json/api/basic_json/parser_callback_t/index.md

nlohmann::basic_json::parser_callback_t

template<typename BasicJsonType>
using parser_callback_t =
    std::function<bool(int depth, parse_event_t event, BasicJsonType& parsed)>;

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.

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.

parameter event	description	parameter depth	parameter parsed
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
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
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
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
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
parse_event_t::value	the parser finished reading a JSON value	depth of the value	the parsed JSON value

Discarding a value (i.e., returning false) has different effects depending on the context in which function was called:

• 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.
• Arrays and objects can be discarded either at their parse_event_t::array_start/parse_event_t::object_start event or at their parse_event_t::array_end/parse_event_t::object_end 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.
• Discarding a parse_event_t::key event discards the whole object member. The callback is still called for the associated value, but its return value has no further effect.
• 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.

Parameters

depth (in) : the depth of the recursion during parsing

event (in) : an event of type parse_event_t indicating the context in the callback function has been called

parsed (in, out) : the current intermediate parse result; note that writing to this value has no effect for parse_event_t::key events

Return value

Whether the JSON value which called the function during parsing should be kept (true) or not (false). In the latter case, it is skipped completely, or replaced by null if it is the top-level value.

Examples

Example

The example below demonstrates the parse() function with and without callback function.

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // a JSON text
    auto text = R"(
    {
        "Image": {
            "Width":  800,
            "Height": 600,
            "Title":  "View from 15th Floor",
            "Thumbnail": {
                "Url":    "http://www.example.com/image/481989943",
                "Height": 125,
                "Width":  100
            },
            "Animated" : false,
            "IDs": [116, 943, 234, 38793]
        }
    }
    )";

    // parse and serialize JSON
    json j_complete = json::parse(text);
    std::cout << std::setw(4) << j_complete << "\n\n";

    // define parser callback
    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
    {
        // skip object elements with key "Thumbnail"
        if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
        {
            return false;
        }
        else
        {
            return true;
        }
    };

    // parse (with callback) and serialize JSON
    json j_filtered = json::parse(text, cb);
    std::cout << std::setw(4) << j_filtered << '\n';
}

Output:

{
    "Image": {
        "Animated": false,
        "Height": 600,
        "IDs": [
            116,
            943,
            234,
            38793
        ],
        "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
    }
}

Example

The example below shows where discarded values are removed. The array and the number are discarded in different ways, but in each case the parse result contains neither the value nor its key.

#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // a JSON text with an array and a number inside an object
    auto text = R"({"IDs": [116, 943], "Width": 800})";

    // discard the array when the parser reads its opening bracket
    json j_array_start = json::parse(text, [](int /*depth*/, json::parse_event_t event, json & /*parsed*/)
    {
        return event != json::parse_event_t::array_start;
    });

    // discard the same array when the parser reads its closing bracket
    json j_array_end = json::parse(text, [](int /*depth*/, json::parse_event_t event, json & /*parsed*/)
    {
        return event != json::parse_event_t::array_end;
    });

    // discard the number, but keep its key
    json j_value = json::parse(text, [](int /*depth*/, json::parse_event_t event, json & parsed)
    {
        return !(event == json::parse_event_t::value && parsed == json(800));
    });

    // discard the key of the number
    json j_key = json::parse(text, [](int /*depth*/, json::parse_event_t event, json & parsed)
    {
        return !(event == json::parse_event_t::key && parsed == json("Width"));
    });

    // discard the top-level object
    json j_root = json::parse(text, [](int /*depth*/, json::parse_event_t event, json & /*parsed*/)
    {
        return event != json::parse_event_t::object_end;
    });

    // in every case, the discarded value is removed together with its key
    std::cout << j_array_start << '\n'
              << j_array_end << '\n'
              << j_value << '\n'
              << j_key << '\n'
              << j_root << '\n';
}

Output:

{"Width":800}
{"Width":800}
{"IDs":[116,943]}
{"IDs":[116,943]}
null

See also

• parse deserialize from a compatible input
• parse_event_t enumeration of parser events

Version history

• Added in version 1.0.0.
• Fixed in version 3.13.0 to also remove discarded values from a parent object; before, discarding an array or a value stored under an object key left a discarded member behind, which made the parse result serialize to invalid JSON.