(j3.2006) deallocating pointer function results

[Bill Long]

On Nov 3, 2015, at 4:34 AM, Robert Corbett <robert.corbett at oracle.com> wrote:

> The following message is based on a thread on comp.lang.fortran.
> All references are with respect to Fortran 2008.
> 
> Consider the following program:
> 
> !-----------------------------------------------------------------
> 
> program main
> 
>    deallocate( new_int() )              ! (1)
>    call my_deallocate( new_int() )      ! (2)
>    call hmm_deallocate( new_int() )     ! (3)
> 
> contains
>    function new_int()
>      integer, pointer :: new_int
>      allocate(new_int)
>      new_int = 3
>    end function
> 
>    subroutine my_deallocate(p)
>      integer, pointer  :: p
>      deallocate( p )
>    end subroutine
> 
>    subroutine hmm_deallocate(p)
>      integer, pointer  :: p, q
>      q => p
>      deallocate( q )
>    end subroutine
> end
> 
> !-----------------------------------------------------------------
> 
> The statement marked (1) is clearly not permitted, because it
> violates syntax rule R632.

Agreed.  I had to comment out this line to get the program to compile and run. 

> 
> The statement marked (2) leads to prohibited behavior.
> Subclause 5.3.10, paragraph 5, states
> 
>       If no INTENT attribute is specified for a dummy argument,
>       its use is subject to the limitations of its effective
>       argument (12.5.2).
> 
> Because a function reference is not allowed as an
> /allocate-object/ in a DEALLOCATE statement, a dummy argument
> corresponding to an actual argument that is a function reference
> is not allowed as an /allocate-object/ in a DEALLOCATE statement.
> (I doubt that any existing implementation catches this error.)

I don?t agree with this.  The actual argument is a function reference that returns a data pointer, which by our definitions, is a ?variable? (R602).   The actual argument looks like any other associated data pointer for the purposes of argument association. 

Cheers,
Bill



> 
> I think that the behavior that results from the statement
> marked (3) is intended not to be permitted.  I base my
> conclusion on a somewhat strained reading of Subclause 12.5.2.13,
> paragraph 1, which states
> 
>       While an entity is associated with a dummy argument,
>       the following restrictions hold.
> 
>           (1) Action that affects the allocation status of
>               the entity or a subobject thereof shall be
>               taken through the dummy argument.
> 
> One issue concerns the meaning of the phrase "the allocation
> status".  I am assuming that it means "the allocation status or
> pointer association status".  In all other cases where both the
> allocation status and the pointer allocation status are meant,
> they are both mentioned.  A reason to believe that both are
> intended here is that the text in the Fortran 90 standard
> (Subclause 12.5.2.9 of the Fortran 90 standard) from which the
> quoted text was derived was described in a way that included
> both cases.
> 
> The dummy argument is not used in the DEALLOCATE statement; a
> local pointer variable is used in its place.  I assume that the
> deallocation is therefore not done "through" the dummy argument.
> An argument can be made against my assumption based on
> Subclause 5.3.10, paragraph 2.  It states
> 
>       The INTENT(IN) attribute for a pointer dummy argument
>       specifies that during the invocation and execution of
>       the procedure its association shall not be changed
>       except that it may become undefined if the target is
>       deallocated other than through the pointer (16.5.2.5).
> 
> If the "allocation status" of the pointer may be changed only
> through the dummy argument, then the last part of that clause
> is unnecessary.
> 
> I would like to know if anyone disagrees with my conclusions.
> 
> Robert Corbett
> 
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Bill Long                                                                       longb at cray.com
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