Let’s Use Procedure Pointers in Object-Oriented Fortran

Author: Amasaki Shinobu (雨崎しのぶ)

Twitter: @amasaki203

Posted on: 2023-08-28 JST Updated on: 2024-0722

Abstract

This article will cover an implementation of “callback” with procedure pointers in modern Fortran. This allows us to write initial and boundary conditions on numerical computation simply.

Contents

Introduction

In classical Fortran, programmers write their code using the procedural programming paradigm. On the other hand, in modern Fortran, the Object-Oriented Programming (OOP) paradigm and procedure pointers are supported starting from Fortran 2003 standards.

In the field of numerical computation, utilizing OOP streamlines source code management, simplifies code modification, and reduces human errors, even though there is some overhead.

This article explores practical implementations of callbacks using procedure pointers in modern Fortran’s object-oriented paradigm.

Preliminary Understanding

In Fortran, object-oriented programming is achieved by declaring subroutines and functions below the contains statement of a derived type, associating them with that type. This is referred to as type-bound procedures. Type-bound procedures, similar to regular procedures, can also accept procedure pointers as arguments.

Procedure pointers are pointers that refer to procedures, allowing for the switching of procedures and invocation of associated procedures using the pointer’s names. In Fortran, they are defined using explicit interface blocks and procedure statements for procedures. For exmple:

interface
   function f(x) result(res)
      real, intent(in) :: x
      real             :: res
   end function f
end interface

procedure(f), pointer :: fptr

where between the interface and end interface keywords, there is a section known as an ‘interface block’, and interface f is then specified within the parentheses of the procedure statement.

By using them, it becomes possible to create callbacks, allowing for concise handling of initial and boundary conditions in Fortran numerical computation codes. This further amplifies the ease of changes brought by OOP, enhancing the overall convenience.

In the following, we will examine the simplicity of performing assignments to array components within a derived type via a procedure pointer.

Usage of Procedure Pointers

For example, let’s consider providing a basic sine function as the array of the initial values.

A sine function with a blue line

To provide this array of initial values, we can achieve it by utilizing the module implemented with OOP, as explained in the following section. This can be accomplished by executing the following statement:

fptr => dsin
call u%allocate()
call u%initialize(fptr)

In the first line, fptr is associated with the intrinsic function dsin, which is the double precision sine function. In the second line, the statement calls the type-bound procedure allocate() of the derived type u. Finally, in the third line, the type-bound procedure initialize(fptr) is invoked with the fptr as its actual argument.

If we want to switch to a cosine function, we can achieve this by simply changing the function that the pointer refers to, as follows:

fptr => dcos
call u%initialize(fptr)

where dcos is the intrinsic function that returns the value of the double precision cosine function.

A cosine function with a red line

Procedure pointers can refer to procedures defined by users.

   ...
   fptr => cliff          ! fptr refers to the user-defined procedure 'cliff'.
   call u%allocate()
   call u%initialize(fptr)
   call data_output()
   
contains

   ! Internal procedure 'cliff'
   function cliff(x)
      implicit none
      real(real64) :: cliff
      real(real64), intent(in) :: x
      real(real64), parameter :: pi_halved = acos(-1d0)/2d0 
      
      ! staircase function
      if (x <= pi_halved ) then
         cliff = 1d0   ! for less than or equal to π/2,
      else
         cliff = 0d0   ! otherwise.
      end if
   
   end function cliff
   ...

Plotting this, we obtain the following a graph of the function.

A staircase function with a green line

From above examples, it becomes evident that using a procedure pointer as an argument for type-bound procedures simplifies code writing and enhances flexibility for initializing arrays.

In the next section, we will discuss modules written in an object-oriented manner that achieve these functionalities.

Modules and Program written in OOP

Here, for simplicity, let’s think about an application composed of the following three Fortran codes:

  1. module kernel performs initialization operation on arrays,
  2. module class_Variable defines the derived type Variable, and
  3. program main is the main program.

The entire three source code files are available on the page of GitHub.

module kernel

This module includes the definition of constant parameters and a callback process. The initialize subroutine, which initializes an array using the passed function pointer as an argument, is implemented as a module procedure.

module kernel
   use :: iso_fortran_env, only: real64
   implicit none
   private
   public :: initialize

   ! Number of discretized points along the computing region
   integer, parameter, public      :: nx = 2**8+1       
   
   ! Value of the mathematical constant π (pi)
   real(real64), parameter, public :: PI = acos(-1d0)
   
   ! Length of the computing region [0, L]
   real(real64), parameter, public :: L = 2*PI
   
   ! Interval between adjacent points
   real(real64), parameter, public :: dx = L/dble(nx-1)

contains
   
   ! Callback: Define a subroutine to initialize
   ! an array using a function pointer.
   subroutine initialize(array, fp)
      use, intrinsic :: iso_fortran_env, only: real64
      implicit none
      real(real64), intent(out) :: array(:)
   
      ! Declare the procedure pointer for the function.
      pointer :: fp
      
      ! Interface block for the function pointer 'fp'.
      interface
         function fp(x) result(res)
            use, intrinsic :: iso_fortran_env, only: real64
            real(real64), intent(in) :: x
            real(real64) :: res
         end function fp
      end interface

      integer :: i
      real(real64) :: x
      !---- End of specification statements for 'initialize' -----!
      !---- Begining executable statements of 'initialize'-----! 
      
