vec3d template

/////////////////////////////////////////////////vec3d.h///////////////
//...some headers
template <class T>
class Vec3D
{
 public:

    Vec3D(void);
    Vec3D(T x, T y, T z);
    Vec3D(const Vec3D<T> &v);
    explicit Vec3D(Vec3D<T> *p);
    //... some more functions

    T _x;
    T _y;
    T _z;
};

template <class T> const Vec3D<T> operator*(const Vec3D<T> &v, T factor);
template <class T> const Vec3D<T> operator*(T factor, const Vec3D<T> &v);
template <class T> const Vec3D<T> operator*(const Vec3D<T> &v1,
                                            const Vec3D<T> &v2);
//... some more functions

//////////////////////////////////////////////////////////////vec3d.cpp//////////////
template<class T>
Vec3D<T>::Vec3D(void):
{
    _x = T();
    _y = T();
    _z = T();
}

template<class T>
Vec3D<T>::Vec3D(T X, T Y, T Z):
{
    _x = X;
    _y = Y;
    _z = Z;
}

template<class T>
Vec3D<T>::Vec3D(const Vec3D<T> &v):
{
    _x = v._x;
    _y = v._y;
    _z = v._z;
}

template<class T>
Vec3D<T>::Vec3D(Vec3D<T> *p):
{
    _x = p->_x;
    _y = p->_y;
    _z = p->_z;
}

template<class T>
const Vec3D<T> operator*(const Vec3D<T> &v, T factor)
{
    return Vec3D<T>(v._x * factor, v._y * factor, v._z * factor);
}

template<class T>
const Vec3D<T> operator*(T factor, const Vec3D<T> &v)
{
    return Vec3D<T>(v._x * factor, v._y * factor, v._z * factor);
}

template<class T>
const Vec3D<T> operator*(const Vec3D<T> &v1, const Vec3D<T> &v2)
{
    return Vec3D<T>(v1._x * v2._x, v1._y * v2._y, v1._z * v2._z);
}

// instantiation
template class Vec3D<double>;