问题

The first output "allocated?= T" is as expected, indicating that the allocatable array a in the main program is allocated, which means it has been dynamically allocated memory.

The second output "allocated?= F" in the subroutine p indicates that the allocatable array c passed as an actual argument to the subroutine is not allocated. This can happen for a couple of reasons:

1. The subroutine p specifies the intent(out) attribute for the dummy argument c. When a dummy argument is declared with intent(out), it means that the subroutine is expected to allocate the memory for the argument. In this case, the subroutine p does not allocate memory for c, which is why it reports that it's not allocated.

2. In the main program, when you call p(a), you pass the allocatable array a to the subroutine. If the subroutine p modifies c in a way that deallocates it or assigns it to a different memory location, it can result in the allocated status of c becoming false. Without seeing the actual code inside the subroutine, it's challenging to pinpoint the exact reason for the deallocation.

If you want c to be allocated within the subroutine p, you should modify the subroutine declaration to remove intent(out) like this:

subroutine p(c)
  implicit none
  real, allocatable :: c(:,:)  ! Remove 'intent(out)'
  write(*,*) 'allocated?=', allocated(c)
end subroutine p

This change will make the subroutine allocate memory for c, and the allocated status should be true when the subroutine is executed.

module m
  real, allocatable :: a(:,:)
end module m

module m2
contains
  subroutine p(c)
    implicit none
    real, allocatable, intent(out):: c(:,:)
    write(*,*) 'allocated?=', allocated(c)
  end subroutine p
end module m2

program main
  use m, only : a
  use m2, only: p
  implicit none

  allocate(a(3,3))
  write(*,*) 'allocated?=', allocated(a)
  call p(a)

end program main

allocated?= T 
allocated?= F 

答案1

得分: 2

Allocatable dummy arguments that have intent(out) are automatically deallocated when the procedure is entered according to thevstandard. The intent also affects the allocation status, not just the values inside.

It actually makes a very good sense. If it is intent(out), any previous allocation status should not matter.