Version 1.1 of ais/ai-00231.txt

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!standard 3.10 (06)          00-04-13 AI95-00231/01
!standard 3.10 (12)
!standard 4.6 (49)
!standard 8.6 (25)
!class amendment 00-04-13
!status work item 00-04-13
!status received 00-04-13
!priority Medium
!difficulty Hard
!subject Access-to-constant parameters
Parameters and discriminants of an anonymous access-to-constant type are introduced. Access parameters and access discriminants are generalized to have a variant that allows a null value.
(See discussion.)
Modify 3.10(6) to:
access_definition ::= access [all | constant] subtype_mark
Modify 3.10(12) to:
An access_definition defines an anonymous general access type; the subtype mark denotes its @i<designated subtype>. If the word @b<all> appears, the type is an access-to-variable type. If the word @b<constant> appears, the type is an access-to-constant type. If neither word appears, the type is an access-to-variable type that does not include a null value. An access_definition is used in the specification of an access discriminant (see 3.7) or an access parameter (see 6.1).
Modify 4.6(49) so that the null check is performed only if neither @b<all> or @b<constant> are given. (Editor's note: We really need a name for such types.)
Modify 8.6(25) so that implicit conversion to any anonymous access type is permitted, with the expected rules (i.e. it is illegal to convert an access-to-constant to access-to-variable).
(See proposal.)
It is surprising that an access-to-constant parameter/discriminant is not available. There are several circumstances where such a parameter/discriminant would be appropriate:
As a controlling parameter of an operation that doesn't modify the
designated object.
As a way to force pass-by-reference when interfacing with a foreign
language, when the external operation does not update the designated object,
As a way to provide read-only access via a discriminant.
The rule disallowing "null" for access parameters and access discriminants has turned out to be confusing, and not really what is wanted typically when interfacing with a foreign language. Therefore, we propose to include the "access all T" as a way to pass a potentially-null pointer which might point to a local object. The new "access constant T" parameters/discriminants also allow null values, to avoid the confusion that occurred with "access T." Also, there is less need for a never-null access-to-constant since a normal "IN" parameter works quite well for that case.

Randy Brukardt  00-04-13

This proposal was split out of the "with type" proposal (AI-00217) in
April 2000. Some early conversation on this feature can be found in that
AI's appendix.


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