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From: "Bader, Reinhold" <Reinhold.Bader@lrz.de>
To: "j3@j3-fortran.org" <j3@j3-fortran.org>,
    WG5 <sc22wg5@open-std.org>
CC: "Rolf Rabenseifner (rabenseifner@hlrs.de)" <rabenseifner@hlrs.de>
Subject: Callback functions with polymorphic arguments
Thread-Topic: Callback functions with polymorphic arguments
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Date: Wed, 31 Aug 2011 19:41:33 +0000
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Hello all,=20

consider the following module:

module mod_callback_up_01
  implicit none
  abstract interface
    subroutine callback(data)
      class(*), dimension(:) :: data    ! (A)
    end subroutine
  end interface
contains
  subroutine foo(data, proc)
    class(*), dimension(:) :: data    ! (B)
    procedure(callback) :: proc
    call proc(data)
  end subroutine
end module

and then the following code which uses this module:=20

module mod_impl
  implicit none
  type :: bar
    integer :: i
  end type
contains
  subroutine callback1(data)
    class(bar) :: data(:)         ! (C)
    integer :: i
    do i=3D1,size(data)
      data(i)%i =3D i
    end do
  end subroutine
end module
program prog
  use mod_callback_up_01
  use mod_impl
  integer, parameter :: dim=3D5
  type(bar) :: x(dim)
  call foo(x, callback1)
end program

Questions:

(1) Is the above code standard conforming?

(2) Assume that at (C), the declaration is replaced by=20
       type(bar) :: data(:)
     Is the resulting code standard conforming?
        =20
(3) Assume that at (A) and (B), class(*) is replaced by
     class(base), where base is some known type, and
    bar is an extension of base.
     Is the resulting code standard conforming?

(4) Apply both assumptions in (2) and (3) - is the
     resulting code then standard conforming?

The background for this is the issue of handling callbacks in MPI-3.=20
For backward compatibility it is considered desirable to have=20
abstract interfaces for callbacks which use arguments declared
as TYPE(*), DIMENSION(*). The dummy arguments of the callbacks=20
of course will have some definite type in existing code. (The=20
programmer may still need to add BIND(C), but this is considered=20
sufficiently low-intrusive). Depending on the answers to above questions,=20
something may need to be done about this in the TR.=20

(For the above code, different compilers give widely varying results
with respect  to both compilation and run time behaviour).=20

Regards
Reinhold