Using select type on allocatable character

Help
1

Hello,
i am trying to reduce some code by writing a generic routine for setting values to variables of different types, using a class(*) argument and performing select type on it. The following code works fine for fixed-length strings, but not for character(:), allocatable, which does not seem to get written at all in the routine.

program main

  implicit none
  character(:), allocatable :: alloc_str
  character(len=20) :: fixed_str

  call set_me( fixed_str )
  write(*,*) fixed_str

  call set_me( alloc_str )
  write(*,*) allocated( alloc_str ), alloc_str

contains

  subroutine set_me( val )
    implicit none
    class(*), intent(out) :: val

    select type( val )
    type is( character(*) )
       val = "a string"
    class default
       write(*,*) "unknown type of val"
    end select

  end subroutine set_me


end program main

output:

 a string            
 F

Is there some workaround for this, or a better way of doing it?
The goal would be to be able to accept and write both, the fixed-length and allocatable strings.

I thought of catching an error with checking if len_trim(val) is zero, which would indicate a non-allocated allocatable, but i can’t seem to find any info about the result of len_trim for a non-allocated string. Testing on gfortran gives zero, but is this true across all the different compilers? (analogous to size() of unallocated arrays being sometimes random)

2

You can pass an unallocated actual string argument to a dummy string argument that is not declared as allocatable, but then you can’t expect it to be allocated at all in the routine.

3

The code i posted is just the minimal example, sorry. In the full code there are more cases in the select type, not only for character.

I am trying to actually avoid having separate routines for each type of variable, if this is what you are suggesting?

4

You can have a single routine, but then the allocatable variables must be allocated befoire calling the routine.

5

Ok, so if i understand correctly, for cases where i want to write a value to a variable, it is actually better to write separate routines because then i can manage the allocation in the routine; while for cases where i read a value from a variable i can use a generic routine with class(*) since the variable is surely already allocated.

6

It is invalid to pass an unallocated actual argument to a non-allocatable dummy argument.

I would caution against where you’re headed with this design. The call site knows the type. You’re making things more difficult by trying to make a single procedure re-determine the type.

7

OK… I thought that specifically for the character type it was somehow equivalent to passing a zero length string.

8

Nope, unallocated and zero length/size are very different things. I.e. my bucket is empty vs I have no bucket. That goes for deferred length character variables as well as arrays.