Rational class, C++

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

using namespace std;

class Rational {
private:
    int _n;
    int _d;

public:
    Rational(int n = 0, int d = 1) {
        int tmp = _gcd(n, d) * d / abs(d);
        _n = n / tmp;
        _d = d / tmp;
    }

    int numerator() const {
        return _n;
    }

    int denominator() const {
        return _d;
    }

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

    const Rational operator-() const {
        return Rational(-_n, _d);
    }

    Rational& operator++() {
        *this += 1;
        return *this;
    }

    Rational operator++(int) {
        Rational copy = *this;
        *this += 1;
        return copy;
    }

    Rational& operator--() {
        *this -= 1;
        return *this;
    }

    Rational operator--(int) {
        Rational copy = *this;
        *this -= 1;
        return copy;
    }

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

private:
    int _gcd(int a, int b) {
        a = abs(a);
        b = abs(b);
        return (a == 0) ? b : _gcd(b % a, a);
    }
};

Rational operator+(const Rational& a, const Rational& b) {
    return Rational(a._n * b._d + b._n * a._d, a._d * b._d);
}

Rational operator-(const Rational& a, const Rational& b) {
    return Rational(a._n * b._d - b._n * a._d, a._d * b._d);
}

Rational operator*(const Rational& a, const Rational& b) {
    return Rational(a._n * b._n, a._d * b._d);
}

Rational operator/(const Rational& a, const Rational& b) {
    return Rational(a._n * b._d, a._d * b._n);
}

Rational& operator+=(Rational& a, const Rational& b) {
    a = a + b;
    return a;
}

Rational& operator-=(Rational& a, const Rational& b) {
    a = a - b;
    return a;
}

Rational& operator*=(Rational& a, const Rational& b) {
    a = a * b;
    return a;
}

Rational& operator/=(Rational& a, const Rational& b) {
    a = a / b;
    return a;
}

bool operator==(const Rational& a, const Rational& b) {
    return (a._n == b._n) & (a._d == b._d);
}

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