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From: Benoit Rochefort <jk791840@er.uqam.ca>
Subject: about abstract data types and pointer
Date: 1996/09/21
Message-ID: <3244B03D.32EA@er.uqam.ca>#1/1
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organization: Universite du Quebec a Montreal
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Date: 1996-09-21T00:00:00+00:00
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Hi! I'm new with ADA but i'm a programmer since a long time.

I use the ADA83 standard (but with the gnat compiler than can compile
ADA95).

I have some questions to start with:

1- Is there a way to define a procedure type like in MODULA-2 in
   a way such:

     TYPE PROC IS PROCEDURE(IN E:ELEMENT);

   Or like in C where we can define a pointer to such a fonction?
   (how can i define an access type in ADA that "points" to such
   a procedure?)

   This would be very helpful in the case of a list type (for example),
   having an operation defined like that:

     PROCEDURE Map(L: IN List; P: PROC);

   Which map the P Procedure to each element of the list. P would
   obviously cause a side effect (like printing an element).

   OR can I define procedure Map something like that:

     PROCEDURE Map(L: IN List; P: PROCEDURE(IN ELEMENT));

   Please tell me how i can call the p procedure passed as a parameter
   to Map in the code; maybe something like:

     P(E);

   Please don't suggest me to use a generic proc, because it's NOT
   what i want; Map could be called for MANY procedures, not just one
   for each instance of a List. example:

     Map(L, Put);    -- Print each element on stdout.
     Map(L, CountE); -- Count the number of elements (side effect).
     Map(L, Draw);   -- Draw the value of L on a graph.

   Of course; Draw and Put have the side effect of "writing".

   Count could be written like this:

   FUNCTION Count(L: IN List) RETURN Natural IS
     Nb : Natural := 0;
     PROCEDURE CountE IS
     BEGIN
       Nb := Nb + 1;
     END CountE;
   BEGIN
     Map(L, CountE);
     RETURN Nb;
   END Count;

2- Is there a way to totally hide the representation of an abstract
   data type in the specification package? I really don't understand
   why we should specify the representation of such a type in the
   PRIVATE part of the specification package (except for compiler
   needs -- that is simpler for the compiler :-) ). Even in C we
   can hide the representation of an abstract data type from the 
   header file. Why not in ADA which is supposed to be better for 
   the information hiding principle?

3- Is there a way to "add" or change attributes to a new type that i 
   have elaborated as an abstract data type (in a package).

   Ex: L'First could have another meaning here if L is a List.
       List'Nb could denote the number of elements of a List L.

   I know i can (and sure i must) code these as functions but i just
   want to use them as attribute; which could be much more natural here.

   Is that what RENAMES is for?

4- Is there a way to overload the ":=" operator if List is a
   limited private type?

   ex:

       L1 := L2;

   could make a copy of all elements of L1 into L2. This is much more
   natural than

       Copy(L1, L2);

5- Do the ADA83 standard specify that the deallocation of memory is
   automatically done? Or must I ensure that my program does it?

   ex:
   
   PROCEDURE Test IS
     L: List;
   BEGIN
     Add(L, 3);
     Add(L, 4);
   END Test;

   If L is a pointer to some NEW element created; will it be freed
   automaticaly on return of the PROCEDURE Test? Or must i provide
   an operation to free memory (like Dispose)?

   The next problem is that probably i would have allocated some extra
   memory for the elements of L; will it be also freed automaticaly?
   Some language do it (SML, SMALLTALK), some don't (MODULA-2, C, C++)

   However, we could define a destructor in C++; is it possible in ADA
   that the destructor would be automatticaly called when the
   object fall out of scope?

6- Now the easy one.
   Is there a function or procedure to read a character on the
   keyboard without the need to press enter WHICH IS DEFINED ON THE
   STANDARD (independant of the used compiler and machine).


I know that's a lot of questions to answer but that would be VERY
appreciated.

Please reply by mail. Thank you.

-- 

Benoit Rochefort
jk791840@er.uqam.ca

-- 

Benoit Rochefort
jk791840@er.uqam.ca