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   Inheritance and subtyping in OO languages

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  • From: Paul Prescod <papresco@technologist.com>
  • To: xml-dev <xml-dev@ic.ac.uk>
  • Date: Mon, 20 Apr 1998 09:20:12 -0400

I've found a good reference to the 8 year old paper that made the
distinction between inheritance and subtyping most explicit. The paper
itself is not online, but this summary is quite good:

"[CCHO89] and [CoHC90] propose an approach based on explicit interfaces
and interface containment. In this system of object interfaces, one type
is considered a subtype of another if some subset of its interface is
identical to that of the second. [...] Hence in this system class-based
inheritance is strictly a reusability mechanism for sharing behaviour
between objects, not to be confused with subtyping. For example two
classes may be equivalent as types, though neither inherits anything
from the other. So class hierarchies are not the same as type
hierarchies, although they may overlap. Object interfaces [as in Java,
C++, etc. - Paul] clarify this distinction between interface containment
(subtyping) and class- based inheritance and give insight into
limitations caused by equating the notions of type and class in many
typed object-oriented programming languages [such as Simula 67 - Paul]."


The paper itself is called: "Inheritance is not subtyping" and is quite
famous, but unfortunately predates the Web.

 Paul Prescod  - http://itrc.uwaterloo.ca/~papresco

"Journalism is good if you follow the rules. Don't allow the human 
rights groups to spoil your profession" 
    - Col. Godwin Ugbo of the  Nigerian military dictatorship

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