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!standard 8.6(17)          05-07-20 AI95-00382/05
!standard 9.1(19)
!standard 9.4(21)
!class amendment 04-08-27
!status Amendment 200Y 04-12-01
!status WG9 Approved 06-06-09
!status ARG Approved 8-0-2 04-11-19
!status work item 04-08-27
!status received 04-08-27
!priority Medium
!difficulty Medium
!subject Current instance rule and anonymous access types
!summary
(See proposal.)
!problem
AI95-00230 introduces the capability to use anonymous access types in a number of new contexts, in particular in component_definitions. It appears however that the current instance rule of 8.6(17) creates illegalities when an anonymous access type is used as the type of a component. This is demonstrated by the following example, excerpted from AI95-00230:
type Obj is record M : Integer; Next : access Obj; end record;
This example is likely to be very typical of user code, as linked structures are ubiquitous. Therefore, one would very much like it to be legal, as it avoids the declaration of a named access type. However, the current instance rule states that Obj in the record type declaration is the name of an object or value, so the construct "access Obj" is meaningless and therefore illegal.
!proposal
We are changing the current instance rule for type so that the type name occurring in a access_definition does not denote the current instance of the type, but rather the type itself.
Note that we are not changing the rules for named access types (the current instance problem for named access types can only occur in task and protected bodies).
A few examples of the effect of the new rule:
type Obj is record M : Integer; Next : access Obj; -- Legal Callback1 : access procedure (X : access Obj); -- Legal Callback2 : access procedure (X : Obj); -- Legal end record;
task type T;
task body T is X : array (1..2) of access T; -- Legal type A is access all T; -- Illegal procedure P (X : access T) is ... end P; -- Legal procedure P (X : T) is ... end P; -- Illegal begin ... end T;
!wording
Replace 8.6(17) by:
If a usage name appears within the declarative region of a type_declaration and denotes that same type_declaration, then it denotes the current instance of the type (rather than the type itself); the current instance of a type is the object or value of the type that is associated with the execution that evaluates the usage name. This rule does not apply if the usage name appears within the subtype_mark of an access_definition for an access-to-object type, or within the subtype of a parameter or result of an access-to-subprogram type.
AARM NOTE: The phrase "within the subtype_mark" is intended to cover a case like "access T'Class" appearing within the declarative region of T: here T denotes the type, not the current instance.
Replace 9.1(19) by:
2 Other than in an access_definition, the name of a task unit within the declaration or body of the task unit denotes the current instance of the unit (see 8.6), rather than the first subtype of the corresponding task type (and thus the name cannot be used as a subtype_mark).
Replace 9.4(21) by:
13 Within the declaration or body of a protected unit other than in an access_definition, the name of the protected unit denotes the current instance of the unit (see 8.6), rather than the first subtype of the corresponding protected type (and thus the name cannot be used as a subtype_mark).
!discussion
(See proposal.)
!example
Here is an example involving a linked structure with an anonymous access type:
type Obj is record M : Integer; Next : access Obj; end record;
!corrigendum 8.6(17)
Replace the paragraph:
If a usage name appears within the declarative region of a type_declaration and denotes that same type_declaration, then it denotes the current instance of the type (rather than the type itself). The current instance of a type is the object or value of the type that is associated with the execution that evaluates the usage name.
by:
If a usage name appears within the declarative region of a type_declaration and denotes that same type_declaration, then it denotes the current instance of the type (rather than the type itself); the current instance of a type is the object or value of the type that is associated with the execution that evaluates the usage name. This rule does not apply if the usage name appears within the subtype_mark of an access_definition for an access-to-object type, or within the subtype of a parameter or result of an access-to-subprogram type.
!corrigendum 9.1(19)
Replace the paragraph:
2 Within the declaration or body of a task unit, the name of the task unit denotes the current instance of the unit (see 8.6), rather than the first subtype of the corresponding task type (and thus the name cannot be used as a subtype_mark).
by:
2 Other than in an access_definition, the name of a task unit within the declaration or body of the task unit denotes the current instance of the unit (see 8.6), rather than the first subtype of the corresponding task type (and thus the name cannot be used as a subtype_mark).
!corrigendum 9.4(21)
Replace the paragraph:
13 Within the declaration or body of a protected unit, the name of the protected unit denotes the current instance of the unit (see 8.6), rather than the first subtype of the corresponding protected type (and thus the name cannot be used as a subtype_mark).
by:
13 Within the declaration or body of a protected unit other than in an access_definition, the name of the protected unit denotes the current instance of the unit (see 8.6), rather than the first subtype of the corresponding protected type (and thus the name cannot be used as a subtype_mark).
!ACATS test
ACATS tests need to be constructed for this rule.
!appendix

