Json::Value as private class member

Question

I'm writing a C++ class for Reading/Writing strings from/to a json file using the jsoncpp lib. My question is if it's possible to create a Json::Value private member for my class and use this every time I need to read/write instead of creating a new Json::Value inside every function?

If so, how can I initialize it in the constructor and access it inside every function?

Answer 1

You don't need any special initialization for the Json::Value . You can make it a private member variable and use it like any other member variable.

Here's an example where I've added support for streaming from/to istream/ostream objects and a few member functions to demonstrate access to specific fields:

#include "json/json.h"

#include <fstream>
#include <iostream>

class YourClass {
public:
    // two public accessors:
    double getTemp() const { return json_value["Temp"].asDouble(); }
    void setTemp(double value) { json_value["Temp"] = value; }

private:
    Json::Value json_value;

    friend std::istream& operator>>(std::istream& is, YourClass& yc) {
        return is >> yc.json_value;
    }
    friend std::ostream& operator<<(std::ostream& os, const YourClass& yc) {
        return os << yc.json_value;
    }
};

int main() {
    YourClass yc;

    if(std::ifstream file("some_file.json"); file) {
        file >> yc;      // read file
        std::cout << yc; // print result to screen

        // use public member functions
        std::cout << yc.getTemp() << '\n';
        yc.setTemp(6.12);
        std::cout << yc.getTemp() << '\n';
    }
}

Edit: I was asked to explain the if statement and it means if( init-statement ; condition ) (added in C++17) which becomes approximately the same as this:

{ // scope of `file`
    std::ifstream file("some_file.json");

    if(file) { // test that `file` is in a good state
        // ... 
    }
} // end of scope of `file`