'friend' functions and << operator overloading: What is the proper way to overload an operator for a class?

Question

In a project I\'m working on, I have a Score class, defined below in score.h. I am trying to overload it so, when a << operation

Answer 1

You're getting compilation errors when operator<< is a member function in the example because you're creating an operator<< that takes a Score as the first parameter (the object the method's being called on), and then giving it an extra parameter at the end.

When you're calling a binary operator that's declared as a member function, the left side of the expression is the object the method's being called on. e.g. a + b might works like this:

A a;
B b

a.operator+(b)

It's typically preferable to use non-member binary operators (and in some cases -- e.g. operator<<for ostream is the only way to do it. In that case, a + b might work like this:

A a;
B b

operator+(a, b);

Here's a full example showing both ways of doing it; main() will output '55' three times:

#include 

struct B
{
    B(int b) : value(b) {}
    int value;
};


struct A
{
    A(int a) : value(a) {}
    int value;

    int operator+(const B& b) 
    {
        return this->value + b.value;
    }
};

int operator+(const A& a, const B& b)
{
    return a.value + b.value;
}

int main(int argc, char** argv)
{
    A a(22);
    B b(33);

    std::cout << a + b << std::endl;
    std::cout << operator+(a, b) << std::endl;
    std::cout << a.operator+(b) << std::endl;

    return 0;
}