Question

I can create function pointers in Fortran 90, with code like

real, external :: f

and then use f as an argument to another function/subroutine. But what if I want an array of function pointers? In C I would just do

double (*f[])(int);

to create an array of functions returning double and taking an integer argument. I tried the most obvious,

real, external, dimension(3) :: f

but gfortran doesn’t let me mix EXTERNAL and DIMENSION. Is there any way to do what I want? (The context for this is a program for solving a system of differential equations, so I could input the equations without having a million parameters in my subroutines.)

Answer 1

The declaration “real, external :: f” doesn’t really make “f” into a full pointer since you can’t change the procedure that it points — it does permit you to pass this single function to another routine., So you also need the “pointer” attribute. There are examples on page 267 of “Fortran 95/2003 explained” by Metcalf, Reid & Cohen — a google search for “fortran procedure pointer” will bring up this page. A simple example close to yours is “real, external, pointer :: f_ptr”. Alternatively: “procedure (f), pointer :: f_ptr”. This is a Fortran 2003 feature — http://gcc.gnu.org/wiki/Fortran2003 and http://gcc.gnu.org/wiki/ProcedurePointers lists partial support in gfortran, best with 4.5. I’m not sure whether “dimension” is directly allowed, but you can assign a procedure to a pointer, which provides a lot of flexibility. You can also put the pointer into a derived type, which could be made into an array.

Edit: here is a code example which works with gfortran 4.5:
Edit 2: line commented out per comments below.

module ExampleFuncs

  implicit none

contains

function f1 (x)
  real :: f1
  real, intent (in) :: x

  f1 = 2.0 * x

  return
end function f1


function f2 (x)
   real :: f2
   real, intent (in) :: x

   f2 = 3.0 * x**2

   return
end function f2


function fancy (func, x)

   real :: fancy
   real, intent (in) :: x

   interface AFunc
      function func (y)
         real :: func
         real, intent (in) ::y
      end function func
   end interface AFunc

   fancy = func (x) + 3.3 * x

end function fancy

end module  ExampleFuncs

program test_proc_ptr

  use ExampleFuncs

  implicit none

  ! REMOVE: pointer :: func
  interface
     function func (z)
        real :: func
        real, intent (in) :: z
     end function func
  end interface

  procedure (func), pointer :: f_ptr => null ()

  type Contains_f_ptr
     procedure (func), pointer, nopass :: my_f_ptr
  end type Contains_f_ptr

  type (Contains_f_ptr), dimension (2) :: NewType


  f_ptr => f1
  write (*, *) f_ptr (2.0)
  write (*, *) fancy (f_ptr, 2.0)

  f_ptr => f2
  write (*, *) f_ptr (2.0)
  write (*, *) fancy (f_ptr, 2.0)

  NewType(1) % my_f_ptr => f1
  NewType(2) % my_f_ptr => f2

  write (*, *) NewType(1) % my_f_ptr (3.0), NewType(2) % my_f_ptr (3.0)

  stop

end program test_proc_ptr