Overload the subscript operator to call a function based on the type assigned

Question

I have an object that has functions like addString and addInteger. These functions add data to a JSON string. At the end, the JSON string can be obtained and sent out. How can this be made easier by overloading subscript operators to do the following?

jsonBuilder builder();

builder[ "string_value" ] = "Hello";
builder[ "int_value" ] = 5;
builder[ "another_string" ] = "Thank you";

Answer 1

You need to have a proxy class that is returned by the operator[] function and which handles the assignment. The proxy class then overloads the assignment operator to handle strings and integers differently.

Something like this:

#include <iostream>
#include <string>

struct TheMainClass
{
    struct AssignmentProxy
    {
        std::string name;
        TheMainClass* main;

        AssignmentProxy(std::string const& n, TheMainClass* m)
            : name(n), main(m)
        {}

        TheMainClass& operator=(std::string const& s)
        {
            main->addString(name, s);
            return *main;
        }

        TheMainClass& operator=(int i)
        {
            main->addInteger(name, i);
            return *main;
        }
    };

    AssignmentProxy operator[](std::string const& name)
    {
        return AssignmentProxy(name, this);
    }

    void addString(std::string const& name, std::string const& str)
    {
        std::cout << "Adding string " << name << " with value \"" << str << "\"\n";
    }

    void addInteger(std::string const& name, int i)
    {
        std::cout << "Adding integer " << name << " with value " << i << "\n";
    }
};

int main(int argc __attribute__((unused)), char *argv[] __attribute__((unused)))
{
    TheMainClass builder;
    builder[ "string_value" ] = "Hello";
    builder[ "int_value" ] = 5;
    builder[ "another_string" ] = "Thank you";
}

See here for a working example.