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<span class=w> </span><span class=nt>"Width"</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
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<span class=w> </span><span class=p>}</span>
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<span class=p>}</span>
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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="July 4, 2026 09:13:25 UTC">July 4, 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 © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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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="July 8, 2026 18:18:24 UTC">July 8, 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 © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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@@ -0,0 +1,155 @@
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# Parser Callbacks
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## Overview
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With a parser callback function, the result of parsing a JSON text can be influenced. When passed to `parse`, it is called on certain events (passed as `parse_event_t` via parameter `event`) with a set recursion depth `depth` and context JSON value `parsed`. The return value of the callback function is a boolean indicating whether the element that emitted the callback shall be kept or not.
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The type of the callback function is:
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```
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template<typename BasicJsonType>
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using parser_callback_t =
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std::function<bool(int depth, parse_event_t event, BasicJsonType& parsed)>;
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```
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## Callback event types
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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 `depth`, `event`, and `parsed`.
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| parameter `event` | description | parameter `depth` | parameter `parsed` |
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| ----------------------------- | --------------------------------------------------------- | ----------------------------------------- | -------------------------------- |
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| `parse_event_t::object_start` | the parser read `{` and started to process a JSON object | depth of the parent of the JSON object | a JSON value with type discarded |
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| `parse_event_t::key` | the parser read a key of a value in an object | depth of the currently parsed JSON object | a JSON string containing the key |
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| `parse_event_t::object_end` | the parser read `}` and finished processing a JSON object | depth of the parent of the JSON object | the parsed JSON object |
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| `parse_event_t::array_start` | the parser read `[` and started to process a JSON array | depth of the parent of the JSON array | a JSON value with type discarded |
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| `parse_event_t::array_end` | the parser read `]` and finished processing a JSON array | depth of the parent of the JSON array | the parsed JSON array |
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| `parse_event_t::value` | the parser finished reading a JSON value | depth of the value | the parsed JSON value |
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Example
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When parsing the following JSON text,
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```
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{
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"name": "Berlin",
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"location": [
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52.519444,
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13.406667
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]
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}
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```
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these calls are made to the callback function:
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| event | depth | parsed |
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| -------------- | ----- | ---------------------------------------------------- |
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| `object_start` | 0 | *discarded* |
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| `key` | 1 | `"name"` |
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| `value` | 1 | `"Berlin"` |
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| `key` | 1 | `"location"` |
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| `array_start` | 1 | *discarded* |
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| `value` | 2 | `52.519444` |
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| `value` | 2 | `13.406667` |
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| `array_end` | 1 | `[52.519444,13.406667]` |
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| `object_end` | 0 | `{"location":[52.519444,13.406667],"name":"Berlin"}` |
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## Return value
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Discarding a value (i.e., returning `false`) has different effects depending on the context in which the function was called:
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- Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read.
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- In case a value outside a structured type is skipped, it is replaced with `null`. This case happens if the top-level element is skipped.
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Example
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The example below demonstrates the `parse()` function with and without callback function.
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```
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#include <iostream>
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#include <iomanip>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// a JSON text
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auto text = R"(
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{
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"Image": {
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"Width": 800,
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"Height": 600,
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"Title": "View from 15th Floor",
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"Thumbnail": {
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"Url": "http://www.example.com/image/481989943",
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"Height": 125,
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"Width": 100
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},
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"Animated" : false,
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"IDs": [116, 943, 234, 38793]
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}
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}
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)";
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// parse and serialize JSON
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json j_complete = json::parse(text);
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std::cout << std::setw(4) << j_complete << "\n\n";
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// define parser callback
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json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
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{
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// skip object elements with key "Thumbnail"
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if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
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{
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return false;
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}
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else
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{
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return true;
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}
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};
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// parse (with callback) and serialize JSON
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json j_filtered = json::parse(text, cb);
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std::cout << std::setw(4) << j_filtered << '\n';
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}
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```
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Output:
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|
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```
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{
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"Image": {
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||||
"Animated": false,
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||||
"Height": 600,
|
||||
"IDs": [
|
||||
116,
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||||
943,
|
||||
234,
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||||
38793
|
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],
|
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"Thumbnail": {
|
||||
"Height": 125,
|
||||
"Url": "http://www.example.com/image/481989943",
|
||||
"Width": 100
|
||||
},
|
||||
"Title": "View from 15th Floor",
|
||||
"Width": 800
|
||||
}
|
||||
}
|
||||
|
||||
{
|
||||
"Image": {
|
||||
"Animated": false,
|
||||
"Height": 600,
|
||||
"IDs": [
|
||||
116,
|
||||
943,
|
||||
234,
|
||||
38793
|
||||
],
|
||||
"Title": "View from 15th Floor",
|
||||
"Width": 800
|
||||
}
|
||||
}
|
||||
```
|
||||
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