Question

I am getting some problems overloading the << operator.
The error is this one: ‘JSON’ is not derived from ‘const std::basic_string<_CharT, _Traits, _Alloc>’

How is the correct method to overload the operator <<.
My objective is to be able to do std::cout<

My code is:
main.cpp

#include <iostream>
#include <string>
#include <fstream>
#include <sstream>

#include "JSON.hpp"

int main()
{
    std::cout<<"JSON V0.1"<<std::endl;

    std::string line;
    std::ifstream file;
    std::stringstream ss;
    JSON obj;

    file.open("test.json");
    if (file.is_open())
    {
        std::cout<<"File opened"<<std::endl;
        ss << file.rdbuf();

        obj.parse(ss);
        file.close();
     }

     std::cout<<obj<<std::endl;

     return 0;
}

JSON.hpp

#ifndef _JSON_H_
#define _JSON_H_

#include <string>
#include <sstream>

#include <boost/property_tree/ptree.hpp>

class JSON
{
    public:
        bool parse(std::stringstream &stream);
        std::string get(std::string &key);

    private:
        boost::property_tree::ptree pt;
};
#endif

JSON.cpp

#include <boost/property_tree/json_parser.hpp>

#include "JSON.hpp"

bool JSON::parse(std::stringstream &stream)
{
    boost::property_tree::read_json(stream, pt);
    return true;
}

std::string JSON::get(std::string &key)
{
    std::string rv = "null";
    return rv;
}

std::ostream& operator<<(std::ostream& out, const JSON& json)
{
    return out << "JSON" << std::endl;
}

Answer 1

The compiler only considers functions that it has seen before when it sees a function call, so it doesn’t see the operator<< you have in the .cpp file.

You must put a forward declaration of your operator<< somewhere so the compiler knows about it, like in the header:

std::ostream& operator<<(std::ostream&, const JSON&);

Then the compiler will know about the function when it sees you call it.