=w> </span><span class=o>-&gt;</span><span class=w> </span><span class=kt>bool</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>depth</span><span class=w> </span><span class=o>&gt;</span><span class=w> </span><span class=n>max_depth</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>&quot;maximum nesting depth exceeded&quot;</span><span class=p>);</span>
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>"maximum nesting depth exceeded"</span><span class=p>);</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=nb>true</span><span class=p>;</span>
<span class=p>};</span>
<span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>input</span><span class=p>,</span><span class=w> </span><span class=n>callback</span><span class=p>);</span>
</code></pre></div> </details> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="August 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="August 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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