Microsoft Word OpenIssuesInOO. doc

4.7 

Types versus classes 

One of the open issues within the object-oriented community is the discussion of type 

versus class. Is it the same or should there be a distinction. Traditionally it has been 

considered a great advantage that class/subclass relations also define  type/subtype relations. 

However, as mentioned above, subclassing and redefinition has been used a lot to obtain 

code reuse, leading to subclasses without any conceptual relations to their superclasses. 

Consequently, it is now more common to distinguish between classes and types. In this 

section we shall try to summaries the discussion and some of the various point of views. 

The main arguments for distinguishing between type and class are: a type defines the 

interface of a set of objects and a class defines its implementation;  for a given type there 

may be many different implementations; a subclass of a given class does not necessarily 

define a subtype of the type implemented by the type of the class; and specification and 

interface should be kept separate.