c++ 深拷贝 重载 = 运算符

深拷贝重载 = 运算符，避免释放空间时，两个指针重复释放内存导致出错。

#include
using namespace std;
class A
{
	friend ostream& operator<<(ostream& cout,A a1);
	int *a;
	public:
		A(int x)
		{
			a= new int(x);
		}
		A& operator=(A& a1)
		{
			if(a!=NULL)
			{
				delete a;
				a=NULL;
			}
			a = new int(*a1.a);
			return *this;
		}
		~A()
		{
			if(a!=NULL)
			{
				delete a;
			}
		}
};
ostream& operator<<(ostream& cout,A a1)
{
	cout<<*a1.a;
}
int main()
{
	A a1(5);
	A a2(6);
	a1=a2;
	cout<