nlohmannjson/doc/examples/sax_parse.cpp

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

using json = nlohmann::json;

// a simple event consumer that collects string representations of the passed
// values; not inheriting from json::json_sax_t is not required, but can
// help not to forget a required function
class sax_event_consumer : public json::json_sax_t
{
  public:
    std::vector<std::string> events;

    bool null() override
    {
        events.push_back("value: null");
        return true;
    }

    bool boolean(bool val) override
    {
        events.push_back("value: " + std::string(val ? "true" : "false"));
        return true;
    }

    bool number_integer(number_integer_t val) override
    {
        events.push_back("value: " + std::to_string(val));
        return true;
    }

    bool number_unsigned(number_unsigned_t val) override
    {
        events.push_back("value: " + std::to_string(val));
        return true;
    }

    bool number_float(number_float_t val, const string_t& s) override
    {
        events.push_back("value: " + s);
        return true;
    }

    bool string(string_t& val) override
    {
        events.push_back("value: " + val);
        return true;
    }

    bool start_object(std::size_t elements) override
    {
        events.push_back("start: object");
        return true;
    }

    bool end_object() override
    {
        events.push_back("end: object");
        return true;
    }

    bool start_array(std::size_t elements) override
    {
        events.push_back("start: array");
        return true;
    }

    bool end_array() override
    {
        events.push_back("end: array");
        return true;
    }

    bool key(string_t& val) override
    {
        events.push_back("key: " + val);
        return true;
    }

    bool binary(json::binary_t& val) override
    {
        events.push_back("binary");
        return true;
    }

    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override
    {
        events.push_back("error: " + std::string(ex.what()));
        return false;
    }
};

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],
            "Distance": 12.723374634
        }
    }
    )";

    // create a SAX event consumer object
    sax_event_consumer sec;

    // parse and serialize JSON
    bool result = json::sax_parse(text, &sec);

    // output the recorded events
    for (auto& event : sec.events)
    {
        std::cout << "(" << event << ") ";
    }

    // output the result of sax_parse
    std::cout << "\nresult: " << std::boolalpha << result << std::endl;
}