Version 1.13 of ais/ai-00127.txt

X'Access

X'Access yields an access value that designates the object denoted by X. The type of X'Access is an access-to-object type, as determined by the expected type. The expected type shall be a general access type. X shall denote an aliased view of an object, including possibly the current instance (see 8.6) of a limited type within its definition, or a formal parameter or generic formal object of a tagged type. The view denoted by the prefix X shall satisfy the following additional requirements, presuming the expected type for X'Access is the general access type A:

X'Access

X'Access yields an access value that designates the object denoted by X. The type of X'Access is an access-to-object type, as determined by the expected type. The expected type shall be a general access type. X shall denote an aliased view of an object, including possibly the current instance (see 8.6) of a limited type within its definition, or a formal parameter or generic formal object of a tagged type. The view denoted by the prefix X shall satisfy the following additional requirements, presuming the expected type for X'Access is the general access type A, with designated type D:

• If the designated type of A is tagged, then the type of the view shall be covered by the designated type; if A's designated type is not tagged, then the type of the view shall be the same, and either A's designated subtype shall statically match the nominal subtype of the view, or the designated subtype shall be discriminated and unconstrained;

• If A is a named access type and D is a tagged type, then the type of the view shall be covered by D; if A is anonymous and D is tagged, then the type of the view shall be either D'Class or a type covered by D; if D is untagged, then the type of the view shall be D, and A's designated subtype shall either statically match the nominal subtype of the view or be discriminated and unconstrained;

!section 3.10.2(27)
!subject Expected type of a 'Access attribute
!reference RM95-3.10.2(27)
!from Pascal Leroy 96-03-12
!reference 96-5444.a Pascal Leroy 96-3-13>>
!discussion

Consider the following code fragment:

        type T is tagged null record;
        procedure P (X : access T);
        Y : aliased T'Class := ...;

        P (Y'Access);

Is the usage of 'Access in the procedure call legal?

RM95 6.4.1(3) says that "the expected type for an actual parameter is the type
of the corresponding formal parameter."  Thus, the expected type for Y'Access
is the anonymous access type "access T".

Now the legality of the 'Access attribute.  RM95 3.10.2(24-28) expresses a
number of rules that apply to the prefix of the attribute: "The view denoted
by the prefix shall satisfy the following requirements, presuming the expected
type ... is the general access type A" (RM95 3.10.2(24)).  In our case, the
expected type is the anonymous access type "access T" as explained above.

One of the requirements applicable to the prefix is stated by RM95 3.10.2(27):
"If the designated type of A is tagged, then the type of the view shall be
covered by the designated type".  Here the designated type of the anonymous
access type is T, which is tagged.  But the type of the view is T'Class, which
is definitely not covered by T.

Following this line of reasoning, the expression Y'Access above would seem
illegal.  But if that is the case, then how are we supposed to use access
parameters for dispatching?

_____________________________________________________________________
Pascal Leroy                                    +33.1.30.12.09.68
pleroy@rational.com                             +33.1.30.12.09.66 FAX

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!section 3.10.2(27)
!subject Expected type of a 'Access attribute
!reference RM95-3.10.2(27)
!reference RM95-3.9.2(7,9)
!reference RM95-8.6(25)
!reference 96-5444.a Pascal Leroy 96-03-12
!from Tucker Taft 93-03-14
!reference 96-5445.a Tucker Taft 96-3-14>>
!discussion

> Consider the following code fragment:
>
>       type T is tagged null record;
>       procedure P (X : access T);
>       Y : aliased T'Class := ...;
>
>       P (Y'Access);
>
> Is the usage of 'Access in the procedure call legal?
>
> ...
> One of the requirements applicable to the prefix is stated by RM95 3.10.2(27):
> "If the designated type of A is tagged, then the type of the view shall be
> covered by the designated type".  Here the designated type of the anonymous
> access type is T, which is tagged.  But the type of the view is T'Class, which
> is definitely not covered by T.

I agree.  This seems like a possible bug in the RM.  If instead of Y'Access,
you had a value of a named access-to-T'Class type, it would be legal
by 8.6(25):

   [the type of the construct shall resolve to T, or], when T
   is an anonymous access type with designated type D, to an access-
   to-variable type whose designated type is D'Class or is covered by D.

and 3.9.2(9):

   ... Similarly, if the expected type for an expression is an anonymous
   access-to-specific tagged type, then the expression shall not be of an
   access-to-class-wide type unless it designates a controlling operand
   in a dispatching operation.

So it makes sense that any access-to-T'Class value should be allowed
as an actual parameter, as part of a dispatching call.  The only
problem is that we don't really have a type for this value.  In some
sense, we don't really need one, since it is about to be implicitly
converted to the anonymous access type, but it is a little awkward
to talk about Y'Access without being able to point to some type
declaration for it.

An alternative, and probably better, way of dealing with this would
be to say the type of Y'Access *is* the anonymous access-to-specific type
of the formal access parameter "X", but add a rule to 3.9.2(7)
defining <class_wide_obj>'Access to be a dynamically tagged operand,
even though its type (as determined by the expected type)
is in fact access-to-specific, and change 3.9.2(9) to disallow this
use unless it is in a call on a dispatching operation, and finally
allowing in 3.10.2(27) that the prefix can be of type A'Class in
addition to being a type covered by A.

The net effect of these changes would be to make the rules
governing an actual "blah'Access" matching a formal of type "access T"
equivalent to the rules coverning an actual "blah" matching a formal
of type "T".  Gaining this equivalence seems like a good thing.

> Following this line of reasoning, the expression Y'Access above would seem
> illegal.  But if that is the case, then how are we supposed to use access
> parameters for dispatching?

They work fine so long as the actual parameter is of a named access-to-
classwide type.  The problem is only with <classwide_obj>'Access.
I agree that it would be nice to fix this problem.

> Pascal Leroy                                    +33.1.30.12.09.68
> pleroy@rational.com                             +33.1.30.12.09.66 FAX

-Tuck

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From: Randy Brukardt, October 7, 1999

The text changes mentioned in my notes from the recent ARG meeting
for paragraphs 3.9.2(7) and 3.9.2(9) don't make sense in English. I've made
what I think are minimal corrections to the text. Here is what my original
notes say:

3.9.2(7)

Replace all but the first sentence:

For an object that is designated by an expression where the expected type is an
anonymous access-to-specific tagged type is dynamically tagged if the
expression, ignoring enclosing parentheses, is of the form X'Access, where X is
of a class-wide type, or is of the form new T'(...), where T denotes a class-wide
subtype. Otherwise, the object is staic or dynamically tagged according to
whether the designated type of the type of the expression is specific or
class-wide, respectively.

-- The first clause here makes no sense as written.

This paragraph was changed because:
    -- Expressions are dynamically or statically tagged, not parameters per se;
    -- We need this to apply to all expressions so 3.9.2(9) works;
    -- A parenthesized expression should be either dynamically or statically
       tagged depending on the contents.

3.9.2(9)

Replace:

...then the object designated by the expression shall not be dynamically tagged
unless it is designated...

-- "it is designated" doesn't match the following text "...a controlling operand...".

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From: Randy Brukardt, January 19, 2000

The (belated) minutes of the meeting make it clear what was intended here, and
this now has been done.

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