      ! Loop through indices 1 to nx of the array.
      do i = 1, nx
      
         ! Compute x-axis value
         x = dble(i-1)*dx
         
         ! Pass the actual argument x to the procedure pointer and
         ! assign the result to array(i).
         array(i) = fp(x)
      end do
   end subroutine initialize

end module kernel

When we write the interface block in the module’s specification statements section, we can declare procedure pointers such as the following statement in internal subprogram section:

procedure(fp), pointer :: fptr

In this case, variable names for function pointers cannot be used if they conflict with names declared in the interface block.

module class_Variable

This module defines the derived type Variable with the component array value(:) and the three type-bound procedures: allocate, initialize, and deallocate.

These procedures perform following processes respectively:

  • Theallocate subroutine allocates the component value as value(nx),
  • The initialize subroutine takes a procedure pointer as its argument, and passes both its own object’s value component and the procedure pointer to the initialize subroutine of the kernel module, and
  • The deallocate subroutine deallocates the component value.
module class_Variable
   use, intrinsic :: iso_fortran_env
   private
   public :: Variable
   
   ! Define the derived type 'Variable'
   type Variable
      ! An component 'value'
      real(real64), allocatable, public :: value(:)
   contains
      ! type-bound procedures
      procedure, public, pass :: allocate
      procedure, public, pass :: initialize
      procedure, public, pass :: deallocate
   end type Variable
 
contains
   
   
   ! Allocate the component of derived type variable 'self'
   subroutine allocate(self)
      use :: kernel, only:Nx
      implicit none
      
      ! Declare the variable named 'self' as the class Variable type.
      class(Variable), intent(inout) :: self
      
      if (.not. allocated(self%value)) then
         allocate(self%value(Nx), source=0d0)
      end if
   end subroutine allocate


   ! A wrapper procedure that invokes initialization by passing
   ! a procedure pointer 'fp' to the 'initialize' procedure of
   ! 'kernel' module.
   subroutine initialize (self, fp)
      use, intrinsic :: iso_fortran_env, only: real64
      
      ! Make alias for the 'initialize' procedure in the 'kernel' moudle
      ! named 'kernel_init'.
      use :: kernel, only: kernel_init => initialize
      
      implicit none
      class(Variable), intent(inout) :: self

      pointer :: fp
      interface
         function fp(x) result(res)
            use, intrinsic :: iso_fortran_env, only: real64
            real(real64), intent(in) :: x
            real(real64) :: res
         end function fp
      end interface

      ! Invoke the 'kernel_init' procedure
      call kernel_init(self%value, fp)
      
   end subroutine initialize


   ! Deallocate the component of variable 'self'
   subroutine deallocate(self)
      implicit none
      class(Variable), intent(inout) :: self

      if (allocated(self%value)) then
          deallocate(self%value)
      end if
   end subroutine deallocate
   
end module class_Variable

In the specification statements section of the initialize, we have to write an interface block about the procedure pointer of the dummy argument. As mentioned in the previous section, we can write an interface blocks in the module’s declaration section and then declare procedure pointers using the procedure(fp) statement.

program main

The following code is the main program of this application.

The module kernel and class_Variable are first imported using the use statements.

In the execution section:

  1. Firstly, a Variable type variable x is created to represent the x-axis coordinates for output.
  2. Next, the procedure pointer fptr is associated with the internal function cliff.
  3. Then, the type-bound procedures u%allocate() and u%initialize(fptr) are invoked, allowing the use of the procedure pointer to assign initial values to the value component of u.
  4. Finally, after calling the subroutine data_output to output the data, the allocations of components for u and x are deallocated using the type-bound procedure deallocate.
program main
   use :: iso_fortran_env, only:real64
   use :: kernel, only: nx, dx
   use :: class_Variable, only: Variable
   implicit none

   ! Declare derived type variable u and x.
   type(Variable) :: u, x
   
   ! Define the interface of the procedure pointer 'fp'
   interface
      function fp(x)
         use, intrinsic :: iso_fortran_env, only: real64
         real(real64), intent(in) :: x
         real(real64) :: fp
      end function fp
   end interface
   
