Overloading operators : const vs non-const return type : any difference of performance?

Question

If we go to the wikipedia article about C++ operators, we have as an example :

Addition : a + b -> T T::operator +(const T& b) const;

S

Answer 1

I would say don't ever implement operator + as member function. Just implement operator += which is definitely not const. Then you provide a global function which takes care about operator +, I think this is the most common way to do it.

T& T::operator +=(const T& t)
{
    // add t to object ...
    return *this;
}

T operator +(T t1, const T& t2)
{
    t1 += t2
    return t1; // RVO will eliminate copy (or move constructor)
}