struct base {virtual void iam(){cout << "base\n";}};
struct der1 : base {void iam(){cout << "der1\n";}};
struct der2 : base {void iam(){cout << "der2\n";}};

    der1 d1;    der2 d2;
    base *p1 = &d1, *p2 = &d2;
    d1.iam(); p1->iam();
    d2.iam(); p2->iam();

#include <iostream>
using namespace std;

class Circle;
class Square;
class Triangle;
class Shape {
    // ...
public:
    virtual bool Check(const Shape&)const = 0;
    virtual bool Check(const Circle&)const = 0;
    virtual bool Check(const Square&)const = 0;
    virtual bool Check(const Triangle&)const = 0;
};
class Circle : public Shape {
    // ...
public:
    bool Check(const Shape& s)const{return s.Check(*this);}
    bool Check(const Circle&)const
        {cout << "Circle & Circle\n"; return true;}
    bool Check(const Square&)const
        {cout << "Circle & Square\n"; return true;}
    bool Check(const Triangle&)const
        {cout << "Circle & Triangle\n"; return true;}
};
class Square : public Shape {
    // ...
public:
    bool Check(const Shape& s)const{return s.Check(*this);}
    bool Check(const Circle& c)const{return c.Check(*this);}
    bool Check(const Square&)const
        {cout << "Square & Square\n"; return true;}
    bool Check(const Triangle&)const
        {cout << "Square & Triangle\n"; return true;}
};
class Triangle : public Shape {
    // ...
public:
    bool Check(const Shape& s)const{return s.Check(*this);}
    bool Check(const Circle& c)const{return c.Check(*this);}
    bool Check(const Square& s)const{return s.Check(*this);}
    bool Check(const Triangle&)const
        {cout << "Triangle & Triangle\n"; return true;}
};
bool intersect(const Shape& s,const Shape& t)
    {return s.Check(t);}

int main()
{
    Circle      c;
    Square      s;
    Triangle    t;
    intersect(c,s);
    intersect(t,c);
}