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!standard 10.02.01 (09)          99-02-23 AI95-00161/03
!class binding interpretation 99-02-23
!status work item 98-04-16
!status received 96-09-10
!priority Medium
!difficulty Medium
!subject Default-initialized objects
!summary 99-02-23
The creation of an object whose nominal subtype has preelaborable initialization is allowed in a preelaborable unit. This makes it possible to declare objects of some private types in preelaborable units.
The notion of a subtype having "preelaborable initialization" is defined by this AI.
!question 99-02-23
The paragraphs RM95 10.2.1(5-9) list a number of constructs which are forbidden in preelaborable units. In particular, RM95 10.2.1(9) states that: "The creation of a default-initialized object (including a component) of a descendant of a private type, private extension, controlled type, task type or protected type with entry declarations."
1. In this rule, what is the meaning of "default-initialized object"?
2. This rule seems extremely restrictive. For instance, if (as recommended) System.Address is a private type, the declaration:
A : System.Address;
makes the enclosing unit non-preelaborable. As another example, the implementation of Ada.Strings.Unbounded suggested in the Rationale is impossible because the initial value for the deferred constant Null_Unbounded_String would have to include an extension aggregate for a controlled type, which is illegal in a preelaborable unit.
!recommendation 98-04-16
(See wording.)
!wording 99-02-23
Change the RM95 10.2.1(9) to read: "The creation of an object (including a component) of a subtype which does not have preelaborable initialization. Similarly the evaluation of an extension_aggregate with an ancestor subtype_mark denoting such a subtype."
Add a new bullet after C.4(4): "No subtype_mark denotes a controlled type, a private type, a private extension, a generic formal private type, a generic formal derived type whose ancestor subtype is tagged, or a descendant of such a type."
!discussion 99-02-23
1. A "default-initialized object" is an object for which the implicit initialization described in RM95 3.3.1(10-14) takes place. Examples of default-initialized objects include an object_declaration without an initialization expression, and the (anonymous) object created by the evaluation of an extension_aggregate whose ancestor_part is a subtype name.
Note that an object of a task type or of a protected type is always default- initialized. Also note that for a private type or a private extension, providing an initialization expression always requires a reference to the name of an object, or a call to a non-static function, which are forbidden by other rules. Finally note that for a controlled type, an initialization expression may also be provided by an extension aggregate (if visibility allows), which must have a controlled subtype name as the ancestor part, which is forbidden.
The conclusion is that the phrase "default-initialized" can be removed from RM95 10.2.1(9).
2. RM95 10.2.1(9) is indeed too restrictive. For preelaboration to be usable (in particular in the context of distributed systems) we must loosen up the rules dramatically. However, we do not want to impose an additional burden on implementations which support annex C and have to comply with the requirements of RM95 C.4. Therefore, each time we remove a restriction from RM95 10.2.1(9), we must add a corresponding restriction to RM95 C.4.
Clearly we must keep task objects and protected objects with entries in RM95 10.2.1(9). Note that the current wording seems to allow the creation of objects of entry-less protected types, which is strange since the component_declarations may include default_expressions which execute non- trivial code.
In discussing private types, private extensions and controlled types, it is useful to introduce the notion of some entities "having preelaborable initialization."
- The partial view of a private type or private extension, a generic formal private type, or a generic formal derived type whose ancestor subtype is tagged, have preelaborable initialization if and only if the pragma Preelaborable_Initialization has been applied to them.
- Task types and protected types do not have preelaborable initialization; controlled types which have an overriden Initialize do not have preelaborable initialization.
- A view of a type has preelaborable initialization if it is an elementary type, an array type whose component subtype has preelaborable initialization, a record type whose components all have preelaborable initialization, a derived type whose parent type has preelaborable initialization and (in the case of a record extension) whose components all have preelaborable initialization.
- A record component (including a discriminant) has preelaborable initialization if its declaration includes a default_expression whose execution does not perform any of the actions listed in RM95 10.2.1(6-9), or if its declaration does not include a default expression and its nominal subtype has preelaborable initialization.
- A constrained subtype of a record type has preelaborable initialization if all its non-discriminant components have preelaborable initialization.
- Other subtypes have preelaborable initialization if their base type has preelaborable initialization.
Having defined the notion of preelaborable initialization, we allow the creation, in preelaborable units, of objects whose nominal subtype has preelaborable initialization. The restrictions which used to appear in RM95 10.2.1(9) are moved to RM95 C.4.
A pragma Preelaborable_Initialization specifies that a type has preelaborable initialization. The form of this pragma is as follows:
pragma Preelaborable_Initialization (direct_name);
This pragma must appear (1) in the visible part of a package, or (2) in the formal part of a generic library package.
In the first case, the enclosing library unit shall be the declaration of a preelaborated package. The direct_name shall denote the first subtype of a private type or a private extension declared within the same package visible part as the pragma. The full view of the type denoted by the direct_name shall have preelaborable initialization. In addition to the places where Legality Rules normally apply, this rule applies also in the private part of an instance of a generic unit.
In the second case, the direct_name shall denote a generic formal private type or a generic formal derived type declared in the same generic formal part as the pragma. Moreover, if the direct_name denotes a formal derived type, the ancestor subtype shall be a tagged type. If the pragma Preelaborable_Initialization is applied to a formal type, in a generic_instantiation the actual type shall have preelaborable initialization.
The following table lists all the private types and private extensions declared in preelaborated predefined packages. For each such type, the table specifies if a pragma Preelaborable_Initialization is applied to the type (yes) or not (no). For those packages that have a Wide_ variant, the types declared in the Wide_ variant have the same classification as the types declared in the 'narrow' variant. The same remark applies to the non-generic equivalents of the various generic packages.
Ada.Calendar.Time yes Ada.Finalization.Controlled yes Ada.Finalization.Limited_Controlled yes Ada.Numerics.Generic_Complex_Types.Imaginary yes Ada.Streams.Root_Stream_Type yes Ada.Strings.Bounded.Generic_Bounded_Length.Bounded_String no Ada.Strings.Maps.Character_Mapping yes Ada.Strings.Maps.Character_Set yes Ada.Strings.Unbounded.Unbounded_String yes Interfaces.C.Strings.Chars_Ptr yes System.Address yes System.Storage_Pool.Root_Storage_Pool yes
Note: The type Bounded_String cannot have a pragma Preelaborable_Initialization, because that would make it impossible to instantiate Generic_Bounded_Length with a non-static expression for the parameter Max.
!appendix