From: Tucker Taft
Sent: Monday, January  2, 2006  9:25 PM

Here is another comment that comes from reviewing
John's book.  I think John may have noted the same
thing himself.  It seems like the current instance
rule for naming the current instance of a type shouldn't
apply in contexts where a subtype name is required.
We made a special exception for "access T",
but "new T" seems like another case, as does "T'(...)".
One of John's examples bumps into the "new T" case
in a task type that is creating new instances of the
task type on the fly.

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From: Jean-Pierre Rosen
Sent: Tuesday, January  3, 2006  7:19 AM

Doesn't seem so necessary. In most cases, you can get around the current
instance rule by declaring a subtype. This would obviously not be
possible inside the type definition itself, but works everywhere else.

****************************************************************

From: Tucker Taft
Sent: Tuesday, January  3, 2006  7:48 AM

Here is an example where you might want the allocator
inside the type declaration:

     type Tree(Is_Leaf : Boolean := True) is record
       case Is_Leaf is
         when True =>
           Val : Integer := 0;
         when False =>
           Left : not null access Tree := new Tree;  -- illegal
           Right : not null access Tree := new Tree; -- illegal
       end case;
     end record;

****************************************************************

From: Randy Brukardt
Sent: Tuesday, January  3, 2006  3:18 PM

That looks pretty dubious to me; there would be no way to recover the
storage for these allocators, so they would be a potential storage leak.

For instance, if you wrote:

        Obj : not null access Tree := new Tree(Is_Leaf => False);

there would be no way to recover the Left and Right objects. [Anonymous
access types can't be used to instantiation Unchecked_Deallocation - ED]
(They're not co-extensions, and they could be changed to other values
anyway, so they couldn't be freed with the outer object.)

"not null" components aren't likely to be used a lot because of issues like
this; usually you need a way to represent "no item", and null is perfect for
that. And when you don't need that, they need an existing item to point at
for initialization - meaning that they can't usefully be used in recursive
situations.

****************************************************************

From: Tucker Taft
Sent: Tuesday, January  3, 2006  3:52 PM

I suspect I could create more convincing examples.
I chose to use anonymous non-null access values, but
I could have used named access types, and I might
still want a component in one variant to be default
initialized with an allocator for some other variant.

Not being able to recover the storage is not always
relevant, since there is a class of programs that
builds up a data structure and uses it, but never
reclaims it before the program ends.

In any case, it seems a bit odd to allow the use of
"access T" inside of T but not "new T".  I should
reiterate I see this as something to consider for
post Ada 2005.

****************************************************************

From: Pascal Leroy
Sent: Wednesday, January  4, 2006  3:15 AM

Now that we allow subtypes of incomplete types, it seems to me that the
following is legal:

     type Tree;
     subtype T is Tree;
     type Tree(Is_Leaf : Boolean := True) is record
       case Is_Leaf is
         when True =>
           Val : Integer := 0;
         when False =>
           Left : not null access Tree := new T;  -- OK?
           Right : not null access Tree := new T; -- OK?
       end case;
     end record;

If this works, it is actually quite similar to the trick you have to use
for task types.

****************************************************************

From: Pascal Leroy
Sent: Wednesday, January  4, 2006  3:55 AM

Note that if you changed the current instance rule for allocators and
qualified expressions, you would end up with rather cryptic expressions
like:

	new T (T.Disc)
	T'(T)

where the first T is a subtype name and the second T an object name (the
current instance).  Also, the meaning of something like T'Size (subtype
size or current instance size?) would become rather unclear to the reader,
even if well-defined in the RM.

****************************************************************


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