C++ / Operator overloading

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

using namespace std;

class Spatiu_Vectorial {
    float a;
    float b;
    float c;
public:
    Spatiu_Vectorial()  {
        a=b=c=0;
    }
    Spatiu_Vectorial(float x, float y, float z) {
        a=x; b=y; c=z;
    }
    void Afisare() {
        cout << "(" << a << ", " << b << ", " << c << ")" << endl;
    }
    float Lungimea() {
        return sqrt(a*a+b*b+c*c);
    }
    friend Spatiu_Vectorial operator + (Spatiu_Vectorial &d1, Spatiu_Vectorial &d2);
    friend Spatiu_Vectorial operator - (Spatiu_Vectorial &d1, Spatiu_Vectorial &d2);
    friend Spatiu_Vectorial operator * (Spatiu_Vectorial &d1, Spatiu_Vectorial &d2);
    friend Spatiu_Vectorial operator / (Spatiu_Vectorial &d1, Spatiu_Vectorial &d2);
    friend float skalar(Spatiu_Vectorial &d1, Spatiu_Vectorial &d2);
    friend float cosinus(Spatiu_Vectorial &d1, Spatiu_Vectorial &d2);
};


Spatiu_Vectorial operator + (Spatiu_Vectorial &d1, Spatiu_Vectorial &d2) {
    return Spatiu_Vectorial(d1.a + d2.a, d1.b + d2.b, d1.c + d2.c);
}
Spatiu_Vectorial operator - (Spatiu_Vectorial &d1, Spatiu_Vectorial &d2) {
    return Spatiu_Vectorial(d1.a - d2.a, d1.b - d2.b, d1.c - d2.c);
}
Spatiu_Vectorial operator * (Spatiu_Vectorial &d1, Spatiu_Vectorial &d2) {
    return Spatiu_Vectorial(d1.a * d2.a, d1.b * d2.b, d1.c * d2.c);
}
float skalar (Spatiu_Vectorial &d1, Spatiu_Vectorial &d2) {
    return ((d1.a * d2.a) + (d1.b * d2.b) + (d1.c * d2.c));
}
float cosinus(Spatiu_Vectorial &d1, Spatiu_Vectorial &d2) {
    return (skalar(d1, d2)) / (d1.Lungimea() * d2.Lungimea());
}


int main() {
    Spatiu_Vectorial a(2, -3, 1), b(1, 4, 2), c, d;
    c=a+b;
    d=a-c;
    a.Afisare();
    b.Afisare();
    c.Afisare();
    d.Afisare();
    cout << cosinus(a, b) << endl;
    cout << cosinus(c, d) << endl;
    return 0;
    }