intent(out) and allocatable Fortran arrays: what is really done?

Question

I read on many posts on Stack Overflow that an allocatable array is deallocated when it is passed in a subroutine where the dummy argument is intent(out).

If I consider the following code :

program main

 real, dimension(:), allocatable :: myArray
 integer :: L=8

 allocate(myArray(1:L))
 call initArray(myArray)
 print *, myArray
 
contains

 subroutine initArray(myArray)
 real, dimension(:), intent(out) :: myArray

 myArray(:) = 10.0
 
 end subroutine initArray
 
end program main

the output is right. So, when deallocation occurs, memory is released but the array shape is kept. Is it exact ? Any detailed explanation would be appreciated.

I read different posts on the subject (Can I use allocatable array as an intent(out) matrix in Fortran?, What is the effect of passing an allocatable variable into a subroutine with non-allocatable argument?, ...). So I understand that the array is deallocated but I would like to understand what does it mean because in my code, the size is kept and I am also surprised that this code works.

Answer 1

The deallocation happens when the dummy argument is allocatable. Try

real, dimension(:), intent(out), allocatable :: myArray

to achieve that.

The fact that the actual argument in the main program is allocatable is immaterial.

I strongly suggest to name the array in the main program and the argument in the subroutine differently to better see the difference.

program main

 real, dimension(:), allocatable :: mainArray
 integer :: L=8

 allocate(mainArray(1:L))
 call initArray(mainArray)
 print *, mainArray
 
contains

 subroutine initArray(argArray)
 real, dimension(:), intent(out) :: argArray

 argArray(:) = 10.0
 
 end subroutine initArray
 
end program main

Now, mainArray is the actual argument, argArray is the dummy argument. The dummy argument must be allocatable for deallocation to happen.