#include <iostream>
using namespace std;

class Foo{
public:
  virtual void foo() = 0;
  //  virtual void foo() = 42; //error in compilation
  //  virtual void foo() = NULL; //error in compilation
};

class Bar : public Foo{
public:
  virtual void foo(){
    cout << "g++" __VERSION__ << endl;
  };
};

int main(){
  Foo * f = new Bar();
  f->foo();
  delete f;
}