I have discovered some strange behavior when using the associate construct compiled with gfortran.
The code implements an integer list supporting insertion at end, and retrieval from any position within the list. It is just an experimental substitute for the string list type being designed by @Arjen at String list new by arjenmarkus · Pull Request #311 · fortran-lang/stdlib · GitHub. Press expand to see the full code:
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module integer_list_mod
implicit none
private
public :: integer_list
public :: list_index, as_int
type :: list_index
private
integer :: idx
end type
! This container has a fixed maximum-size of 100 elements
type :: integer_list
integer :: n = 0
!! Actual size
integer :: values(100)
!! The underlying integer storage
integer :: index_ref = 0
!! Integer pointer to index array.
contains
procedure :: insert_int
procedure :: insert_index
generic :: insert => insert_int, insert_index
procedure :: get_int
procedure :: get_index
generic :: get => get_int, get_index
procedure :: end => list_end
end type
type(list_index), allocatable, target :: index_array(:)
integer :: next_free = 0
contains
subroutine insert_int(list,num)
class(integer_list), intent(inout) :: list
integer, intent(in) :: num
if (list%index_ref > 0) then
associate(pos => list%n + 1)
if (pos > 100) then
print *, "List full."
return
end if
list%n = pos
list%values(pos) = num
index_array(list%index_ref)%idx = list%n
end associate
else
list%index_ref = free_index_array_position()
list%n = 1
list%values(1) = num
index_array(list%index_ref)%idx = list%n
end if
end subroutine
integer function free_index_array_position() result(ref)
if (allocated(index_array)) then
if (next_free > size(index_array)) then
resize: block
type(list_index), allocatable :: new_index_array(:)
allocate(new_index_array(2*size(index_array)))
new_index_array(1:size(index_array)) = index_array
call move_alloc(from=new_index_array,to=index_array)
end block resize
end if
ref = next_free
next_free = next_free + 1
else
allocate(index_array(10))
next_free = 1
ref = next_free
end if
end function
subroutine insert_index(list,idx)
class(integer_list), intent(inout) :: list
type(list_index), intent(in) :: idx
call insert_int(list,idx%idx)
end subroutine
integer function get_int(list,i)
class(integer_list), intent(in) :: list
integer, intent(in) :: i
get_int = list%values(i)
end function
integer function get_index(list,idx)
class(integer_list), intent(in) :: list
type(list_index), intent(in) :: idx
get_index = get_int(list,idx%idx)
end function
function list_end(list) result(index_ptr)
class(integer_list), intent(inout) :: list
type(list_index), pointer :: index_ptr
if (list%index_ref == 0) then
list%index_ref = free_index_array_position()
end if
index_ptr => index_array(list%index_ref)
end function
pure integer function as_int(self)
type(list_index), intent(in) :: self
as_int = self%idx
end function
end module
The main thing I was interested in was using the associate construct to retrieve a pointer to an index object, which could be used to access the last value in the list:
program test_integer_list
use integer_list_mod
implicit none
type(integer_list) :: ilist
integer :: i
associate(end => ilist%end())
call ilist%insert(1)
call ilist%insert(2)
call ilist%insert(3)
print '(A,I0)', "Last element: ", ilist%get(end) ! prints "3"
call ilist%insert(4)
print '(A,I0)', "Last element (index): ", ilist%get(end)
print '(A,I0)', "Last element (int): ", ilist%get(as_int(end))
print '(A,I0)', "End index: ", as_int(end)
do i = 1, as_int(end)
call ilist%insert(i+4)
end do
print '(A,*(I0,2X))', "Full list: ", ilist%values(1:as_int(end))
end associate
! More dangerous way of accesing end index
block
type(list_index), pointer :: end => null()
end => ilist%end()
do i = 1, as_int(end)
print *, ilist%get(i)
end do
end block
end program
With ifort I get the following output in agreement with my expectations:
~/fortran$ ifort -warn all integer_list_mod.f90 test_integer_list.f90
~/fortran$ ./a.out
Last element: 3
Last element (index): 4
Last element (int): 4
End index: 4
Full list: 1 2 3 4 5 6 7 8
1
2
3
4
5
6
7
8
However, with gfortran (v 10.1.0) I get
~/fortran$ gfortran -Wall integer_list_mod.f90 test_integer_list.f90
~/fortran$ ./a.out
Last element: 3
Last element (index): 4
Last element (int): 4
End index: 0
Full list:
1
2
3
4
Interestingly, the as_int(end) appears to be evaluated correctly when used in the call to get. When used locally within the scope of the main program scope the value is frozen to the initial value of zero and the do insertion loop is never executed.
Which compiler is correct?