Type Casting in Callbacks#
There are essentially five different ways to do type casting, each with its own advantages and disadvantages.
The methods I, II and V can be used both in C and Fortran. The methods III and IV are only available in Fortran. The method VI is obsolete and should not be used.
Work Arrays#
Pass a «work array» which is packed with everything needed by the caller and unpacked by the called routine. This is the old way – e.g., how LAPACK does it.
Integrator:
module integrals
use types, only: dp
implicit none
private
public simpson
contains
real(dp) function simpson(f, a, b, data) result(s)
real(dp), intent(in) :: a, b
interface
real(dp) function func(x, data)
use types, only: dp
implicit none
real(dp), intent(in) :: x
real(dp), intent(inout) :: data(:)
end function
end interface
procedure(func) :: f
real(dp), intent(inout) :: data(:)
s = (b-a) / 6 * (f(a, data) + 4*f((a+b)/2, data) + f(b, data))
end function
end module
Usage:
module test
use types, only: dp
use integrals, only: simpson
implicit none
private
public foo
contains
real(dp) function f(x, data) result(y)
real(dp), intent(in) :: x
real(dp), intent(inout) :: data(:)
real(dp) :: a, k
a = data(1)
k = data(2)
y = a*sin(k*x)
end function
subroutine foo(a, k)
real(dp) :: a, k
real(dp) :: data(2)
data(1) = a
data(2) = k
print *, simpson(f, 0._dp, pi, data)
print *, simpson(f, 0._dp, 2*pi, data)
end subroutine
end module
General Structure#
Define a general structure which encompass the variations you actually need (or are even remotely likely to need going forward). This single structure type can then change if needed as future needs/ideas permit but won’t likely need to change from passing, say, real numbers to, say, and instantiation of a text editor.
Integrator:
module integrals
use types, only: dp
implicit none
private
public simpson, context
type context
! This would be adjusted according to the problem to be solved.
! For example:
real(dp) :: a, b, c, d
integer :: i, j, k, l
real(dp), pointer :: x(:), y(:)
integer, pointer :: z(:)
end type
contains
real(dp) function simpson(f, a, b, data) result(s)
real(dp), intent(in) :: a, b
interface
real(dp) function func(x, data)
use types, only: dp
implicit none
real(dp), intent(in) :: x
type(context), intent(inout) :: data
end function
end interface
procedure(func) :: f
type(context), intent(inout) :: data
s = (b-a) / 6 * (f(a, data) + 4*f((a+b)/2, data) + f(b, data))
end function
end module
Usage:
module test
use types, only: dp
use integrals, only: simpson, context
implicit none
private
public foo
contains
real(dp) function f(x, data) result(y)
real(dp), intent(in) :: x
type(context), intent(inout) :: data
real(dp) :: a, k
a = data%a
k = data%b
y = a*sin(k*x)
end function
subroutine foo(a, k)
real(dp) :: a, k
type(context) :: data
data%a = a
data%b = k
print *, simpson(f, 0._dp, pi, data)
print *, simpson(f, 0._dp, 2*pi, data)
end subroutine
end module
There is only so much flexibility really needed. For example, you could define two structure types for this purpose, one for Schroedinger and one for Dirac. Each would then be sufficiently general and contain all the needed pieces with all the right labels.
Point is: it needn’t be «one abstract type to encompass all» or bust. There are natural and viable options between «all» and «none».
Private Module Variables#
Hide the variable arguments completely by passing in module variables.
Integrator:
module integrals
use types, only: dp
implicit none
private
public simpson
contains
real(dp) function simpson(f, a, b) result(s)
real(dp), intent(in) :: a, b
interface
real(dp) function func(x)
use types, only: dp
implicit none
real(dp), intent(in) :: x
end function
end interface
procedure(func) :: f
s = (b-a) / 6 * (f(a) + 4*f((a+b)/2) + f(b))
end function
end module
Usage:
module test
use types, only: dp
use integrals, only: simpson
implicit none
private
public foo
real(dp) :: global_a, global_k
contains
real(dp) function f(x) result(y)
real(dp), intent(in) :: x
y = global_a*sin(global_k*x)
end function
subroutine foo(a, k)
real(dp) :: a, k
global_a = a
global_k = k
print *, simpson(f, 0._dp, pi)
print *, simpson(f, 0._dp, 2*pi)
end subroutine
end module
However it is best to avoid such global variables – even though really just semi-global – if possible. But sometimes it may be the simplest cleanest way. However, with a bit of thought, usually there is a better, safer, more explicit way along the lines of II or IV.
