void sum(const Complex& a, const Complex& b, Complex& c)
{
c.repart = a.repart + b.repart;
c.impart = a.impart + b.impart;
}
An alternative is
void sum(const Complex& a, const Complex& b, Complex& c)
{
c = Complex(a.repart + b.repart, a.impart + b.impart);
}