Problems with default =

class Vector3
{
    float           _x;
    float           _y;
    float           _z;

public :
 /// constructors and stuff

}

class Point : public Vector3
{
// some BS
    Point(float _x):Vector3(float _x)
    {}
}

main()
{
    Point aPoint(3);
    Point anotherPoint(4);

    // WHY DOES THIS WORK and copy _x,_y & _z properly
    aPoint = anotherPoint;

}