Pointers in Fortran

Point with =>

A Fortran pointer is an alias: it refers to another variable instead of owning storage of its own.

Only a variable declared with the target attribute can be pointed at, and the link is made with the => operator.

The program below points p at val, so a change through p is visible in val.

program demo implicit none real, target :: val real, pointer :: p val = 10.0 p => val p = p + 5.0 print *, val end program demo

Associated test

The associated intrinsic reports whether a pointer currently points at something, which prevents accidental use of a dangling alias.

Check association before reading through a pointer that may still be empty, especially after optional setup steps.

This fragment prints one message when p is linked and another when it is not.

program demo implicit none real, target :: val real, pointer :: p nullify(p) if (associated(p)) print *, "linked" p => val if (associated(p)) print *, "linked" end program demo

Nullify

Nullify breaks the link between a pointer and its target without touching the target value itself.

A freshly declared pointer starts in an undefined state, so nullify it or point it at something before the first associated test.

The example links p, uses the target, and then safely disconnects the pointer.

program demo implicit none real, target :: val real, pointer :: p val = 3.0 p => val print *, p nullify(p) print *, associated(p) end program demo

Prefer allocatables

Allocatables are the safer default: they own their memory, clean up on their own, and cannot dangle the way pointers can.

Reach for a pointer only when aliasing is truly required, such as walking a linked structure or retargeting one name to several targets in turn.

The same storage below is managed with an allocatable and needs no nullify or association test at all.

program demo implicit none real, allocatable :: v(:) allocate(v(3)) v = [1.0, 2.0, 3.0] print *, v deallocate(v) end program demo

Next: DO CONCURRENT

Article author: Arthur Isaev