!section 10.2.1(9)
!subject Default-initialized objects
!reference RM95-10.2.1(9)
!from Pascal Leroy 96-08-30
!reference 96-5697.a Pascal Leroy 96-9-10>>
!discussion

The referenced paragraph says that a construct is not preelaborable if it
performs "The creation of a default-initialized object (including a
component)
of a descendant of a private type, private extension, controlled type, task
type or protected type with entry declarations;"

In this rule, what is the meaning of "default-initialized object"?  I
couldn't
find a definition of this phrase.

Bob Duff (private communication) tells me that this phrase is intended to
cover (among other things?) an object declaration which doesn't have an
initialization expression.

Even accepting this interpretation, there remain open questions.  For
instance: is an object to which a pragma Import is applied a
default-initialized object?  B.1(38) and B.1(24) say that no initialization
occurs in this case, so it would seem that the answer is 'no'.

It is interesting to note that, with this interpretation, 10.2.1(9) becomes
very restrictive: for a private type, a private extension or a controlled
type, I am not sure how one could write an initial value without either
referencing the name of an object, calling a non-static function, or writing
an extension aggregate.  But these things are forbidden by 10.2.1(7-9).  So
10.2.1 seems to state that (except in the case of imported objects) a
preelaborable units can never declare an object of a private type, a private
extension, or a controlled type.  Is it the intent?

(It looks like all objects of task or protected types are
"default-initialized", so the rule is redundant.  But that's not important:
it
is clear that we don't want objects of such types in preelaborated units.)

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From:   Randy Brukardt [SMTP:Randy@rrsoftware.com]
Sent:   Thursday, May 07, 1998 8:12 PM
Subject:        Re: AI95-00161/02

This AI has class "Confirmation", yet proposes language which extends the
language (allowing things previously disallowed).  I wouldn't expect
"Confirmation" Ais to have wording changes.  Shouldn't this AI be
re-classified?

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From:   Pascal Leroy[SMTP:phl@Rational.Com]
Sent:   Friday, May 08, 1998 6:04 AM
Subject:        Re: AI95-00161/02

> This AI has class "Confirmation", yet proposes language which extends the
> language (allowing things previously disallowed).  I wouldn't expect
> "Confirmation" Ais to have wording changes.  Shouldn't this AI be
> re-classified?

Surely it should be a binding interpretation.  Thanks for noticing.

Pascal

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From: 	Tucker Taft
Sent: 	Friday, February 26, 1999 9:47 AM
Subject: 	Re: Updated AIs

In AI 131, you have proposed that entry-less protected
objects are not preelaborable.  This is a troublesome
change, as Shared_Passive packages are required to
be preelaborable, and entry-less protected objects are
the only means of synchronization available via a
Shared_Passive partition.

Hence, the AI needs to accommodate entry-less protected objects
somehow, ideally without causing incompatibilities with
existing code which uses Shared_Passive packages, though
that goal may not be achievable...

-Tuck

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From: 	Randy Brukardt
Sent: 	Friday, February 26, 1999 3:06 PM
Subject: 	RE: Updated AIs

Tucker writes:

>In AI 131, you have proposed that entry-less protected
>objects are not preelaborable.

You mean AI 161. 131 is on interfacing to C.

Randy's comments on AI-161:

There is something wrong with the Wording change in this AI, as it depends on the
(undefined) term "preelaboratable initialization". I think the entire definition of this
term, and the pragma, needs to be moved to the "Wording" section. That would
leave just the actual discussion behind, not the technical definition.

				Randy.

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