C++对Point类重载“++”（自增）运算符，请问怎么改

#include
using namespace std;

class Point{
private:
int _x;
int _y;
public:
Point &operator++();//前++
Point operator++(int);//后++
Point &operator--();
Point operator--(int);
};

Point Point::operator++(int){
Point old=*this;
++(*this);
return old;
}

Point &Point operator++(){
_x++;
return this;
}

Point Point::operator--(int){
Point old=*this;
--(*this);
return old;
}

Point& Point operator--(){
_y--;
return this;
}

 #include <iostream>
using namespace std;

class Point{
private:
    int _x;
    int _y;
public:
    Point &operator++();//前++
    Point operator++(int);//后++
    Point &operator--();
    Point operator--(int);
};

Point Point::operator++(int){
    Point old=*this;
    ++(*this);
    return old;
}

Point& Point::operator++(){
    _x++;
    return *this;
}

Point Point::operator--(int){
    Point old=*this;
    --(*this);
    return old;
}

Point& Point::operator--(){
    _y--;
    return *this;
}

#include
using namespace std;

class Point{
private:
int _x;
int _y;
public:
Point &operator++();//前++
Point operator++(int);//后++
Point &operator--();
Point operator--(int);
};

Point Point::operator++(int){
Point old=*this;
++(*this);
return old;
}

Point& Point::operator++(){
_x++;
return *this;
}

Point Point::operator--(int){
Point old=*this;
--(*this);
return old;
}

Point& Point::operator--(){
_y--;
return *this;
}

Point &operator++(); //前++
Point operator++(int); //后++