Complex class, C++

#include <iostream>
#include <math.h>

using namespace std;

class Complex {
private:
    double re;
    double im;
public:
    Complex(double _re, double _im = 0) : re(_re), im(_im) {}

    double Re() const {
        return re;
    }

    double Im() const {
        return im;
    }

    const Complex& operator+ () const {
        return *this;
    }

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

    friend Complex operator+(const Complex& a, const Complex& b);
    friend Complex operator-(const Complex& a, const Complex& b);
    friend Complex operator*(const Complex& a, const Complex& b);
    friend Complex operator/(const Complex& a, const Complex& b);
    friend bool operator==(const Complex& a, const Complex& b);
    friend bool operator!=(const Complex& a, const Complex& b);
};

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

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

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);
}

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

bool operator==(const Complex& a, const Complex& b) {
    if (a.re == b.re && a.im == b.im) {
        return true;
    }
    else {
        return false;
    }
}

bool operator!=(const Complex& a, const Complex& b) {
    return !(a == b);
}

double abs(const Complex& a) {
    return sqrt(a.Re() * a.Re() + a.Im() * a.Im());
}