The solution to this is to inline functions. We discussed inlining in chapter 2 of the lecture notes. Inlining means that the function body is copied directly into the calling code, thus avoiding calling the function. Inlining is enabled by the inline keyword
inline Complex operator+ (const Complex& a, const Complex& b)
{ return Complex (a.re + b.re, a.im + b.im); }
Inline functions, with complete bodies must be written in the header file complex.h.