void CComplex::multiply(CComplex &rhs)
{
    double tmp = m_real;
    m_real = m_real*rhs.m_real - m_imaginary*rhs.m_imaginary;
    m_imaginary = tmp*rhs.m_imaginary + m_imaginary*rhs.m_real; 
}

This routine will be correct if

CComplex x(1,2), y(3, 4);
x.multiply(y);

but will be wrong if 

x.multiply(x);