Question

Refer C++ FAQ lite

It states for binary infix arithmetic operators :

member functions don't allow promotion of the left hand argument, since that 
would change the class of the object that is the recipient of the member 
function invocation

Can someone please explain why this is so ? Why is there a constrain on the type of first argument ?

Thank you.

Answer 1

Consider this:

struct Number {
    Number(int val) : val(val) {  }    // *Not* explicit
    int val;                           // Everything public to simplify example

    Number operator+(Number const& other) const { return val + other.val; }
};

Number n(42);
Number result = 32 + n;    // Doesn't compile

But if we removed the member operator and made it a free function instead:

Number operator+(Number const& a, Number const& b) { return a.val + b.val; }

Number n(42);
Number result = 32 + n;    // Compiles!