Re: Which is right here - GNAT or OA ?

[Tucker Taft <stt@averstar.com>]

Florian Weimer wrote:
> ...
> Another but related thing surprises me---it's AARM 12.5.4(4.a):
> 
> | If no _modifier applies to the formal, then the actual type may be
> | either a pool-specific or a general access-to-variable type.
> 
> I can't imagine any reason why an access-to-variable type should be
> allowed as an actual parameter for a formal access type without an
> `all' modifier.  If this was changed, an access-to-variable would not be
> allowed as a parameter in an instantiation of Ada.Unchecked_Deallocation,
> which seems quite reasonable to me.  (I don't claim that the language
> definition inadequate here, I simply can't make sense out of it, which
> is very likely my own fault. ;)

You may be parsing the sentence incorrectly.  It is saying that the
actual type may be a (pool-specific or general) access-to-variable
type.  It certainly must be an access-to-variable type, unless
the "constant" modifier is present.  By omitting the "all" modifier,
you are allowing either kind of access-to-variable type, pool-specific
or general.  Within the generic unit, the formal type is considered
pool-specific.  This works because the capabilities of a pool-specific
access type are a subset of the capabilities of a general access type.

This is analogous to the way "limited" is used in generic formal types.
If present, either a limited or a non-limited actual type is used.
If the word "limited" is absent, then only a non-limited actual
type may be used.  In general, a formal type specifies the minimum
set of properties required.  It is safe to provide an actual type
with more properties than what is presumed for the formal.

-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Distributed IT Solutions  (www.averstar.com/tools)
AverStar (formerly Intermetrics, Inc.)   Burlington, MA  USA