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.
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.
An area of dynamically allocated memory.
A data object of the same type as the pointer, with the target attribute.
A data object of the same type as the pointer, with the target attribute.
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.
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.
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
0
0
10
10

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

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