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:
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. |
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's association status is defined.
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 an EQUIVALENCE or NAMELIST statement. A pointer in a DATA statement can only be associated with NULL().
Fortran 95/90 pointers are not the same as integer pointers. For more information, see the POINTER - Integer statement.
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.F90in the TUTORIAL sample programs.
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