Сравнение между объектами разных классов

A a;
B b;
 
if(a==b) dosomething();

if(a.return_var()==b.return_var()) dosomething();

#include <iostream>
using namespace std;
class B;
class A {
    private:
        int v1;
    public:
        A(int x=0):v1(x) {}
        friend bool operator==(const A& a, const  B &b);
};
class B {
    private:
        int v2;
    public:
        B(int x=0):v2(x) {}
        friend bool operator==(const A& a, const  B &b) ;
 
};
bool operator==(const A& a, const  B &b) {
    return a.v1==b.v2;
}
int main() {
    A a(5);
    B b(7);
    if(a==b) {
        cout<<"Equal";
    } else {
        cout<<"Not yet";
    }
    return 0;
}

#include <iostream>
 
class A {};
class B : public A {};
class C : public A {};
 
int main()
{
    A *b = new B, *c = new C;
    if (b == c)
        std::cout << "It works!";
    delete b;
    delete c;
}

class Base
{
protected:
    int x, y; //коодрдинаты
public:
    ...
    friend bool EqualCoord(Base* left, Base* right);
};
...
bool EqualCoord(Base* left, Base* right)
{
    return left->x == right->x && left->y == right->y;
}
 
class Player : public Base {...};
class Enemy : public Base {...};
 
int main()
{
    Base* unit1 = new Player;
    Base* unit2 = new Enemy;
    ...
    if (EqualCoord(unit1, unit2)) { ... }
}

if(*base_[0]==*base_[1]) cout<<"something"<<endl;

friend bool operator==(const Base& b1,const Base& b2)
{
  cout<<"test"<<endl;
  return(b1.x==b2.x&&b1.y==b2.y);
}

class Base
{
protected:
//.......
public:
//virtual bool foo()=0; убрали
}
//////////////////////////////////////
class der1:public Base
{
private:
//.....
public:
bool foo()
{
//dosomething
}
}
//////////////////////////////////////
class der2:public Base
{
private:
//.......
public:
//здесь мне не нужен метод foo()
//other funcs
}