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From: franka@mmintl.UUCP (Frank Adams)
Newsgroups: comp.lang.c++
Subject: Re: C++ libraries...
Message-ID: <2443@mmintl.UUCP>
Date: Wed, 30-Sep-87 20:06:52 EDT
Article-I.D.: mmintl.2443
Posted: Wed Sep 30 20:06:52 1987
Date-Received: Tue, 6-Oct-87 04:05:22 EDT
References: <1096@g.gp.cs.cmu.edu>
Reply-To: franka@mmintl.UUCP (Frank Adams)
Organization: Multimate International, E. Hartford, CT.
Lines: 43

In article <1096@g.gp.cs.cmu.edu> dld@g.gp.cs.cmu.edu (David Detlefs) writes:
>  How do inheritance and parameterized classes interact?  I think that
>if a class is not originally parameterized, you probably should not be
>able to introduce parameters for subclasses.

This would prevent having a class Object from which all other classes are
derived.  I would probably want to define a parameterized hash table class
as a subclass of an Array class; the Array class might be parameterized, but
one would be adding new parameters (the hash function) which the superclass
did not have.  Given the ability to add new parameters, there seems no
reason to forbid adding parameters to unparameterized classes.

>Can one derive a class directly from a parameterized class, getting a
>parameterized subclass?

I would think so.
-------------------------
One question needs to be asked here.  How are the parameters of the class
any different from the instance variables of the class?  It seems like the
real problem here is that many of the things one wishes to parameterize over
are not first-class objects.

Instead of a class array[element_type, size], parameterized by the type and
number of objects stored in the array, one would like to have a definition
like the following:

class array {
	type element_type;
	int size;
	element_type contents[size];
	...
}

There are a lot of problems with implementing something like this, of
course.  But then, parameterized classes are not trivial, either.

(One might wish to require that element_type and size be declared const in
the above example, meaning that they can only be set upon initialization.)

-- 

Frank Adams                           ihnp4!philabs!pwa-b!mmintl!franka
Ashton-Tate          52 Oakland Ave North         E. Hartford, CT 06108