Xref: utzoo comp.object:662 comp.lang.c++:6015
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From: sakkinen@tukki.jyu.fi (Markku Sakkinen)
Newsgroups: comp.object,comp.lang.c++
Subject: Re: Dumb question
Message-ID: <2625@tukki.jyu.fi>
Date: 4 Jan 90 07:13:15 GMT
References: <61737@aerospace.AERO.ORG> <2122@tukki.jyu.fi> <1990Jan2.112028.19312@xanadu.com>
Reply-To: sakkinen@jytko.jyu.fi (Markku Sakkinen)
          SAKKINEN@FINJYU.bitnet (alternative)
Organization: University of Jyvaskyla, Finland
Lines: 39

In article <1990Jan2.112028.19312@xanadu.com> michael@xanadu.UUCP (Michael McClary) writes:
>In article <2122@tukki.jyu.fi> sakkinen@jytko.jyu.fi (Markku Sakkinen)
>SAKKINEN@FINJYU.bitnet (alternative) writes:
>>
>>Note 2: An old favourite idea of mine is that classes should offer,
>>as a primitive operation, rather one comparison operator (values:
>>equal, less, greater, incomparable, equivalent; possibly others
>>in special cases) than several relational operators. The latter
>>can be derived from the former, and class interfaces would be simpler.
>
>I recommend against this approach.  Such an operator must fully
>characterize the relationship between two objects to answer any
>question about that relationship.  Then the bulk of this work is
>discarded.
> ...

No, I did not mean that the one operator should be able to fully
characterise relatioships between objects in complicated cases.
Rather, it should express some "canonical" order (or quasi-order)
within the class. If there are several interesting order relations
in a class, each of them should be represented by its own
comparison-valued function.

Of course, there are cases in which it is easier e.g. to test whether
two objects are equal than to find out their exact relationship.
I did not want to _prohibit_ the definition of conventional relational
operators when deemed useful.

For a more thorough treatment, see my article in ACM SIGPLAN Notices
22:8 (August 1987). The term 'quasi-order' is, however, used there
incorrectly: I should have written 'weak order'; quasi-order is
more general.

Markku Sakkinen
Department of Computer Science
University of Jyvaskyla (a's with umlauts)
Seminaarinkatu 15
SF-40100 Jyvaskyla (umlauts again)
Finland