Difference between nullify(pointer) and pointer => null()

Question

What is the difference between

procedure(some_routine), pointer :: ptr
ptr => null()

and

procedure(some_routine), pointer :: ptr
nullify(ptr)

Does nullify do something behind the scenes? Or is it just two different ways of doing the same thing?

score 9 · Accepted Answer · answered Oct 31 '14 at 13:08

9

The result is completely identical. The assignment sign => can be also used for variable initialization or derived type components default initialization, where the nullify statement is unusable, but that is only a syntactic thing, it is not a proper assignment in fact.

For example

  type t
    real, pointer :: ptr => null()
  end type

is the default initialization of a component, and

 program p
   real, pointer :: ptr2 => null()

is initialization of a variable. The variable ptr2 is implicitly save as any other initialized variable (a common source of errors).

answered Oct 31 '14 at 13:08

Vladimir F Героям слава

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But please be aware of the implicit `save` when combining declaration and initialization! – Alexander Vogt Oct 31 '14 at 13:09
I took that into account in the answer now. – Vladimir F Героям слава Oct 31 '14 at 13:13
Thanks. I knew about the variable assignment, actually, we often use it that way, but was unsure if it is the way of choice. Especially mixing both looked kind of fishy to me. But as it both is the same, there shouldn't be any problems with that. – PVitt Oct 31 '14 at 13:20
It is completely the same. For the two types initializations I showed as an addition be aware they are Fortran 95 and not 90. – Vladimir F Героям слава Oct 31 '14 at 13:23

Difference between nullify(pointer) and pointer => null()

1 Answers1