The meaning of the addition operator \( + \) for Complex objects is defined in the
function
Complex operator+ (const Complex& a, const Complex& b); // a+b
The compiler translates c = a + b; into c = operator+ (a, b);.
Since this implies the call to function, it brings in an additional overhead. If speed
is crucial and this function call is performed inside a loop, then it is more difficult for a
given compiler to perform optimizations of a loop.