I'm trying to write a simple code, which takes some objects with the same parental abstract class, stores them into a binary file and reads them back.
My code looks like this:
module m
implicit none
type :: container
class(a), allocatable :: item
end type container
type, abstract :: a
character(20), public :: obj_type
integer, public :: num
contains
procedure :: write_impl => write_a
procedure :: read_impl => read_a
generic :: write(unformatted) => write_impl
generic :: read(unformatted) => read_impl
end type a
type, extends(a) :: b
integer, public :: num2
contains
procedure :: write_impl => write_b
procedure :: read_impl => read_b
end type b
type, extends(a) :: c
end type c
contains
subroutine write_a(this, unit, iostat, iomsg)
class(a), intent(in) :: this
integer, intent(in) :: unit
integer, intent(out) :: iostat
character(*), intent(inout) :: iomsg
write(unit, iostat=iostat, iomsg=iomsg) this%num
end subroutine write_a
subroutine read_a(this, unit, iostat, iomsg)
class(a), intent(inout) :: this
integer, intent(in) :: unit
integer, intent(out) :: iostat
character(*), intent(inout) :: iomsg
read(unit, iostat=iostat, iomsg=iomsg) this%num
end subroutine read_a
subroutine write_b(this, unit, iostat, iomsg)
class(b), intent(in) :: this
integer, intent(in) :: unit
integer, intent(out) :: iostat
character(*), intent(inout) :: iomsg
write(unit, iostat=iostat, iomsg=iomsg) this%num, this%num2
end subroutine write_b
subroutine read_b(this, unit, iostat, iomsg)
class(b), intent(inout) :: this
integer, intent(in) :: unit
integer, intent(out) :: iostat
character(*), intent(inout) :: iomsg
read(unit, iostat=iostat, iomsg=iomsg) this%num, this%num2
end subroutine read_b
end module m
program mwe
use m
implicit none
class(a), allocatable :: o1, o2, o3, o4
class(container), allocatable :: arr(:)
integer :: i, arr_size, tmp
character(20) :: str_tmp
o1 = b('b', 1, 2)
o2 = c('c', 3)
allocate(arr(2))
arr(1)%item = o1
arr(2)%item = o2
select type(t => o1)
type is(b)
write(*,*) t%num, t%num2
end select
select type(t => arr(1)%item)
type is(b)
write(*,*) t%num, t%num2
end select
write(*,*) 'Write into binary'
! WRITE size
open(123, file='test5.dat', form='unformatted')
write(123) SIZE(arr)
do i=1,2
write(123) arr(i)%item%obj_type
if(arr(i)%item%obj_type .eq. 'b') then
select type(t => arr(i)%item)
type is(b)
write(123) t
end select
else if(arr(i)%item%obj_type .eq. 'c') then
select type(t => arr(i)%item)
type is(c)
write(123) t
end select
end if
end do
close(123)
write(*,*) 'Read from binary'
open(123, file='test5.dat', form='unformatted')
read(123) arr_size
write(*,*) 'array size: ', arr_size
do i=1,2
read(123) str_tmp
write(*,*) str_tmp
if(allocated(o3)) deallocate(o3)
if(str_tmp .eq. 'b') then
allocate(b :: o3)
select type(t => o3)
type is(b)
read(123) t
write(*,*) t%num, t%num2 ! BAD OUTPUT
end select
else if(str_tmp .eq. 'c') then
allocate(c :: o3)
select type(t => o3)
type is(c)
read(123) t
write(*,*) t%num
end select
end if
end do
end program mwe
The problem is, with reading of o1 - it's of the type b, i.e. this object has two components - num and num2. I store it and I would naturally expect the read values to be the same, as the written ones.
But I'm getting the same strange behavior as described in Variables being deleted in Fortran Arrays? . In that question it was caused by the bad assignment syntax while initializing an array, but in this case I'm completely clueless why my output looks like this:
1 2
1 2
Write into binary
Read from binary
array size: 2
b
1 0
c
3
The values under b should be obviously 1 and 2, not 1 and 0. What am I doing wrong?
The problem here is that in
select type(t => arr(i)%item)
type is(b)
write(123) t
end select
ifort is not actually selecting the procedure write_b to process the defined output. It is also not selecting the procedure read_b when it comes to processing the defined input.
Instead the procedures write_a and read_a are selected.
This is a problem with the compiler and should be reported to Intel. A rather tedious workaround is to select type in those procedures.
来源：https://stackoverflow.com/questions/50774846/reading-bad-values-from-binary-file-in-fortran-with-a-defined-input-procedure