构造函数和析构函数，拷贝构造，赋值重载，调用练习

#include<iostream>

using namespace std;
class Demo
{
private:
int dwint;
public:
Demo(int i=0):dwint(i)
{
cout<<"default constructor called !"<<endl;
}
Demo(const Demo& d )
{
dwint=d.dwint;
cout<<"copy constructor called !"<<endl;
}
~Demo()
{
cout<<"desconstructor called !"<<endl;
}
const Demo& operator=(const Demo& a )
{
this->dwint=a.dwint;
cout<<"assignment operator used"<<endl;
return *this;
}
};
Demo func(Demo d)
{
Demo demo;
demo=d;
return demo;
}
int main()
{
Demo a(4);
{
Demo b;
b=func(a);
}
Demo c=a;
return 0;
}