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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Ada is getting more popular!
Date: Sun, 31 Oct 2010 15:57:21 +0200
Organization: Tidorum Ltd
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Date: 2010-10-31T15:57:21+02:00
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Dmitry A. Kazakov wrote:
> On Sun, 31 Oct 2010 11:38:02 +0100, J-P. Rosen wrote:
> 
>> FWIW, here are my criteria for using inheritance:
>> 1) you have several types, that are too different to be modelled as
>> variations of a single type (use discriminated types otherwise);
>> 2) however, these types have some common logical properties, with
>> different implementations
>> 3) you have to maintain an heterogenous container of these various types
>> 4) you want to call the common operations on all objects in the container
> 
> 5) you want to postpone some design decision/implementation to a later
> point. So you describe/implement what you know for certain right now, in
> the form of an abstract type, and let a derived type to define the rest
> when more information becomes available.

I have a similar criterion with a different interpretation of "later 
point", assuming that Dmitry's "later" means "later in time". My 
criterion is:

5') you want to declare the existence and common properties of these 
types relatively early in the package dependency graph, with the minimal 
set of dependencies, but postpone implementations to a deeper point (ie. 
a "later point") in the dependency graph.

In my designs this aspect of inheritance and extension has greatly 
helped to decouple packages. However, I believe that Jean-Paul's four 
criteria 1-4 have also been satisfied, so 5' is perhaps rather an effect 
or benefit of using extension and inheritance than an independent criterion.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .