TITLE: Hiding information in classes

PROBLEM: Often designers want to hide detailed internal information from
clients of a class. This reduces intermodule dependencies.


RESPONSE: Bjarne Stroustrup (bs@alice.att.com), 12 May 92


Often, but of course not always, when people have problems with too much
information in header files the problem can be attacked by the use of
abstract classes. For example:

	class interface {
		// pure virtual functions
		~interface() {}
	};

This defines the interface, has no data members, and is defined somewhere
like interface.h. The implementations are elsewhere:

	// my_implementation.h:

	class my_implementation : public interface {
		// definitions
	};

and

	// your_implementation.h:

	class your_implementation : public interface {
		// definitions
	};

A user need include interface.h only. People who are interested in a particular
implementation, say because they want to create objects of that type, include
the appropriate implementation headers also.

See Chapter 13 of my 2nd edition for more details.