Re: Rules for Representation of Subtypes

[dewar@cs.nyu.edu (Robert Dewar)]

I meant to add more on the issue of Size, but accidentally termianted
transmission early!

One thing we have done in GNAT is the following:

First, objects of a subtype by default have the same size as the size
of the first subtype, even though Ada 95 does NOT require this, and in
fact makes an implementation suggestion to the contrary:

50   If the Size of a subtype is specified, and allows for efficient
independent addressability (see 9.10) on the target architecture, then the
Size of the following objects of the subtype should equal the Size of the
subtype:

   51  Aliased objects (including components).

   52  Unaliased components, unless the Size of the component is
       determined by a component_clause or Component_Size clause.


We at first followed this advice and found it caused compatibility chaos!
So, that's the first point, David's guess as the the behavior of subtypes
is in fact corect WITH RESPECT TO OBJECTS in GNAT. This is of course in the
absence of a specific attribute representation clause. 

Second, we have provided two new attributes

Value_Size is just like Size in the RM, except that it can be applied to
subtypes other than the first subtype. 

Object_Size is the size that will be used by default for objects of the
type or subtype. It need not be the same as Value_Size. The default value
of Object_Size is close to what most Ada 83 compilers provided for Size.
It can be specified for subtypes other than the first subtype.

Value_Size, like Size in the RM, is used for packing and for unchecked
conversion, but not much else.