続・fortranでもオブジェクト指向したい！「継承と抽象クラス」 ref: https://qiita.com/Bluepost59/items/8d4b7d7713676fe06472

file0.f

type,extends(parent_class) :: child_class
   integer param1
   !...
contains
   procedure :: method1 => child_method1
   !...
end type child_class

file1.f

type,abstract :: absclass
   integer param2
   !...
contains
   procedure(absmethod1),deferred :: method1
   procedure(absmethod2),deferred :: method2
   !...
end type absclass

!引用仕様（戻り値、引数）の定義
interface
   subroutine absmethod1(self,subparam)
      import absclass
      class(absclass) self
      integer subparam
   end subroutine absmethod1

   double precision function absmethod2(self,subparam)
      import absclass
      !...
   end function absmethod2
end interface

main.f90

program main
  use mod_idol
  implicit none

  type(idol) :: myidol

  character(len=60) :: myname = "Momoka"
  character(len=60) :: myunit = "momo_pear_berry"

  call myidol%set_name(myname)
  call myidol%join(myunit)

  write(6,*) "name:", myidol%get_name()
  write(6,*) "unit:", myidol%get_group()

end program main

mod_idol.f90

module mod_idol
  implicit none

!---------------------------------------------------
! 抽象クラス girl
  type,abstract :: girl
     private
     character(len=60) :: name = "noname"
   contains
     procedure(girl_set_name),deferred :: set_name
     procedure(girl_get_name),deferred :: get_name
  end type girl

! 引用仕様の宣言
  interface
     subroutine girl_set_name(self,str)
       import girl
       class(girl) self
       character(len=60) str
     end subroutine girl_set_name

     character(len=60) function girl_get_name(self)
       import girl
       class(girl) self
     end function girl_get_name
  end interface
!---------------------------------------------------
! クラス idolの宣言部 
  type,extends(girl) :: idol
     private
     character(len=60) group
   contains
     procedure :: set_name => idol_set_name
     procedure :: get_name => idol_get_name
     procedure :: join => idol_join
     procedure :: get_group => idol_get_group
  end type idol

contains
!---------------------------------------------------
! クラス idolの実装部
  subroutine idol_set_name(self,str)
    class(idol) self
    character(len=60) str
    self%name = str
  end subroutine idol_set_name

  character(len=60) function idol_get_name(self)
    class(idol) self
    idol_get_name = trim(self%name)
  end function idol_get_name

  subroutine idol_join(self,group)
    class(idol) self
    character(len=60) group
    self%group = group
  end subroutine idol_join

  character(len=60) function idol_get_group(self)
    class(idol) self
    idol_get_group = trim(self%group)
  end function idol_get_group

end module mod_idol