Call overridden procedure of abstract parent type

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22

I had guessed as much and that’s why I suggested the alternative above and where I mentioned further subclassing.

Owing to certain other gaps and issues with the language such as with generics and lack of built-in standard intrinsic derived types for utilities (a la STL “classes”), it incentivizes library and solution developers with Fortran to veer toward deeper hierarchies when it comes to OOP and which then require certain flexibility in the language and also an awareness of them to use the facility adequately.

23

I’m not sure what the standard says about this. (Not that I’m suggesting you use it, though it sort of does what you want, and sort of doesn’t.) The Intel compiler is happy with it and it runs ok unless I enable null pointer checking when it fails with “Attempt to use pointer PARENT when it is not associated with a target” in the call to self%parent%say_hello. Presumably without the check the parent component is automatically allocated before the call.

module parent_m
   implicit none
   private

   type, abstract, public :: parent_t
      type(concrete_parent_t), allocatable :: parent
   contains
      private
      procedure, public :: say_hello => say_hello_parent
   end type
   
   type, extends(parent_t), public :: concrete_parent_t
   end type
   
contains

   subroutine say_hello_parent(self)
      class(parent_t), intent(in) :: self

      print *, "Hello from parent_t"
   end subroutine
end module

module child_m
   use parent_m, only: parent_t

   implicit none
   private

   type, extends(parent_t), public :: child_t
   contains
      private
      procedure, public :: say_hello => say_hello_child !<-- override the "generic"
   end type

contains

   subroutine say_hello_child(self)
      class(child_t), intent(in) :: self

      print *, "Hello from child_t"
      call self%parent%say_hello      
   end subroutine
end module

program main
   use child_m, only: child_t

   implicit none

   type(child_t) :: child
   
   call child%say_hello()
end program
24

@Nocaster60 ,

You would need to explicitly allocate the parent concrete component, say along the following lines, in order for the code to conform:

...
   type(child_t) :: child
   
   allocate( child%parent ) !<-- an option
   call child%say_hello()
...
25

Yes. I should have checked in the debugger. Of course there is no allocation. Parent remains undefined. I suppose the compiler knows where the bound procedure is whether parent is allocated or not and calls it without checking. No doubt things would start to get ugly if other components of parent were referenced. (Initially I had allocated parent in a constructor for child_t but found it didn’t make any difference whether the constructor was used or not so left it out.)