   ! Declare the procedure pointer variable 'fptr',
   ! initializing it refers to null(). 
   procedure(fp), pointer ::  fptr => null()

   integer :: i
   
   ! Generate the x-axis coordinate values for data output.
   call x%allocate()
   do i = 1, nx
      x%value(i) = 0d0 + dx*dble(i-1)
   end do

   call u%allocate()

   ! Associate the procedure pointer 'fptr' with the internal function 'cliff'.
   fptr => cliff
!  fptr => dsin
!  fptr => dcos
   
   ! Invoke the type-bound procedure 'initialize' of the variable 'u'
   ! of the 'Variable' derived type, passing the procedure pointer 'fptr'
   ! as an actual argument.
   call u%initialize(fptr)
  
   call data_output(u)
  
   ! Call the procedure for deallocation.
   call u%deallocate()
   call x%deallocate()

contains

   ! User-defined internal procedure 'cliff'
   pure function cliff(x)
      implicit none
      real(real64) :: cliff
      real(real64), intent(in) :: x
      
      if (x <= 1.57d0) then
         cliff = 1d0
      else
         cliff = 0d0
      end if
      
   end function cliff
   

	! Output data with formatted output.
   subroutine data_output(v)
      use :: kernel, only: nx
      implicit none
      type(Variable), intent(in) :: v
      
      integer, save :: uni = 10
      integer, save :: count = 1
      character(7), save :: filename = 'out.dat'
      logical :: isOpened

      inquire(file=filename, opened=isOpened)

      if (.not. isOpened) then
      	open(uni, file=filename, form='formatted', &
              action='write', status='replace')
      end if
      
		write(uni, '(a, i0)') '> -Z', count  ! header for GMT6

      do i = 1, nx
         write(uni, '(e10.3, 1x, e10.3)') x%value(i), v%value(i)
      end do
      
      count = count + 1
      
   end subroutine data_output
end program main

Sequence diagram

The above application is represented by the following sequence diagram.

A staircase function with a green line

Compilation and Execution

Compilation

Compile the code using the GNU Fortran Compiler gfortran:

% gfortran -c kernel.f90
% gfortran -c class_Variable.f90
% gfortran -c main.f90
% gfortran kernel.o class_Variable.o main.o -o a.out

Alternatively, compile with the Intel Fortran Compiler Classic ifort from the Intel oneAPI HPC Toolkit:

% source /opt/intel/oneapi/setvars.sh
% ifort -c kernel.f90
% ifort -c class_Variable.f90
% ifort -c main.f90
% ifort kernel.o class_Variable.o main.o -o a.out

Execution

Upon running the application, a data file named out.dat will be generated, completing the preparations for plotting the aforementioned figures.

% ./a.out
% head -n 10 out.dat
> -Z1
 0.000E+00  0.100E+01
 0.245E-01  0.100E+01
 0.491E-01  0.100E+01
 0.736E-01  0.100E+01
 0.982E-01  0.100E+01
 0.123E+00  0.100E+01
 0.147E+00  0.100E+01
 0.172E+00  0.100E+01
 0.196E+00  0.100E+01

Conclusion

We have discussed constructing Fortran programs using object-oriented programming techniques and explored the method of passing procedure pointers for initializing arrays. This approach helps clarify the roles of the main program and modules, making it easier to reuse code effectively.

There is very limited information available in both English and Japanese about Fortran procedure pointers. I hope this article serves as a helpful resource for you to enhance your codes.

Appendixes

Notes

Execution of Fortran programs containing pointers compiled with the Intel Fortran Compiler might not be as fast, so it’s recommended to use the -O2 optimization compilation option whenever possible, unless it causes any issues.

The figures of the graphs of functions were created using Generic Mapping Tools version 6.4.0. I will include a Bash script for plotting in the following:

#!/bin/bash

### Use GMT6 on Gentoo Linux
type gmt6 > /dev/null
if [[ $? -eq 0 ]]; then
   shopt -s expand_aliases
   alias gmt='gmt6'
fi
###

proj="X4i"
stem="out"
range="0/6.283/-2/2"

gmt begin $stem png
   gmt basemap -J$proj -R$range -Bya1f0.5g1+l"u"  -Bxa1.57f1.57g1.57+l"x" -BWeSn
   gmt plot out.dat -J$proj -R$range -Cgreen -W1p -Z1 -l"u = cliff(x)"
gmt end

Tested Environments

The Operating Systems of my machines are Gentoo Linux (kernel version 6.1.31), and compilers’ version are following:

% gfortran --version
GNU Fortran (Gentoo 12.3.1_p20230526 p2) 12.3.1 20230526
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

% ifort --version
ifort (IFORT) 2021.9.0 20230302
Copyright (C) 1985-2023 Intel Corporation.  All rights reserved.