storing a procedure pointer within a type in Fortran

Question

I would like to store a pointer to a function as an attribute w/in a type. But the compiler complains that the first argument to the function ought to be the type (like the self or this argument in languages like Python or C++). But the attribute is just a pointer - it is not part of the type.

Here is some code:

module my_mod

  implicit none
  public :: my_type

  abstract interface
    ! INTERFACE I WANT TO POINT TO
    subroutine my_interface(arg)
      real, intent(in) :: arg
    end subroutine my_interface
  end interface

  type :: my_type
    ! MY TYPE DEFINITION
    real :: my_variable
    procedure(my_interface), pointer :: my_fn_pointer
  end type my_type

  interface my_type
    ! "CONSTRUCTOR" INTERFACE FOR MY TYPE
    module procedure init_my_type
  end interface

  contains

  type(my_type) function init_my_type(arg1, arg2)
    ! ACTUAL "CONSTRUCTOR" FOR MY TYPE
    real, intent(in) :: arg1
    procedure(my_interface) :: arg2

    init_my_type%my_variable = arg1
    init_my_type%my_fn_pointer => arg2
  end function init_my_type

end module my_mod

The actual error I get (ifort compiler) is:

error #8262: For a type-bound procedure that has the PASS binding attribute, the first dummy argument must have the same declared type as the type being defined.

I don't understand why it thinks that the interface I want to point to is a type-bound procedure.

Any hints?

Answer 1

You are, reasonably, misled by the wording of the error message as it relates to type-bound procedures. This is a problem of the error message itself which is unclear.

Your derived type has no binding.

However, the "passed-object dummy argument" concept applies to a procedure pointer component (which you do have) just as much as it does to type-bound procedures.

You should use the NOPASS attribute when using procedure pointer components and type-bound procedures if you want to avoid the passed-object dummy.