Item 27. Minimizing Compile-time Dependencies—Part 2

Difficulty: 6

Now that the unnecessary headers have been removed, it's time for Phase 2: How can you limit dependencies on the internals of a class?

Below is how the header from Item 26 looks after the initial clean-up pass. What further #includes could be removed if we made some suitable changes, and how?

This time, you may make changes to X as long as X's base classes and its public interface remain unchanged; any current code that already uses X should not be affected beyond requiring a simple recompilation.

//  x.h: sans gratuitous headers 
//
#include <iosfwd>
#include <list>
// None of A, B, C or D are templates.
// Only A and C have virtual functions.
#include "a.h"  // class A
#include "b.h"  // class B
#include "c.h"  // class C
#include "d.h"  // class D
class E;
class X : public A, private B
{
public:
     X( const C& );
  B  f( int, char* );
  C  f( int, C );
  C& g( B );
  E  h( E );
  virtual std::ostream& print( std::ostream& ) const;
private:
  std::list<C> clist_;
  D            d_;
};
inline std::ostream& operator<<( std::ostream& os, const X& x )
{
  return x.print(os);
}