Re: private type discriminants ignored?

[Joel VanLaven <jvl@ocsystems.com>]

Jay Sachs <sachs@cs.williams.edu> wrote:
[message trimmed for content]
:        type Stack(Size : Integer := 100) is record 
: 	  Top : Natural := 0;
: 	  Rep : stackrep(1..Size);
:        end record;

: I get warnings (from gnat 3.10) saying that I may get a storage_error
: by creating a variable of type stack.  I do in fact get such an error
: if I declare

:   s : Stacks4.Stack;

: However, supplying the initialization as in

:   s2 : Stacks4.Stack(75);

: with no error.  What is the reason that the default initialization
: seems to be ignored in the discriminant for the private type?

Default initializations of the type you are using have a very special
meaning in Ada.  They indicate that the constraint can be changed.  Objects
declared like s are sometimes referred to as "mutable" meaning that one can
assign to them a value with a constraint other than 100.  Essentially,
there are three ways objects get their constriant:

explicitly (forever constained to that value like 75 above)
by initialization (forever constained like a : string := "abcd" will
                   always have length 4)

by default (mutable, can be changed, un-constrained like s above)

Many (most, all?) compilers implement this by allocating (I'm using that
term loosely) the maximum possible space needed to represent an object of
the type with any constraint.  So, s is of size on the order of magnitude
of integer'last.  Not good.  If size was of some type that ranged from
1 to 200 GNAT would likely make s of size on the order of 200 and 
everything would be fine.

I don't think you really want mutable objects here.  It can be useful
but is one of the trickier and fancier features in Ada, and I don't think
it is what you are looking for.

-- Joel VanLaven
-- (I just happen to work for OC Systems)