Another possibility is
Complex:: Complex () {}
which means that there is no initialization of the real and imaginary parts. The drawback is that a given compiler can then assign random values to a given variable.
A call like Complex a(0.1,1.3); means that we could call the member function `Complex(double, double)`as
Complex:: Complex (double re_a, double im_a) {
re = re_a; im = im_a; }