Fortran Wiki
Strategy Pattern

Strategy Pattern

Strategy Pattern is a design pattern whereby the algorithm (“strategy”) is selected at run-time, instead of embedding it as a type-bound procedure. Fortran 2003 introduces the concept of procedure pointers and abstract interfaces, which can be used to select the procedure at runtime (polymorphism).

Here is an example:

module m_strategy_pattern
implicit none

type :: Button
    character(len=20) :: label
    procedure(generic_function), pointer, nopass :: on_submit
    procedure :: init
end type Button

abstract interface
    subroutine generic_function(numbers)
        integer, dimension(:), intent(in) :: numbers
    end subroutine
end interface


    subroutine init(self, func, label)
        class(Button), intent(inout) :: self
        procedure(generic_function) :: func
        character(len=*) :: label

        self%on_submit => func      !! Procedure pointer
        self%label = label

    end subroutine init

    subroutine summation(array)
        integer, dimension(:), intent(in) :: array
        integer :: total

        total = sum(array)
        write(*,*) total
    end subroutine summation

    subroutine join(array)
        integer, dimension(:), intent(in) :: array
        write(*,*) array        !! Just write out the whole array
    end subroutine join

end module m_strategy_pattern

program test_strategy
use m_strategy_pattern
implicit none

    type(Button) :: button1, button2
    integer :: i

    call button1%init(summation, "Add them")
    call button2%init(join, "Join them")

    call button1%on_submit([(i, i=1,10)])   !! Array constructor syntax
    call button2%on_submit([(i, i=1,10)])

end program test_strategy