// Example program
#include <iostream>
using std::cout;

class B;
class A {
    int age;
    B foo(); // even if undefined type 'B' is used by Value, not Reference of pointer,
             // this works.
};
class B : public A {

};
B A::foo() {
    return B();
}
int main()
{
  B b;
  cout << "compiled.\n";
  return 0;
}
/*
Output:
  compiled.
*/