POINTER - Fortran 95/90

Statement and Attribute: Specifies that an object is a pointer (a dynamic variable). A pointer does not contain data, but points to a scalar or array variable where data is stored. A pointer has no initial storage set aside for it; memory storage is created for the pointer as a program runs.

The POINTER attribute can be specified in a type declaration statement or a POINTER statement, and takes one of the following forms:

Syntax

Type Declaration Statement:

type, [att-ls,] POINTER [, att-ls] :: ptr [(d-spec)] [, ptr [(d-spec)]] ...

Statement:

POINTER [::] ptr [(d-spec)] [, ptr [(d-spec)]] ...

type-spec
Is a data type specifier.

att-ls
Is an optional list of attribute specifiers.

ptr
Is the name of the pointer. The pointer cannot be declared with the INTENT or PARAMETER attributes.

d-spec
(Optional) Is a deferred-shape specification (: [, :] ...). Each colon represents a dimension of the array.

Rules and Behavior

No storage space is created for a pointer until it is allocated with an ALLOCATE statement or until it is assigned to a allocated target. A pointer must not be referenced or defined until memory is associated with it.

Each pointer has an association status, which tells whether the pointer is currently associated with a target object. When a pointer is initially declared, its status is undefined. You can use the ASSOCIATED intrinsic function to find the association status of a pointer.

If the pointer is an array, and it is given the DIMENSION attribute elsewhere in the program, it must be declared as a deferred-shape array.

A pointer cannot be specified in a DATA, EQUIVALENCE, or NAMELIST statement.

Fortran 90 pointers are not the same as integer pointers. For more information, see the POINTER - Compaq Fortran statement.

Compatibility

CONSOLE STANDARD GRAPHICS QUICKWIN GRAPHICS WINDOWS DLL LIB

See Also: ALLOCATE, ASSOCIATED, DEALLOCATE, NULLIFY, TARGET, Deferred-Shape Arrays, Pointer Assignments, Pointer Association, Pointer Arguments, NULL, Compaq Fortran POINTER statement, Type Declarations, Compatible attributes

Examples

The following example shows type declaration statements specifying the POINTER attribute:

  TYPE(SYSTEM), POINTER :: CURRENT, LAST
  REAL, DIMENSION(:,:), POINTER :: I, J, REVERSE

The following is an example of the POINTER statement:

  TYPE(SYSTEM) :: TODAYS
  POINTER :: TODAYS, A(:,:)

See also the examples POINTER.F90 and POINTER2.F90 in /DF98/SAMPLES/TUTORIAL.

The following shows another example:

 REAL, POINTER :: arrow (:)
 REAL, ALLOCATABLE, TARGET :: bullseye (:,:)

 ! The following statement associates the pointer with an unused
 ! block of memory.

 ALLOCATE (arrow (1:8), STAT = ierr)
 IF (ierr.eq.0) WRITE (*,'(/1x,a)') 'ARROW allocated'
 arrow = 5.
 WRITE (*,'(1x,8f8.0/)') arrow
 ALLOCATE (bullseye (1:8,3), STAT = ierr)
 IF (ierr.eq.0) WRITE (*,*) 'BULLSEYE allocated'
 bullseye = 1.
 bullseye (1:8:2,2) = 10.
 WRITE (*,'(1x,8f8.0)') bullseye

 ! The following association breaks the association with the first
 ! target, which being unnamed and unassociated with other pointers,
 ! becomes lost. ARROW acquires a new shape.

 arrow => bullseye (2:7,2)
 WRITE (*,'(/1x,a)') 'ARROW is repointed & resized, all the 5s are lost'
 WRITE (*,'(1x,8f8.0)') arrow

 NULLIFY (arrow)
 IF (.NOT.ASSOCIATED(arrow)) WRITE (*,'(/a/)') ' ARROW is not pointed'

 DEALLOCATE (bullseye, STAT = ierr)
 IF (ierr.eq.0) WRITE (*,*) 'Deallocation successful.'
 END