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! @(#) Example of overloading operators in Fortran
!
! The following MODULE allows for using the syntax
! L1 == L2
! where L1 and L2 are LOGICAL
! as an alternative to
! L1 .EQV. L2
! (or the function call
! BOOLEAN_EQUAL (L1, L2))
! or
! L1 .BOOLEAN_EQUAL. L2
! for that matter); and likewise allows
! L1 /= L2
! as equivalent to
! L1 .neqv. L2
!
! This is a simplistic example. Two significant real-world examples of
! operator overloading to look for use operator overloading to provide
! modules that allow many existing Fortran routines to be used with almost
! no source file changes to produce versions using arbitrary precision
! arithmetic, or to provide cumulative error bounds on floating-point
! calculations.
!
! THOSE features would make interesting additions to the Fortran standard.
!
! The other purpose of this example is to point out that
! L1 == L2 !! should be L1 .eqv. L2
! and
! L1 /= L2 !! should be L1 .neqv. L2
! should NOT work by default; but often do (probably because the compiler
! silently converts LOGICAL to INTEGER when a LOGICAL appears where a
! numeric value is required?). If you can comment out the USE statement
! below and still run this example, your compiler supports this
! non-standard (but intuitive-looking) syntax. Using it makes your code
! much less portable.
!
!-----------------------------------------------------
MODULE logical_ops
!-----------------------------------------------------
INTERFACE OPERATOR ( == )
MODULE PROCEDURE boolean_equal
END INTERFACE OPERATOR ( == )
!-----------------------------------------------------
INTERFACE OPERATOR ( /= )
MODULE PROCEDURE boolean_notequal
END INTERFACE OPERATOR ( /= )
!-----------------------------------------------------
CONTAINS
!-----------------------------------------------------
LOGICAL FUNCTION boolean_equal(logical_val1,logical_val2)
IMPLICIT NONE
LOGICAL, INTENT (IN) :: logical_val1
LOGICAL, INTENT (IN) :: logical_val2
IF (logical_val1 .EQV. logical_val2 )THEN
boolean_equal=.TRUE.
ELSE
boolean_equal=.FALSE.
ENDIF
END FUNCTION boolean_equal
!-----------------------------------------------------
LOGICAL FUNCTION boolean_notequal(logical_val1,logical_val2)
IMPLICIT NONE
LOGICAL, INTENT (IN) :: logical_val1
LOGICAL, INTENT (IN) :: logical_val2
IF (logical_val1 .EQV. logical_val2 )THEN
boolean_notequal=.FALSE.
ELSE
boolean_notequal=.TRUE.
ENDIF
END FUNCTION boolean_notequal
!-----------------------------------------------------
END MODULE logical_ops
!-----------------------------------------------------
PROGRAM testit
USE logical_ops
IF (.TRUE. .EQV. .TRUE. )THEN ! standard
WRITE(*,*)'hello world .eqv.'
ENDIF
IF (.TRUE. .NEQV. .FALSE. )THEN ! standard
WRITE(*,*)'hello world .neqv.'
ENDIF
IF (.TRUE. == .TRUE. )THEN ! using overloaded operator
WRITE(*,*)'hello world =='
ENDIF
IF (.TRUE. /= .FALSE. )THEN ! using overloaded operator
WRITE(*,*)'hello world /='
ENDIF
END PROGRAM testit