Purpose
The OPTIONAL attribute specifies that a dummy argument need not be associated with an actual argument in a reference to the procedure.
Rules
A procedure that has an optional dummy argument must have an explicit interface in any scope in which the procedure is referenced.
Use the PRESENT intrinsic function to determine if an actual argument has been associated with an optional dummy argument. Avoid referencing an optional dummy argument without first verifying that the dummy argument is present.
A dummy argument is considered present in a subprogram according to the rules described in the section: Restrictions on optional dummy arguments not present.
An optional dummy argument that is not present may be used as an actual argument corresponding to an optional dummy argument, which is then also considered not to be associated with an actual argument. An optional dummy argument that is not present is subject to the restrictions specified in the section: Restrictions on optional dummy arguments not present
The OPTIONAL attribute cannot be specified for dummy arguments in an interface body that specifies an explicit interface for a defined operator or defined assignment.
| ALLOCATABLE 1 | EXTERNAL | TARGET |
| ASYNCHRONOUS | INTENT | VALUE 1 |
| CONTIGUOUS 2 | POINTER | VOLATILE |
| DIMENSION | ||
Note:
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- Fortran 2008.
Examples
SUBROUTINE SUB (X,Y)
INTERFACE
SUBROUTINE SUB2 (A,B)
OPTIONAL :: B
END SUBROUTINE
END INTERFACE
OPTIONAL :: Y
IF (PRESENT(Y)) THEN ! Reference to Y conditional
X = X + Y ! on its presence
ENDIF
CALL SUB2(X,Y)
END SUBROUTINE
SUBROUTINE SUB2 (A,B)
OPTIONAL :: B ! B and Y are argument associated,
IF (PRESENT(B)) THEN ! even if Y is not present, in
B = B * A ! which case, B is also not present
PRINT*, B
ELSE
A = A**2
PRINT*, A
ENDIF
END SUBROUTINE