Pointers to Arrays vs Allocatable Arrays

We can create space dynamically using allocatable arrays, or using pointer allocation. In general, allocatable arrays are more efficient, while pointer allocation is more flexible.

There are two main restrictions imposed upon allocatable arrays which do not apply to pointer arrays:

• ALLOCATABLE arrays cannot be used as components of derived types (to be discussed later);
• unallocated ALLOCATABLE arrays cannot be pased as actual arguments to procedures.

For example,

program ptralloc
implicit none
real, dimension(:), allocatable :: a
real, dimension(:), pointer :: b
  allocate(a(4)) 
  call t1(a)
  print*, a
  call t2(b)
  print*, b
contains
  subroutine t1(a)
    real, dimension(:) :: a
    a = reshape((/1.,2.,3.,4./),(/4/))
  end subroutine t1
  subroutine t2(b)
    real, dimension(:), pointer :: b
    allocate(b(4));
    b = reshape((/5.,6.,7.,8./),(/4/))
  end subroutine t2
end program ptralloc