Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!bony1!richieb
From: richieb@bony1.bony.com (Richard Bielak)
Newsgroups: comp.lang.eiffel
Subject: Re: A question on generic parameters
Message-ID: <1991Apr15.131054.110@bony1.bony.com>
Date: 15 Apr 91 13:10:54 GMT
References: <1203@nikhefh.nikhef.nl>
Reply-To: richieb@bony1.UUCP (Richard Bielak)
Organization: Bank of New York
Lines: 45

In article <1203@nikhefh.nikhef.nl> a38@nikhefh.nikhef.nl (James Barr) writes:

[...]

>I noticed that you could only do the bare minimum of operations
>on generic class parameters, which seemed a bit limiting.
>In the case of an ordered list class you would need a class
>parameter that could support a comparision function, but there
>seems no way of doing this.
>
[...]
>
>class OrderedList [T heir of Ordered]
> ...
>
>deferred class Ordered
>  -- make this an heir of any class/type whose instances/values
>  -- can be ordered.
>  export compare;
>  compare (a,b : Ordered) : Boolean
>  ...
>

Although not discussed in "Object Oriented Software Construction",
Eiffel does support such constructs. This feature is called
"constrained genericity". Using the actual syntax, your example would
be written as:

  class OrderedList [ T -> COMPARABLE ] is
     [...whatever...]

Then within this class you could use any routine or attribute from
COMPARABLE on things declared with type T.

Hope this answers your question.


...richie


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