Несмотря на то, что в С++ есть стандартный шаблон complex для представления комплекных чисел, я решил сделать свой класс для того, чтобы было проще и понятнее. В результате получилось следующее:

class Complex
{
public:
    double re, im;

    Complex () {}
    Complex ( double r           ) : re(r), im(0) {}
    Complex ( double r, double i ) : re(r), im(i) {}

    bool operator ! () const
    {
        return ! re && ! im;
    }

    Complex operator ~ () const
    {
        return Complex ( re, -im );
    }

    Complex operator - () const
    {
        return Complex ( -re, -im );
    }

    Complex & operator = ( const double & b )
    {
        re = b;
        im = 0.;
        return *this;
    }

    Complex & operator += ( const double & b )
    {
        re += b;
        return *this;
    }

    Complex & operator -= ( const double & b )
    {
        re -= b;
        return *this;
    }

    Complex & operator *= ( const double b )
    {
        re *= b;
        im *= b;
        return *this;
    }

    Complex & operator /= ( const double b )
    {
        re /= b;
        im /= b;
        return *this;
    }

    Complex & operator += ( const Complex & b )
    {
        re += b.re;
        im += b.im;
        return *this;
    }

    Complex & operator -= ( const Complex & b )
    {
        re -= b.re;
        im -= b.im;
        return *this;
    }

    Complex & operator *= ( const Complex & b )
    {
        const double rr = re * b.re - im * b.im;
        const double ii = re * b.im + im * b.re;
        re = rr;
        im = ii;
        return *this;
    }

    Complex & operator /= ( const Complex & b )
    {
        const double rr = re * b.re + im * b.im;
        const double ii = im * b.re - re * b.im;
        const double sq = b.re * b.re + b.im * b.im;
        re = rr / sq;
        im = ii / sq;
        return *this;
    }
};

////////////////////////////////////////////////////////////////////////

inline Complex operator + ( const double & a, const Complex & b )
{
    return Complex ( a + b.re, b.im );
}

inline Complex operator - ( const double & a, const Complex & b )
{
    return Complex ( a - b.re, - b.im );
}

inline Complex operator * ( const double & a, const Complex & b )
{
    return Complex ( a * b.re, a * b.im );
}

inline Complex operator / ( const double & a, const Complex & b )
{
    const double sq = b.re * b.re + b.im * b.im;
    return Complex ( a * b.re / sq, - a * b.im / sq );
}

////////////////////////////////////////////////////////////////////////

inline Complex operator + ( const Complex & a, const double & b )
{
    return Complex ( a.re + b, a.im );
}

inline Complex operator - ( const Complex & a, const double & b )
{
    return Complex ( a.re - b, a.im );
}

inline Complex operator * ( const Complex & a, const double & b )
{
    return Complex ( a.re * b, a.im * b );
}

inline Complex operator / ( const Complex & a, const double & b )
{
    return Complex ( a.re / b, a.im / b );
}

////////////////////////////////////////////////////////////////////////

inline Complex operator + ( const Complex & a, const Complex & b )
{
    return Complex ( a.re + b.re, a.im + b.im );
}

inline Complex operator - ( const Complex & a, const Complex & b )
{
    return Complex ( a.re - b.re, a.im - b.im );
}

inline Complex operator * ( const Complex & a, const Complex & b )
{
    return Complex ( a.re * b.re - a.im * b.im, a.re * b.im + a.im * b.re );
}

inline Complex operator / ( const Complex & a, const Complex & b )
{
    const double re = a.re * b.re + a.im * b.im;
    const double im = a.im * b.re - a.re * b.im;
    const double sq = b.re * b.re + b.im * b.im;
    return Complex ( re / sq, im / sq );
}

////////////////////////////////////////////////////////////////////////

inline bool operator == ( const Complex & a, const Complex & b )
{
    return ( a.re == b.re ) && ( a.im == b.im );
}

inline bool operator != ( const Complex & a, const Complex & b )
{
    return ( a.re != b.re ) || ( a.im != b.im );
}

//************************ 08.04.2006 *************************//
//
//                    Квадрат модуля числа
//
//************************ 08.04.2006 *************************//

inline double qmod ( const Complex & c )
{
    return c.re * c.re + c.im * c.im;
}

//************************ 08.04.2006 *************************//
//
//             Абсолютное значение ( модуль числа )
//
//************************ 08.04.2006 *************************//

double abs ( const Complex & );


//************************ 18.11.2018 *************************//
//
//                         Экспонента
//
//************************ 18.11.2018 *************************//

Complex exp ( const Complex & );


//************************ 08.04.2006 *************************//
//
//                     Квадратный корень
//
//************************ 08.04.2006 *************************//

Complex sqrt ( const Complex & );


//************************ 24.12.2011 *************************//
//
//                     Кубические корни
//
//************************ 24.12.2011 *************************//

void root3 ( Complex x, Complex & r1, Complex & r2, Complex & r3 );

Исходники находятся в файле complex.cpp.