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Date: 9 Sep 92 05:26:34 GMT
From: munnari.oz.au!yoyo.aarnet.edu.au!huon.itd.adelaide.edu.au!cs.adelaide.edu
.au!andrewd@uunet.uu.net  (Andrew Dunstan)
Subject: Re: Generics?
Message-ID: <18k1uaINN6su@huon.itd.adelaide.edu.au>
List-Id: <comp.lang.ada>

In article <6978@stan.xx.swin.oz.au>, rob@saturn.cs.swin.OZ.AU (Rob Allen) writ
es:
|> tp923021@jarrah.canberra.edu.au (Ben Elliston) writes:
|> >I've since moved onto making an array a generic type (such as this):
|> >GENERIC
|> >Type Index IS (<>);
|> >[..]
|> >Type Stack_Array IS Array(Index) of Character;
|> 
|> >Type Stack_Type IS RECORD
|> >  Data_Store: Stack_Array;
|> >  Pointer: Natural;
|> >END RECORD;
|> 
|> >As it stands now, it seems that since the "pointer" variable (which
|> >indicates the current "highwater" mark of the array) is a natural, that
|> >Ada won't allow a natural to access an array indexed by some unknown
|> >(generic) type.

[..]

|> If however you decide to keep Pointer as a Natural where 1 corresponds to
|> the first element then always use it as 
|>   X.Data_Store(Index'val(X.Pointer - 1 + Pos_1st))
|> where
|>   Pos_1st : constant := Index'pos(Index'first);
|> is to account for the likelihood that the actual Index is an arbitrary
|> subtype of a discrete type.  Using this approach the 'full' test is
|> X.Pointer = Index'length
|> Though none of the above code has been checked, I hope that helps.  rob


I'd like to see the code from which this is taken. If what is really wanted
is a stack, why is the index of the stack array a generic parameter? It
seems to me that the only generic type parameter for a stack type should
be the type stored - after all, the stack might not be implemented as an array
at all. I agree that Rob Allen's answer to the question is correct, but I
still want to know why the above is being attempted.

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