单目运算符重载为友元函数

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#include<iostream>
using namespace std;
class point
{
private:
    int x,y;
public:
    point(int xx=0,int yy=0)
    {x=xx;y=yy;}   
void display()
{cout<<"x="<<x<<",y="<<y<<endl;}
friend point operator++(point &p);
};
point operator ++(point &p)
{
    ++p.x;
    ++p.y;
    return p;
}
int main()
{    
    point a(1,2);
    ++a;
    a.display();
    return 0;

}