Allocatable Arrays

The allocatable attribute provides a safe way for memory handling. In comparison to variables with pointer attribute the memory is managed automatically and will be deallocated automatically once the variable goes out-of-scope. Using allocatable variables removes the possibility to create memory leaks in an application.

They can be used in subroutines to create scratch or work arrays, where automatic arrays would become too large to fit on the stack.

real(dp), allocatable :: temp(:)
allocate(temp(10))

The allocation status can be checked using the allocated intrinsic to avoid uninitialized access

subroutine show_arr(arr)
  integer, allocatable, intent(in) :: arr(:)

  if (allocated(arr)) then
    print *, arr
  end if
end subroutine show_arr

To allocate variables inside a procedure the dummy argument has to carry the allocatable attribute. Using it in combination with intent(out) will deallocate previous allocations before entering the procedure:

subroutine foo(lam)
  real(dp), allocatable, intent(out) :: lam(:)
  allocate(lam(5))
end subroutine foo

The allocated array can be used afterwards like a normal array

real(dp), allocatable :: lam(:)
call foo(lam)

An already allocated array cannot be allocated again without prior deallocation. Similarly, deallocation can only be invoked for allocated arrays. To reallocate an array use

if (allocated(lam)) deallocate(lam)
allocate(lam(10))

Passing allocated arrays to procedures does not require the allocatable attribute for the dummy arguments anymore.

subroutine show_arr(arr)
  integer, intent(in) :: arr(:)

  print *, arr
end subroutine show_arr

subroutine proc
  integer :: i
  integer, allocatable :: arr

  allocate(arr(5))

  do i = 1, size(arr)
    arr(i) = 2*i + 1
  end do
  call show_arr(arr)
end subroutine proc

Passing an unallocated array in this context will lead to an invalid memory access. Allocatable arrays can be passed to optional dummy arguments -- if they are unallocated the argument will not be present. The allocatable attribute is not limited to arrays and can also be associated with scalars, which can be useful in combination with optional dummy arguments.

Allocations can be moved between different arrays with allocatable attribute using the move_alloc intrinsic subroutine.

subroutine resize(var, n)
  real(wp), allocatable, intent(inout) :: var(:)
  integer, intent(in), optional :: n
  integer :: this_size, new_size
  integer, parameter :: inital_size = 16

  if (allocated(var)) then
    this_size = size(var, 1)
    call move_alloc(var, tmp)
  else
    this_size = initial_size
  end if

  if (present(n)) then
    new_size = n
  else
    new_size = this_size + this_size/2 + 1
  end if

  allocate(var(new_size))

  if (allocated(tmp)) then
    this_size = min(size(tmp, 1), size(var, 1))
    var(:this_size) = tmp(:this_size)
  end if
end subroutine resize

Finally, allocations do not initialize the array. The content of the uninitialized array is most likely just the bytes of whatever was previously at the respective address. The allocation supports initialization using the source attribute:

real(dp), allocatable :: arr(:)
allocate(arr(10), source=0.0_dp)

The source keyword supports scalar and array valued variables and constants.