With this function, statements like
Complex d = b; or Complex d(b);
make a new object d, which becomes a copy of b.
We can make simple implementations in terms of the assignment
Complex:: Complex (const Complex& c)
{ *this = c; }
which is a pointer to "this object", *this is the present object,
so *this = c; means setting the present object equal to c, that is
this->operator= (c);.