20. Implicit conversion, upcast

• An object of a derived type will be implicitly converted to an object of the base type when necessary:

  
  class Base {};
  
  class Derived : public Base {};
  
  void e( Base  b  );
  void f( Base* bp );
  void g( Base& br );
  void h( Derived* dp );
  
  Base b;
  Derived d;
  
  e(  d );  // OK, implicit conversion via "upcast"
  f( &d );  // OK, implicit conversion
  g(  d );  // OK, implicit conversion
  h( &b );  // Error