Is it possible in Fortran to determine if two polymorphic objects are the same derived type?

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Is it possible to take two polymorphic objects and determine if they are of the same derived type (or class)? The intention is to use it as a clean way to filter a generic linked list.

Something like the following mock code.

function isSameType(a, b) result(itIs)

    !arguments
    class(*), intent(in) :: a
    class(*), intent(in) :: b

    !output
    logical :: itIs

    !return true if a and b are both the same type
    !return false if a and b are not

end function isSameType
derived-types filter fortran polymorphism
1 Answers

The standard inquiry function same_type_as tests equality of dynamic type of two objects:

program typetest
  implicit none

  type t1
  end type t1

  type t2
  end type t2

  class(*), allocatable :: a, b

  allocate(t1 :: a)
  allocate(t2 :: b)

  print *, SAME_TYPE_AS(a,b)   ! False
end program

same_type_as does not test declared type (except where this is the same thing). It does not consider kind parameters:

program partest
  implicit none

  type :: t1(n)
     integer, kind :: n
  end type t1

  type(t1(1)) :: a
  type(t1(2)) :: b

  print *, SAME_TYPE_AS(a,b)   ! True
end program

Further, to get a useful result you'll be wanting (at least) one of a and b to be of extensible dynamic type. While you can ask

program intrinsictest

  implicit none

  class(*), allocatable :: a, b

  allocate(real :: a)
  allocate(double precision :: b)

  print *, SAME_TYPE_AS(a,b)  ! ...
end program

the result is processor dependent (could be true or false).

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