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From: bertrand@eiffel.UUCP (Bertrand Meyer)
Newsgroups: comp.lang.misc,comp.object
Subject: Re: CHALLENGE: heterogeneous collections
Summary: Object-oriented programming usually includes inheritance
Message-ID: <521@eiffel.UUCP>
Date: 30 Mar 91 01:41:37 GMT
References: <1991Mar25.220525.11087@leland.Stanford.EDU> <CIMSHOP!DAVIDM.91Mar28192651@uunet.UU.NET>
Distribution: comp
Organization: Interactive Software Engineering, Santa Barbara CA
Lines: 54

From <CIMSHOP!DAVIDM.91Mar28192651@uunet.UU.NET> by
cimshop!davidm@uunet.UU.NET (David S. Masterson):

[Quoting from my earlier posting:]

   >> To handle heterogeneous collections without endangering static
   >> typing, use genericity (Eiffel syntax):

   >>       class COLLECTION [G] ...

[Mr. Masterson's reply:]

   > Is this a heterogeneous collection (one collection object
   > containing many objects of many types) or a generic collection
   > class (a collection class that may be defined to contain any
   > *single* type of object [which may have multiple parent types])?
   > 
   > Next contestant?

Although this is probably clear to most people reading this newsgroup,
it may be worth confirming that the collection described by the above
COLLECTION class is indeed heterogeneous (if so desired by
clients). If a client declares

   tax_collection: COLLECTION [TAX]

and then uses the routines of class COLLECTION to insert objects into
`tax_collection' and retrieve them, these objects may be direct instances
not just of class TAX but of any of its proper descendants.
(``Proper descendant'' means direct or indirect heir, according
to the inheritance relation.)

This is a direct result of the type rules: if COLLECTION [G]
has an insertion procedure

   put (new: G) is
         -- Add `new' to the collection
      do
         ...
      ensure
         has (new)
      end -- put

then in a call of the form

   tax_collection.put (tx)

the actual argument tx may be of any type which is a descendant of TAX.

In different calls, we may use actual arguments of different types,
inserting objects of different types into the same collection.
-- 
-- Bertrand Meyer
bertrand@eiffel.uucp