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Parser Callbacks

Overview

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.

The type of the callback function is:

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

Callback event types

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
Example

When parsing the following JSON text,

{
    "name": "Berlin",
    "location": [
        52.519444,
        13.406667
    ]
}

these calls are made to the callback function:

event depth parsed
object_start 0 discarded
key 1 "name"
value 1 "Berlin"
key 1 "location"
array_start 1 discarded
value 2 52.519444
value 2 13.406667
array_end 1 [52.519444,13.406667]
object_end 0 {"location":[52.519444,13.406667],"name":"Berlin"}

Return 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.
  • 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.
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
    }
}