Visitor

#include <iostream>
#include <string>
#include <vector>

using namespace std;

struct Base;
struct A;
struct B;

struct Visitor {
    virtual void visit (Base& b) = 0;
    virtual void visit (A& a) = 0;
    virtual void visit (B& b) = 0;
    };

struct Base {
    int av;
    virtual void accept (Visitor& v) { v.visit (*this); }

    Base () { av = 1;}
};

struct A : public Base {
    int bv;

    void accept (Visitor& v) { v.visit (*this); }

    A () { av = 2; bv = 3; }
};

struct B : public Base {
    string cv;

    void accept (Visitor& v) { v.visit (*this); }

    B () { av = 4; cv = "s5"; }
};

struct Print : Visitor {
    void visit (Base& b) { cout << "av " << b.av << endl; }
    void visit (A& b) { cout << "bv " << b.bv << endl; }
    void visit (B& b) { cout << "cv " << b.cv << endl; }
} print;

struct data {
    vector <Base*> basev;

    Base* findb (int value) {
        static Base base; Base* pBase = &base;

        for (int i = 0; i < basev.size (); ++i) {
            if (basev[i]->av == value) return basev[i];
            }
            return pBase;
        }
} d;


int main() {

d.basev.push_back (new Base);
d.basev.push_back (new A);
d.basev.push_back (new B);

Base* found = d.findb (4);

found->accept (print);

cout << "\nDone";
}