How to make a generic procedure pointer in Fortran? - fortran

Question

I am a low level Fortran programmer. I am trying to make subroutines as generic as possible and I wonder if I can send an information of which subroutine another subroutine should acces from a given information in the main program. For instance, if I have:
program main
use test
implicit none
real(8) X
end program main
And:
module test
contains
subroutine A (X)
real(8) X
X = 2*X
end subroutine A
subroutine B (X)
real(8) X
X = 3*X
end subroutine B
end module test
I would like to, given the subroutine name 'A' or 'B' in the main program, transfer this to a third subroutine C inside the module test, which in turn would make some calculations and use the transferred name to choose between A and B in its calculations.
I know if I build the calculations of that subroutine C inside the main program, I can use a procedure pointer to access either A or B subroutines. However, I need to have the subroutine C. So I guess I will have a subroutine C with built in procedure pointer and it will take the name given in the main program as an argument. But I do not know how to do that. Neither if it is possible. If it is not, is there any other way? I do not know, maybe the subroutine C reading a txt file associating the read name to the procedure pointer. But, how?
Thank you, in advance!

Answer 1

I think that what you want is this: you first define subroutines A and B
module ab_m
implicit none
contains
subroutine A(X)
real, intent(inout) :: X
X = 2 * X
end subroutine A
subroutine B(X)
real, intent(inout) :: X
X = 3 * X
end subroutine B
end module ab_m
Then subroutine C uses a dummy procedure, specified by an interface,
module c_m
implicit none
contains
subroutine C(sub,y)
interface
subroutine sub(p)
implicit none
real, intent(inout) :: p
end subroutine sub
end interface
real, intent(inout) :: y
y = y + 1
call sub(y)
end subroutine C
end module c_m
and the main program chooses what procedure to use in C:
program p
use ab_m
use c_m
implicit none
real :: z(2)
z = 1.0
call C(B, z(1))
call C(A, z(2))
write(*,*) z ! writes 6.0, 4.0 [ (1+1)*3, (1+1)*2) ]
end program p