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Date: 9 Sep 92 01:34:35 GMT
From: munnari.oz.au!goanna!minyos.xx.rmit.oz.au!stan!saturn!rob@uunet.uu.net  (
Rob Allen)
Subject: Re: Generics?
Message-ID: <6978@stan.xx.swin.oz.au>
List-Id: <comp.lang.ada>

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.
>For example, if the array was instantiated using a subtype of naturals
>(0 .. 100), allowing a natural to access elements of the array could
>lead to errors (of course, stack.data_store(103) would cause problems).
>Basically, I need to be able to "look at" any element in the array
>regardless of how large an array is defined in the given procedure using
>this package.
>I'm not too familiar with generics, and if someone could shed some light
>on this, I would really appreciate it.
>------------------------------------------------------------------------------
Why not declare Pointer to be of type Index ?  You can use it directly as
a subscript, initiallize it to  Index'first  and check for full against
Index'last.  However if the store is allowed to be logically empty then
there is a classic problem: it is illegal to initiallise Pointer to the 
item before the first [Index'pred(Index'first)].  The easiest solution is
to waste the first array element.
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
-- 
+---------------------------------------------------------------------+
| Robert K Allen                     email: rob@saturn.cs.swin.oz.au  |
| Computer Science,                  snail: PO Box 218, Hawthorn,     |
| Swinburne University of Technology,       VIC 3122 Australia        |