怎么调用析构函数??

#include<iostream.h>
class A
{
public:
A(int i=0)
{m=i;cout<<"Constructor called."<<m<<endl;}
void Print() const
{cout<<m<<endl;}
~A()
{cout<<"Destructor called."<<m<<endl;}
private:
int m;
};
void main()
{
const int N=5;
A my;
my=N;//该处为什么调用了一次析构函数??
my.Print();
}