I found a book "Design patterns explained" by Alan Shalloway and James
Trott (Addison-Wesley, 2002)
that answers a lot of questions for me on design patterns.

Among other things it points out the general trend in the GoF patterns
is that objects are to be made
up of other objects to describe their properties rather than use a
complex inheritance tree to achieve
specialisation.

I find it a very worthwhile book as it explains the reasoning behind
these patterns and how to apply
them. (And I have started thinking about ways to apply these ideas to
Fortran programs. It might
form a nice subject for an article in Fortran Forum ...)

Regards,

Arjen