Fortran/OOP in Fortran

Object-oriented programming

Module

Overview

Data can be gathered in modules. The general form is given by

module <name>
  <use statements>
  <declarations>
contains
  <subroutines and functions>
end module

Data access

There are three possible access properties: public, private, protected.

public: Outside code has read and write access.
private: Outside code has no access.
public, protected: Outside code has read access.

Using module in other code

One can include the module's public data in outside code. There are three ways.

use <moduleName>: includes all public data and methods
use <moduleName>, <renames>: includes all public data and methods, but renames some public data or methods
use <moduleName>, only : <subset>: includes only some public data and methods

Example

General overview

module test_m
  implicit none

  private                                   ! all data is by default private
  public print_coords, set_coords           ! these procedures are set public -> accessible outside

  real :: x, y                               ! not accessible outside

contains
  subroutine print_coords
    print *, "x, y", x, y
  end subroutine

  subroutine set_coords(new_x, new_y)
    real, intent(in) :: new_x, new_y

    x = new_x
    y = new_y
  end subroutine
end module

program main
  use test_m                                ! import the "test_m" module

  implicit none

  call set_coords(1.0, 1.0)                 ! call the public procedure from test_mod
  call print_coords
end program

Data access

module data_access_m
  implicit none

  private

  public    a, b
  protected b
  private   c

  integer :: a=1
  integer :: b=1
  integer :: c=1
end module

program main
  use data_access_m

  ! accessing public object works
  print *, a

  ! editing public object works
  a = 2

  ! accessing protected object works
  print *, b

  ! editing protected object does not work
  !b = 2 <- ERROR

  ! accessing private object does not work
  !print *, c <- ERROR

  ! editing private object does not work
  !c = 2 <- ERROR
end program

Using modules

module test_module
  implicit none
  private
  integer, public :: a=1
  integer, public, protected :: b=1
  integer, private :: c=1
end module test_module

!> import all public data of test_module
program main
  use test_module

  print *, a, b
end program main

!> import all data, and rename
program main
  use test_module, better_name => a

  ! new name use available
  print *, better_name

  ! old name is not available anymore
  !print *, a  <- ERROR
end program main

!> import only a subset of the public data
program main
  use test_module, only : a

  ! only a is loaded
  print *, a

  ! b is not loaded
  !print *, b  <- ERROR
end program main

Submodule

Modules can be extended using submodules. Multiple advantages arise

splitting of large modules
splitting of interface definitions and implementations such that dependent modules do not need to be recompiled if the implementations change
two modules need data from each other.

Example

Splitting of definitions and implementations

!> simple module about circles
module circle_mod
  implicit none
  private
  public :: area, radius

  real            :: radius
  real, parameter :: PI = 3.1415

  interface                              ! interface block needed. each function implemented via submodule needs an entry here.
    module function area()               ! important note the "module" keyword
      real :: area
    end function 
  end interface
end module 

submodule (circle_mod) circle_subm       ! submodule (parent_mod) child_mod
contains
  module function area()                 ! again "module" keyword
    area = PI*radius**2
  end function 
end submodule

program main
  use circle_mod
  implicit none

  radius = 1.0
  print *, "area:", area()
end program

Derived data types

In Fortran one can derive structures off of other structures, so called derived data types. The derived types will have the features of the parent type as well as the newly added ones and the general syntax is given by:

type, extends(<parentTypeName>) :: <newTypeName>
  <definitions>
end type

The following example shows different types of people within a company.

module company_data_mod

  implicit none

  private
  public phone_type, address_type, person_type, employee_type, salaried_worker_type, hourly_worker_type

  type phone_type
    integer :: area_code, number
  end type

  type address_type
    integer                       :: number
    character(len=:), allocatable :: street, city
    character(len=2)              :: state
    integer                       :: zip_code
  end type

  type person_type
    character(len=:), allocatable :: name
    type(address_type)            :: address
    type(phone_type)              :: phone
    character(len=:), allocatable :: remarks
  end type

  type, extends(person_type) :: employee_type
    integer :: phone_extension, mail_stop, id_number
  end type

