Question

In Fortran, you can pass a function/subroutine A as an argument to another function/subroutine B, but can you store A for later retrieval and use?

for example, this is allowed in C

int foo(float, char, char) { /*whatever*/};

int (*pointerToFunction)(float, char, char);
pointerToFunction = foo;

In Fortran you can pass a subroutine as an argument

subroutine foo
! whatever
end subroutine foo

subroutine bar(func)
    call func
end subroutine bar

program x

    call bar(foo)

end program

but how can you store the address of foo in a similar way to C ?

Answer 1

Starting from so-called “Fortran 2003” (ISO/IEC 1539-2004) procedure pointers is a part of the Fortran language. It’s definitely of the major new features of Fortran language.

Usage example from Fortran Wiki.

---

Stefano, you mentioned strategy design pattern. In Fortran 2003 you can use pure OOP way to implement it (without procedure pointers). Offhand example:

strategies.f90

module strategies

  implicit none

  private

  public :: strategies_transportation_strategy, &
            strategies_by_taxi_strategy, &
            strategies_by_bus_strategy

  type, abstract :: strategies_transportation_strategy
  contains
    procedure(transportation_strategy_go), deferred :: go
  end type strategies_transportation_strategy

  type, extends(strategies_transportation_strategy) :: strategies_by_taxi_strategy
  contains
    procedure :: go => strategies_by_taxi_strategy_go
  end type strategies_by_taxi_strategy

  type, extends(strategies_transportation_strategy) :: strategies_by_bus_strategy
  contains
    procedure :: go => strategies_by_bus_strategy_go
  end type strategies_by_bus_strategy

  abstract interface
    subroutine transportation_strategy_go(this)
      import strategies_transportation_strategy
      class(strategies_transportation_strategy), intent(in) :: this
    end subroutine transportation_strategy_go
  end interface

  contains

    subroutine strategies_by_taxi_strategy_go(this)
      class(strategies_by_taxi_strategy), intent(in) :: this

      print *, "We are using taxi."

    end subroutine strategies_by_taxi_strategy_go

    subroutine strategies_by_bus_strategy_go(this)
      class(strategies_by_bus_strategy), intent(in) :: this

      print *, "We are using public transport."

    end subroutine strategies_by_bus_strategy_go

end module strategies

vehicles.f90

module vehicles

  use strategies

  implicit none

  private

  public :: vehicles_vehicle, &
            vehicles_taxi, &
            vehicles_bus

  type, abstract :: vehicles_vehicle
    private
    class(strategies_transportation_strategy), allocatable :: transportation_strategy
  contains
    procedure :: set_transportation_strategy => vehicle_set_transportation_strategy
    procedure :: go => vehicle_go
  end type vehicles_vehicle

  type, extends(vehicles_vehicle) :: vehicles_taxi
  contains
    procedure :: init => taxi_init
  end type vehicles_taxi

  type, extends(vehicles_vehicle) :: vehicles_bus
  contains
    procedure :: init => bus_init
  end type vehicles_bus

  contains

    subroutine vehicle_go(this)
      class(vehicles_vehicle), intent(in) :: this

      call this%transportation_strategy%go()

    end subroutine vehicle_go

    subroutine vehicle_set_transportation_strategy(this, new_transportation_strategy)
      class(vehicles_vehicle), intent(inout) :: this
      class(strategies_transportation_strategy), intent(in) :: new_transportation_strategy

      if (allocated(this%transportation_strategy)) then
        deallocate (this%transportation_strategy)
      end if

      allocate (this%transportation_strategy, source=new_transportation_strategy)

    end subroutine vehicle_set_transportation_strategy

    subroutine taxi_init(this)
      class(vehicles_taxi), intent(out) :: this

      type(strategies_by_taxi_strategy) :: by_taxi_strategy

      call this%set_transportation_strategy(by_taxi_strategy)

    end subroutine taxi_init

    subroutine bus_init(this)
      class(vehicles_bus), intent(out) :: this

      type(strategies_by_bus_strategy) :: by_bus_strategy

      call this%set_transportation_strategy(by_bus_strategy)

    end subroutine bus_init

end module vehicles

main.f90

program main

  use vehicles

  implicit none

  type(vehicles_taxi) :: taxi
  type(vehicles_bus) :: bus

  call taxi%init()
  call bus%init()

  call taxi%go()
  call bus%go()

end program main

At least works using gfortran 4.6 (20100925).