Fortran - Pointers

In most programming languages, a pointer variable stores the memory address of an object. However, in Fortran, a pointer is a data object that has more functionalities than just storing the memory address. It contains more information about a particular object, like type, rank, extents, and memory address.
A pointer is associated with a target by allocation or pointer assignment.

Declaring a Pointer Variable

A pointer variable is declared with the pointer attribute.
The following examples shows declaration of pointer variables:

integer, pointer :: p1 ! pointer to integer  
real, pointer, dimension (:) :: pra ! pointer to 1-dim real array  
real, pointer, dimension (:,:) :: pra2 ! pointer to 2-dim real array

A pointer can point to:

• an area of dynamically allocated memory
• a data object of the same type as the pointer, with the target attribute

Allocating Space for a Pointer

The allocate statement allows you to allocate space for a pointer object. For example:

program pointerExample
implicit none

   integer, pointer :: p1
   allocate(p1)
   
   p1 = 1
   Print *, p1
   
   p1 = p1 + 4
   Print *, p1
   
end program pointerExample

When the above code is compiled and executed, it produces the following result:

1
5

You should empty the allocated storage space by the deallocate statement when it is no longer required and avoid accumulation of unused and unusable memory space.

Targets and Association

A target is another normal variable, with space set aside for it. A target variable must be declared with the target attribute.
You associate a pointer variable with a target variable using the association operator (=>).
Let us rewrite the previous example, to demonstrate the concept:

program pointerExample
implicit none

   integer, pointer :: p1
   integer, target :: t1 
   
   p1=>t1
   p1 = 1
   
   Print *, p1
   Print *, t1
   
   p1 = p1 + 4
   
   Print *, p1
   Print *, t1
   
   t1 = 8
   
   Print *, p1
   Print *, t1
   
end program pointerExample

When the above code is compiled and executed, it produces the following result:

1
1
5
5
8
8

A pointer can be:

• Undefined
• Associated
• Disassociated

In the above program, we have associated the pointer p1, with the target t1, using the => operator. The function associated, tests a pointer’s association status.
The nullify statement disassociates a pointer from a target.
Nullify does not empty the targets as there could be more than one pointer pointing to the same target. However, emptying the pointer implies nullification also.
Example 1
The following example demonstrates the concepts:

program pointerExample
implicit none

   integer, pointer :: p1
   integer, target :: t1 
   integer, target :: t2
   
   p1=>t1
   p1 = 1
   
   Print *, p1
   Print *, t1
   
   p1 = p1 + 4
   Print *, p1
   Print *, t1
   
   t1 = 8
   Print *, p1
   Print *, t1
   
   nullify(p1)
   Print *, t1
   
   p1=>t2
   Print *, associated(p1)
   Print*, associated(p1, t1)
   Print*, associated(p1, t2)
   
   !what is the value of p1 at present
   Print *, p1
   Print *, t2
   
   p1 = 10
   Print *, p1
   Print *, t2
   
end program pointerExample

When the above code is compiled and executed, it produces the following result:

1
1
5
5
8
8
8
T
F
T
952754640
952754640
10
10

Please note that each time you run the code, the memory addresses will be different.
Example 2

program pointerExample
implicit none

   integer, pointer :: a, b
   integer, target :: t
   integer :: n
   
   t= 1
   a=>t
   t = 2
   b => t
   n = a + b
   
   Print *, a, b, t, n 
   
end program pointerExample

When the above code is compiled and executed, it produces the following result:

2  2  2  4