  type, extends(employee_type) :: salaried_worker_type
    real :: weekly_salary
  end type

  type, extends(employee_type) :: hourly_worker_type
    real :: hourly_wage, overtime_factor, hours_worked
  end type

end module

program main

  use company_data_mod

  implicit none

  type(hourly_worker_type) :: obj

end program

Destructors

One can define procedures which will be invoked before the object is automatically deleted (out of scope). This is done with the statement final. The following example illustrates it

module person_m
  implicit none

  type person
    integer, allocatable :: numbers(:)
  contains
    final :: del
  end type

contains

  subroutine del(this)
    !! example for a derived type's destructor. allocatables are deallocated automatically anyways. this is just to show the usage of "final".

    type(person), intent(inout) :: this

    if (allocated(this%numbers)) deallocate (this%numbers)
  end subroutine 

end module

Abstract base type and deferred procedure

One can set the base type as abstract such that one cannot initialize objects of that type but one can derive sub-types of it (via extends). Specific procedures which should be defined in the sub-type need the property deferred as well as an explicit interface.

The following example illustrates their use.

module shape_m
  implicit none

  type, abstract :: shape
    real :: a, b
  contains
    procedure                       :: print => shape_print
    procedure(area_shape), deferred :: area
  end type

  interface
    real function area_shape(this)
      import :: shape
      class(shape), intent(in) :: this
    end function
  end interface
contains
  subroutine shape_print(this)
    class(shape), intent(in) :: this

    print *, 'a,b', this%a, this%b
  end subroutine
end module

module line_m
  use shape_m
  implicit none
  private
  public line

  type, extends(shape) :: line
  contains
    procedure :: area
  end type
contains
  real function area(this)
    class(line), intent(in) :: this

    area = abs(this%a - this%b)
  end function
end module

module rectangle_m
  use shape_m
  implicit none
  private
  public rectangle

  type, extends(shape) :: rectangle
  contains
    procedure :: area
  end type
contains
  real function area(this)
    class(rectangle), intent(in) :: this

    area = this%a * this%b
  end function
end module

program main
  use line_m
  use rectangle_m

  implicit none

  type(line)      :: l
  type(rectangle) :: r

  ! line
  l%a = 2.0
  l%b = 4.0
  print *, "line ...  "
  call l%print
  print *, "-> from:  ", l%a
  print *, "-> to:    ", l%b
  print *, "-> length:", l%area()

  ! rectangle
  r%a = 3.0
  r%b = 5.0
  print *
  print *, "rectangle ..."
  call r%print
  print *, "-> side a:", r%a
  print *, "-> side b:", r%b
  print *, "-> area:  ", r%area()
end program

Polymorphic Pointer

One can create pointers to child classes by using type definitions in allocate statements and the select type environment. The following example highlights its use.

module shape_m
  implicit none

  type, abstract :: shape
    ! just an empty class used to implement a parent class.
    ! reason for abstract: there shouldnt be objects of TYPE(!) shape, just polymorphic CLASS instances.
  end type
end module

module line_m
  use shape_m
  implicit none

  type, extends(shape) :: line
    ! a child class w/ one attribute
    ! reason for extends(shape): polymorphic shape pointer can point to objects of this type
    real :: length
  end type
end module

module rectangle_m
  use shape_m
  implicit none

  type, extends(shape) :: rectangle
    ! a child class w/ another attribute
    ! reason for extends(shape): (see explanation in line type)
    real :: area
  end type
end module

program main
  use rectangle_m
  use line_m

  implicit none

  class(shape), allocatable :: sh     ! pointer to parent class

  ! allocate (line :: sh)
  allocate (rectangle :: sh)             ! allocate using child types

  select type (x => sh)               ! associate block. "x" will be a pointer to the child object and of its type(!!)
    type is (line)                    ! select the right child type (the one we used in the allocate statement)
      x%length = 1.0
      print *, 'line length', x%length
    type is (rectangle)
      x%area = 2.0
      print *, 'rectangle area', x%area
    ! class is ()                     ! select by using classes
    class default                     ! if nothing of the above applied
      error stop 'class/type not specified!'
  end select
end program

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