C++ overloaded operator with reverse order of associativity

Question

It was very hard to come up with a title... (I'm not a native English speaker.)

struct A
{
    int value;
    A operator+(int i) const
    {
        A a;
        a.value=value+i;
        return a;
    };
};
int main(){
    A a;
    a.value=2;
    a=a+2;
    return 0;
}

This code compiles/works as expected, but when I change a=a+2 to a=2+a, it won't compile anymore. GCC gives me this error:

no match for ”operator+” in ”2 + a”

Is there any way to somehow make 2+a work just like a+2?

Answer 1

You need a free function, defined after the class

struct A
{
   // ...
};

A operator+(int i, const A& a)
{
  return a+i; // assuming commutativity
};

also, you might consider defining A& operator+=(int i); in A an implement both versions of operator+ as free functions. You might also be interested in Boost.Operators or other helpers to simplify A, see my profile for two options.