visitor

#include <iostream>
#include <vector>

using namespace std;

class Base;
class A;
struct BaseVisitor
{
  virtual void visit (Base&) = 0;
  virtual void visit (A&) = 0;
};
struct Base {
  int av;
  virtual void accept (BaseVisitor& v) {
    v.visit (*this);
  }
};

struct A : public Base {
  int bv;
  virtual void accept (BaseVisitor& v) {
    v.visit (*this);
  }
};

struct ValueFinder: public BaseVisitor {
  int value;
  bool found;
  ValueFinder (int value) : value(value) {}
  virtual void visit (Base& b) {
    found = value == b.av;
  }
  virtual void visit (A& b) {
    found = value == b.bv;
  }
};

struct database {
  vector <Base*> basev;
  Base NBase;

  database () { NBase.av = 0; }

  Base* findb (int value) {
    ValueFinder finder(value);
    for (int i = 0; i < basev.size (); ++i) {
      basev[i]->accept (finder);
      if(finder.found)
    return basev[i];
    }
    return &NBase;
  }
} db;

struct BasePrinter: public BaseVisitor {
  virtual void visit (Base& b) {
    cout << b.av << endl;
  }
  virtual void visit (A& b) {
    cout << b.bv << endl;
  }

};
int main( ) {

  Base der;
  der.av = 5;
  db.basev.push_back (&der);

  Base* found = db.findb (5);
  BasePrinter printer;
  found->accept(printer);
}