nlohmannjson/doc/examples/parse__istream__parser_callback_t.cpp

#include "json.hpp"
#include <iomanip> // for std::setw

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]
        }
    }
    )";

    // fill a stream with JSON text
    std::stringstream ss;
    ss << text;

    // parse and serialize JSON
    json j_complete = json::parse(ss);
    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;
        }
    };

    // fill a stream with JSON text
    ss.clear();
    ss << text;

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