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!standard 3.10(6)          05-02-14 AI95-00404/03
!standard 3.9.2(11)
!class amendment 05-01-28
!status work item 05-01-28
!status received 05-01-28
!status received
!priority High
!difficulty Easy
!subject Not null and all in access parameters and types
!summary
Not null is implicit, but may be given explicitly, for controlling access parameters and results. When defined by a renaming or a generic instantiation, the controlling access parameters and results must be null excluding.
All is not permitted as a general access modifier for anonymous access types.
!problem
The current proposal for Ada 2005 permits the specification of not null for controlling access parameters even though they have to be not null anyway. Similarly it permits the specification of all in anonymous access types even though they never are pool specific and so all is superfluous. This provides the user with several different ways of saying the same thing which is confusing.
However, specifying not null for controlling access parameters provides helpful documentation, and which parameters are controlling might change, or might not be obvious, so this sort of specification should be permitted, despite being redundant.
!proposal
(See summary.)
!wording
Modify 3.10(6) [as previously modified by AI-231 and AI-254] to
... access_definition ::=
[null exclusion] access [constant] subtype_mark |
| [null exclusion] access [protected] procedure parameter_profile | [null exclusion] access [protected] function parameter_and_result_profile
Add after 3.9.2(11):
If a dispatching operation is defined by a subprogram_renaming_declaration or the instantiation of a generic subprogram, any access parameter or access result of the renamed subprogram or the generic subprogram that corresponds to a controlling access parameter or controlling access result of the dispatching operation, shall be null excluding.
AARM Note on incompatibility:
This rule will require the addition of an explicit "not null" on non-dispatching operations that are later renamed to be dispatching, or on a generic that is used to define a dispatching operation.
!discussion
The draft rationale says:
For uniformity, Ada 2005 permits all three forms with anonymous access types. And we can also add not null so we can write all the following in Ada 2005
procedure P1(X: access T) procedure P2(X: access constant T); procedure P3(X: access all T); procedure P4(X: not null access T); procedure P5(X: not null access constant T); procedure P6(X: not null access all T);
Note that two anomalies remain. One is that access T is deemed to be short for access all T when it occurs in an anonymous access type in order to permit compatibility between Ada 95 and Ada 2005. And so P1 and P3 are the same.
To have required all existing users to insert all in their access parameters would have been too much of a burden. Similarly P4 and P6 are the same as well.
Moreover, as mentioned above, there is also the anomaly that controlling parameters are still always null excluding and so in that case P1, P3, P4, and P6 are all the same.
It is very confusing to have unnecessary ways of saying the same thing.
We are stuck with access T as a parameter as being general because of backwards compatibility with Ada 95.
It is therefore proposed that anonymous access types should just take the form
AV: access T; AC: access constant T;
and that
AV: access all T;
should not be permitted.
This makes them behave like normal object declarations where we either have constant or nothing.
The anomaly is with named access types having to distinguish between pool-specific and general access types. There is no reason why this should be extended to anonymous access types.
Similarly, controlling access parameters have to be null excluding but for compatibility with Ada 95 we cannot require the programmer to insert not null everywhere. We considered disallowing an explicit not null where they are implicit, but it was anticipated that after a transition period, explicit not null would be used everywhere it applies, and disallowing its use for controlling access parameters or results would defeat this.
For a dispatching operation defined by renaming or instantiation, we require that the renamed subprogram or generic subprogram be null excluding for each access parameter or access result that ends up controlling in the renaming or the instance. For example:
generic type GT is private; procedure Gen_Subp_1(Y : access GT);
generic type GT is private; procedure Gen_Subp_2(Y : not null access GT);
with Gen_Subp_1; with Gen_Subp_2; package P is type T is tagged ....
package Inner is procedure Not_Disp_1(X : access T); procedure Not_Disp_2(X : not null access T); end Inner;
procedure Ren1(X : access T) renames Not_Disp_1; -- illegal procedure Ren2(X : access T) renames Not_Disp_2; -- legal
procedure Inst1 is new Gen_Subp_1(T); -- illegal procedure Inst2 is new Gen_Subp_2(T); -- legal end P;
!example
(See discussion.)
--!corrigendum
!ACATS test
Create B-Tests to check that the renames and instance rule is checked. Create a B-Test to insure that "all" is not allowed in anonymous access types.
!appendix

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