Read & Parse a JSON file c++ BOOST

Question

As there is a lot of questions already on this, I am kind of apprehensive about asking... but

I've looked at many different Questions and nothing from any of them is working for me. I have this code as my attempt but it doesn't work:

#include "boost/property_tree/ptree.hpp"
#include "boost/property_tree/json_parser.hpp"
using namespace boost::property_tree;
...
std::ifstream jsonFile("test_file.json");
if (!jsonFile){
    std::cerr << "Error opening file\n";
    return -1;
}
ptree pt;
json_parser::read_json(jsonFile, pt);
for (auto& array_element : pt) {
    for (auto& property : array_element.second) {
        std::cout << property.first << " = " << property.second.get_value<std::string>() << "\n";
    }
}

Its contents are in the following format:

[{"number": 1234,"string": "hello world"}, {"number": 5678,"string": "foo bar"}, ... etc }]

I can't get it to read out the 1234 and then the hello world. Infact, it just does nothing. How can I read out from my .JSON file?

Answer 1

I'm not completely sure what the problem is. It seems to work (once you make the JSON valid):

UPDATE: Boost JSON

Boost 1.75.0 introduced Boost JSON, far superior way to actually deal with Json: Live On Wandbox

#include <boost/json.hpp>
#include <boost/json/src.hpp>
#include <iostream>
#include <iterator>
#include <fstream>
namespace json = boost::json;

struct Rec {
    int64_t number;
    std::string string;

    friend Rec tag_invoke(json::value_to_tag<Rec>, json::value const& v) {
        auto& o = v.as_object();
        return {
            o.at("number").as_int64(),
            value_to<std::string>(o.at("string")),
        };
    }

    friend void tag_invoke(json::value_from_tag, json::value& v, Rec const& rec)
    {
        v = json::object{
            {"number", rec.number},
            {"string", rec.string},
        };
    }
};

int main() {
    std::ifstream ifs("input.txt");
    std::string   input(std::istreambuf_iterator<char>(ifs), {});

    using Recs = std::vector<Rec>;
    Recs recs  = value_to<std::vector<Rec>>(json::parse(input));

    for (auto& [n, s] : recs) {
        std::cout << "Rec { " << n << ", " << std::quoted(s) << " }\n";

        // some frivolous changes:
        n *= 2;
        reverse(begin(s), end(s));
    }

    std::cout << "Modified json: " << json::value_from(recs) << "\n";
}

Printing

Rec { 1234, "hello world" }
Rec { 5678, "foo bar" }
Modified json: [{"number":2468,"string":"dlrow olleh"},{"number":11356,"string":"rab oof"}]

Live On Coliru

#include "boost/property_tree/ptree.hpp"
#include "boost/property_tree/json_parser.hpp"

int main() {
    using boost::property_tree::ptree;

    std::ifstream jsonFile("input.txt");

    ptree pt;
    read_json(jsonFile, pt);

    for (auto & array_element: pt) {
        for (auto & property: array_element.second) {
            std::cout << property.first << " = " << property.second.get_value < std::string > () << "\n";
        }
    }
}

With input.txt containing:

[{"number": 1234, "string": "hello world"},{"number": 5678, "string": "foo bar"}]

Prints

number = 1234
string = hello world
number = 5678
string = foo bar