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From: bertrand@eiffel.UUCP (Bertrand Meyer)
Newsgroups: comp.lang.eiffel
Subject: Re: redefining constant features
Summary: Constants don't change
Message-ID: <183@eiffel.UUCP>
Date: 16 Jul 89 19:49:26 GMT
References: <976@clyde.Concordia.CA>
Organization: Interactive Software Engineering, Santa Barbara CA
Lines: 34

From <976@clyde.Concordia.CA> by marcap@concour.CS.Concordia.CA
(Marc Pawlowsky):

> Why is it not legal to redefine a constant feature?
> [The following produces a compilation error]:
>     
>               class IMAGE ... feature
>                   nb_directions: INTEGER is 4; ...
>             
>               class HEX_IMAGE ... inherit
>                    IMAGE redefine nb_directions ....
>               feature
>                   nb_directions: INTEGER is 6; ...

    A constant attribute is what the name indicates: its value does
not change, either in the class or in any descendant.

    Actually the same message contains the solution:
     
>  Yet if I change the code [as follows] the program compiles:
>
>        in IMAGE: nb_directions: INTEGER is do Result := 4 end;
>        in HEX_IMAGE: nb_directions: INTEGER is do Result := 6 end;

This is the appropriate way to go: in such a case, use a function.
My only pet complaint is that constant attributes should be used
in each class rather than the manifest constants 4 and 6.




-- Bertrand Meyer
bertrand@eiffel.com