(j3.2006) Question about pointer components of coindexed objects

[Malcolm Cohen]

Hi John,

 

Yes I understand that pointer association is local, duh.

 

But this is NULLIFY.  It is breaking a pointer association, not creating one.  Making a pointer disassociated is not difficult.  I don?t care about the redundant useless ?pointer => Null()? that we added in F95.

 

Furthermore, the ?pointer becomes undefined? rule for cross-image pointer assignment (available by derived-type assignment) is that it becomes undefined if ?the pointer is pointer-assigned to a target on a different image?.  That does not do any damage to a disassociated pointer, so we KNOW that pointer-assigning a disassociated pointer on another image causes disassociation, not undefinement.

 

Finally, your suggestion would be an INCOMPATIBLE TECHNICAL CHANGE.

 

I see no merit in your suggestion.

 

Cheers,

-- 

..............Malcolm Cohen, NAG Oxford/Tokyo.

 

From: j3-bounces at mailman.j3-fortran.org [mailto:j3-bounces at mailman.j3-fortran.org] On Behalf Of John Reid
Sent: Monday, July 24, 2017 5:59 PM
To: fortran standards email list for J3 <j3 at mailman.j3-fortran.org>
Subject: Re: (j3.2006) Question about pointer components of coindexed objects

 

Malcolm,

This looks to me like an oversight in F2008 that we have carried over to 
F2015. We had a design objective that pointer association be local. For 
allocate and deallocate, we have

C949 (R933) An allocate-object shall not be a coindexed object.

For pointer assignment, we have

C1024 (R1034) A data-pointer-object shall not be a coindexed object.
C1026 (R1037) A data-target shall not be a coindexed object.

We seem to have forgotten to have such a constraint for nullify. Note that

x[1]%p => null()

is not permitted.

John.



Malcolm Cohen wrote:
> Hi folks,
>
>
>
> Consider:
>
>
>
> Program n9a
>
> Type t
>
> Integer,Pointer :: p
>
> End Type
>
> Type(t) x[*]
>
> Nullify(x[1]%p)
>
> Print *,'ok if F?',Associated(x[1]%p)
>
> End Program
>
>
>
> There are two questions:
>
>
>
> (1) Is the NULLIFY conforming? It certainly looks conforming to me.
> However another compiler claims that it is not permitted.
>
>
>
> (2) If the ASSOCIATED conforming? Since we?re allowed to follow
> pointers on other images, e.g. ?x[1]%p = 3?, I don?t see why it should
> be a problem. However, one compiler thinks it falls foul of
>
> C1542 The actual argument corresponding to a dummy
> pointer shall not be a coindexed object.
>
> It is not at all obvious that C1542 should apply to intrinsic
> procedures? A second compiler claimed that x[1]%p was not a pointer,
> throwing further confusion onto the fire. Anyway, I think that either
> ASSOCIATED should say that you can?t use it on coindexed objects, or
> C1542 should be followed by a Note saying it does not apply to the
> ASSOCIATED intrinsic, to clarify the situation.
>
>
>
> Any comments about these cases would be welcome (especially from Bill!).
>
>
>
> Cheers,
>
> --
>
> ..............Malcolm Cohen, NAG Oxford/Tokyo.
>
>
>
>
>
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