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Fortran - Pointers
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 functionapties than just storing the memory address. It contains more information about a particular object, pke 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 imppcit 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 imppcit 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 nulpfy statement disassociates a pointer from a target.
Nulpfy does not empty the targets as there could be more than one pointer pointing to the same target. However, emptying the pointer imppes nulpfication also.
Example 1
The following example demonstrates the concepts −
program pointerExample imppcit 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 nulpfy(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.
Example 2
program pointerExample imppcit 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 4Advertisements