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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/api/basic_json/parser_callback_t/" rel="canonical"><link href=../parse_event_t/ rel=prev><link href=../patch/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>parser_callback_t - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> parser_callback_t </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch 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4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 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<span class=k>using</span><span class=w> </span><span class=n>parser_callback_t</span><span class=w> </span><span class=o>=</span>
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<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>function</span><span class=o><</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>&</span><span class=w> </span><span class=n>parsed</span><span class=p>)</span><span class=o>></span><span class=p>;</span>
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</code></pre></div> <p>With a parser callback function, the result of parsing a <abbr title="JavaScript Object Notation">JSON</abbr> text can be influenced. When passed to <a href=../parse/ ><code>parse</code></a>, it is called on certain events (passed as <a href=../parse_event_t/ ><code>parse_event_t</code></a> via parameter <code>event</code>) with a set recursion depth <code>depth</code> and context <abbr title="JavaScript Object Notation">JSON</abbr> value <code>parsed</code>. The return value of the callback function is a boolean indicating whether the element that emitted the callback shall be kept or not.</p> <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> <p><img alt="Example when certain parse events are triggered" src=../../../images/callback_events.png></p> <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 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. This holds for every value type and for both kinds of parent: a discarded element is removed from the surrounding array, and a discarded member is removed from the surrounding object together with its key.</li> <li>Arrays and objects can be discarded either at their <code>parse_event_t::array_start</code>/<code>parse_event_t::object_start</code> event or at their <code>parse_event_t::array_end</code>/<code>parse_event_t::object_end</code> event, and both remove the whole value. Discarding it at the start event also means the callback is called neither for the content of the value nor for its matching end event.</li> <li>Discarding a <code>parse_event_t::key</code> event discards the whole object member. The callback is still called for the associated value, but its return value has no further effect.</li> <li>In case a value outside a structured type is skipped, it is replaced with <code>null</code>. This case happens if the top-level element is skipped.</li> </ul> <h2 id=parameters>Parameters<a class=headerlink href=#parameters title="Permanent link">¶</a></h2> <dl> <dt><code>depth</code> (in)</dt> <dd>the depth of the recursion during parsing</dd> <dt><code>event</code> (in)</dt> <dd>an event of type <a href=../parse_event_t/ ><code>parse_event_t</code></a> indicating the context in the callback function has been called</dd> <dt><code>parsed</code> (in, out)</dt> <dd>the current intermediate parse result; note that writing to this value has no effect for <code>parse_event_t::Line truncated
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