Nested functions#
Integrator:
module integrals
use types, only: dp
implicit none
private
public simpson
contains
real(dp) function simpson(f, a, b) result(s)
real(dp), intent(in) :: a, b
interface
real(dp) function func(x)
use types, only: dp
implicit none
real(dp), intent(in) :: x
end function
end interface
procedure(func) :: f
s = (b-a) / 6 * (f(a) + 4*f((a+b)/2) + f(b))
end function
end module
Usage:
subroutine foo(a, k)
use integrals, only: simpson
real(dp) :: a, k
print *, simpson(f, 0._dp, pi)
print *, simpson(f, 0._dp, 2*pi)
contains
real(dp) function f(x) result(y)
real(dp), intent(in) :: x
y = a*sin(k*x)
end function f
end subroutine foo
Using type(c_ptr) Pointer#
In C, one would use the void * pointer. In Fortran, one can use
type(c_ptr) for exactly the same purpose.
Integrator:
module integrals
use types, only: dp
use iso_c_binding, only: c_ptr
implicit none
private
public simpson
contains
real(dp) function simpson(f, a, b, data) result(s)
real(dp), intent(in) :: a, b
interface
real(dp) function func(x, data)
use types, only: dp
implicit none
real(dp), intent(in) :: x
type(c_ptr), intent(in) :: data
end function
end interface
procedure(func) :: f
type(c_ptr), intent(in) :: data
s = (b-a) / 6 * (f(a, data) + 4*f((a+b)/2, data) + f(b, data))
end function
end module
Usage:
module test
use types, only: dp
use integrals, only: simpson
use iso_c_binding, only: c_ptr, c_loc, c_f_pointer
implicit none
private
public foo
type f_data
! Only contains data that we need for our particular callback.
real(dp) :: a, k
end type
contains
real(dp) function f(x, data) result(y)
real(dp), intent(in) :: x
type(c_ptr), intent(in) :: data
type(f_data), pointer :: d
call c_f_pointer(data, d)
y = d%a * sin(d%k * x)
end function
subroutine foo(a, k)
real(dp) :: a, k
type(f_data), target :: data
data%a = a
data%k = k
print *, simpson(f, 0._dp, pi, c_loc(data))
print *, simpson(f, 0._dp, 2*pi, c_loc(data))
end subroutine
end module
As always, with the advantages of such re-casting, as Fortran lets you do if you really want to, come also the disadvantages that fewer compile- and run-time checks are possible to catch errors; and with that, inevitably more leaky, bug-prone code. So one always has to balance the costs and benefits.
Usually, in the context of scientific programming, where the main thrust is to represent and solve precise mathematical formulations (as opposed to create a GUI with some untold number of buttons, drop-downs, and other interface elements), simplest, least bug-prone, and fastest is to use one of the previous approaches.
transfer() Intrinsic Function#
Before Fortran 2003, the only way to do type casting was using the
transfer intrinsic function. It is functionally equivalent to the
method V, but more verbose and more error prone. It is now obsolete and
one should use the method V instead.
Examples:
http://jblevins.org/log/transfer
http://jblevins.org/research/generic-list.pdf
http://www.macresearch.org/advanced_fortran_90_callbacks_with_the_transfer_function
Object Oriented Approach#
The module:
module integrals
use types, only: dp
implicit none
private
public :: integrand, simpson
! User extends this type
type, abstract :: integrand
contains
procedure(func), deferred :: eval
end type
abstract interface
function func(this, x) result(fx)
import :: integrand, dp
class(integrand) :: this
real(dp), intent(in) :: x
real(dp) :: fx
end function
end interface
contains
real(dp) function simpson(f, a, b) result(s)
class(integrand) :: f
real(dp), intent(in) :: a, b
s = ((b-a)/6) * (f%eval(a) + 4*f%eval((a+b)/2) + f%eval(b))
end function
end module
The abstract type prescribes exactly what the integration routine needs, namely a method to evaluate the function, but imposes nothing else on the user. The user extends this type, providing a concrete implementation of the eval type bound procedure and adding necessary context data as components of the extended type.
Usage:
module example_usage
use types, only: dp
use integrals, only: integrand, simpson
implicit none
private
public :: foo
type, extends(integrand) :: my_integrand
real(dp) :: a, k
contains
procedure :: eval => f
end type
contains
function f(this, x) result(fx)
class(my_integrand) :: this
real(dp), intent(in) :: x
real(dp) :: fx
fx = this%a*sin(this%k*x)
end function
subroutine foo(a, k)
real(dp) :: a, k
type(my_integrand) :: my_f
my_f%a = a
my_f%k = k
print *, simpson(my_f, 0.0_dp, 1.0_dp)
print *, simpson(my_f, 0.0_dp, 2.0_dp)
end subroutine
end module
Complete Example of void * vs type(c_ptr) and transfer()#
Here are three equivalent codes: one in C using void * and two codes
in Fortran using type(c_ptr) and transfer():
Language |
Method |
Link |
|---|---|---|
C |
|
|
Fortran |
|
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Fortran |
|
The C code uses the standard C approach for writing extensible libraries
that accept callbacks and contexts. The two Fortran codes show how to do
the same. The type(c_ptr) method is equivalent to the C version and
that is the approach that should be used.
The transfer() method is here for completeness only (before Fortran
2003, it was the only way) and it is a little cumbersome, because the
user needs to create auxiliary conversion functions for each of his
types. As such, the type(c_ptr) method should be used instead.