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!standard 11.4.1 (14)          01-09-11 AI95-00241/03
!class binding interpretation 00-11-19
!status Amendment 200Y 02-05-09
!status WG9 Approved 01-10-05
!status ARG Approved 9-0-1 00-11-19
!status work item 00-10-04
!status received 00-10-04
!priority Medium
!difficulty Easy
!subject Testing for Null_Occurrence
!summary
The function Exception_Identity applied to Null_Occurrence returns Null_Id, which can then be tested using the equality operation.
!question
Ada.Exceptions exports the constant Null_Occurrence, but doesn't provide any way to test whether a given exception occurrence is Null_Occurrence (type Exception_Occurrence is limited). Is this intended? (No.)
!recommendation
The call Exception_Identity(X), where X has the value Null_Occurrence, should not raise Constraint_Error, but return the value of Null_Id which can then be tested against Null_Id using the equality operation.
!wording
Replace 11.4.1(14) by
Raise_Exception and Reraise_Occurrence have no effect in the case of Null_Id or Null_Occurrence. Exception_Name raises Constraint_Error for a Null_Id. Exception_Message, Exception_Name, and Exception_Information raise Constraint_Error for a Null_Occurrence. Exception_Identity applied to Null_Occurrence returns Null_Id.
!discussion
When logging information, an application probably will use Null_Occurrence to represent the absence of an exception.
When displaying the log, it is necessary to determine whether a particular occurrence is Null_Occurrence or an occurrence containing information about an exception.
Currently, it is difficult to test for Null_Occurrence because, being of a limited type, it is not possible to compare with the constant Null_Occurrence. Moreover, the function Exception_Identity applied to Null_Occurrence raises Constraint_Error. An exception handler can be used to detect this but it is awkward, does not reflect that this is a normal case, and potentially can mask other errors.
Two alternatives were considered. One was to add a function Is_Null_Occurrence and the other was to define Exception_Identity to return Null_Id when applied to Null_Occurrence rather than raise Constraint_Error.
The second approach was taken because it avoids adding a further function and moreover it seems unnatural that Exception_Id(Null_Occurrence) should raise Constraint_Error.
An instance of Null_Occurrence can be tested for as in the following:
procedure Process_Exception(X: Exception_Occurrence) is begin if Exception_Identity(X) = Null_Id then -- process the case of a Null_Occurrence else -- process other exception occurrences end if; end;
!corrigendum 11.4.1(14)
Replace the paragraph:
Raise_Exception and Reraise_Occurrence have no effect in the case of Null_Id or Null_Occurrence. Exception_Message, Exception_Identity, Exception_Name, and Exception_Information raise Constraint_Error for a Null_Id or Null_Occurrence.
by:
Raise_Exception and Reraise_Occurrence have no effect in the case of Null_Id or Null_Occurrence. Exception_Name raises Constraint_Error for a Null_Id. Exception_Message, Exception_Name, and Exception_Information raise Constraint_Error for a Null_Occurrence. Exception_Identity applied to Null_Occurrence returns Null_Id.
!ACATS test
Add a test case to CB41004 to check that Exception_Identity(Null_Occurrence) does return Null_Id.
!appendix

!topic Testing for Null_Occurrence
!reference RM95-11.4.1(3)
!from Gary Dismukes (for Ada Core Technologies)
!keywords Ada.Exceptions, Null_Occurrence
!discussion

Ada.Exceptions exports the constant Null_Occurrence, but doesn't
provide any way to test whether a given exception occurrence is
Null_Occurrence (type Exception_Occurrence is limited).

We propose that a function should be added to Ada.Exceptions that
would allow testing whether an exception occurrence is
Null_Occurrence.
In GNAT, we have added a child function
Ada.Exceptions.Is_Null_Occurrence
for this purpose:

--  This is a GNAT-specific child function of Ada.Exceptions. It
provides
--  clearly missing functionality for its parent package, and most
reasonably
--  would simply be an added function to that package, but this
change cannot
--  be made in a conforming manner.

function Ada.Exceptions.Is_Null_Occurrence
  (X    : Exception_Occurrence)
   return Boolean;
   --  This function yields True if X is Null_Occurrence, and False
otherwise

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From: Tucker Taft
Sent: Friday, September 22, 2000 9:19 PM

I believe the original intent was that:

   Exception_Identity(Null_Occurrence) = Null_Id

providing a mechanism for testing for Null_Occurrence,
but I see that 11.4.1(14) contradicts that by indicating that
Exception_Identity raises Constraint_Error in this case.  I think
that is a mistake, though others might remember otherwise.

> function Ada.Exceptions.Is_Null_Occurrence
>   (X    : Exception_Occurrence)
>    return Boolean;
> --  This function yields True if X is Null_Occurrence, and False
otherwise

This seems like a reasonable proposal, in any case, although it
is obviously an amendment, not an interpretation.

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

From: Robert Dewar
Sent: Friday, September 22, 2000 10:03 PM

<<I believe the original intent was that:

   Exception_Identity(Null_Occurrence) = Null_Id

providing a mechanism for testing for Null_Occurrence,
but I see that 11.4.1(14) contradicts that by indicating that
Exception_Identity raises Constraint_Error in this case.  I think
that is a mistake, though others might remember otherwise.
>>

Well I would be happy to allow this identity, though this would be a
change
(in GNAT we could add a child, but not change clear RM semantics),
but
if one is in the amendment business, perhaps allowing the above
identity
is the cleanest approach.

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