什么时候调用C ++析构函数类？

Question

有人在C ++析构函数类调用时向我解释吗？它何时自动调用，何时需要显式调用？ 我尝试了下面的代码，但结果出乎意料。

class V{
    private:double x,y,z;
    public:
        V(){
            x=y=x=0.0;
        }
        V(double x=0.0,double y=0.0,double z=0.0):z(z){
            this->x=x;
            (*this).y=y;
            return;
        }
        V(const V &src){
            x=src.x;
            y=src.y;
            z=src.z;
            return;
        }
        void print(){
            cout<<x<<" "<<y<<" "<<z<<endl;
        }
        ~V(){
            cout<<"Destructor called"<<endl;
        }
};
int main(){ 
    V *p;
    p=new V(1,2,3);
    /*delete p;
    Why is the destructor not called automatically for dynamically allocated object p after the program ends? If delete p is included destructor is called. Why is it so?*/   

    V(1,2,3).print();
    /*Here the object does not have any name but still the destructor is called. Why does this happen?*/

    V vec3(V(1,2,3));
    /*Here only one destructor was called but I was expecting two.One for object object **vec3** and other for the nameless object  **V(1,2,3)** as in above case. Why did this happen?*/
}