opertator ++ (and maybe others) on interface

[FrankMaraite]

Hi all,
this is not really an issue of the compiler. C# does it the same. But something worth to discuss.

I got
error XS0563: One of the parameters of a binary operator must be the containing type
when defining ++ operator to classes derived from interface.
on the commented code below.

Interesting: a missing != operator gives warning
warning XS0216: The operator 'FrankMaraite.SimplePoint.operator ==(FrankMaraite.ISimplePoint, FrankMaraite.ISimplePoint)' requires a matching operator '!=' to also be defined

Of course, this operator, when added, is also not allowed.

The C# (5) compiler doen't allow this too. A solution may be to have my own method like IsSamePositon() in this case.

Or let the compiler call the == operator on the left class. Hm ... confusing. Maybe this is something for the new C# compiler with standard implemantions of interfaces.

Here a small sample together with test code:

`INTERFACE ISimplePoint
PROPERTY East AS REAL8 GET SET
PROPERTY North AS REAL8 GET SET
METHOD Equals( point AS ISimplePoint ) AS LOGIC
END INTERFACE

CLASS SimplePoint IMPLEMENTS ISimplePoint, IEquatable

PROPERTY East AS REAL8 AUTO
PROPERTY North AS REAL8 AUTO

PUBLIC OVERRIDE METHOD Equals( point AS ISimplePoint ) AS LOGIC

  IF !( point IS SimplePoint )
     RETURN FALSE
  ELSE
     IF SELF:East == point:East ;
     .AND.SELF:North == point:North
        RETURN TRUE
     ELSE
        RETURN FALSE
     ENDIF
  ENDIF

OPERATOR == ( point1 AS SimplePoint, point2 AS SimplePoint ) AS LOGIC
RETURN point1:Equals( point2 )
END OPERATOR

OPERATOR == ( point1 AS SimplePoint, point2 AS ISimplePoint ) AS LOGIC
RETURN point1:Equals( Point2 )
END OPERATOR

OPERATOR == ( point1 AS ISimplePoint, point2 AS SimplePoint ) AS LOGIC
RETURN point1:Equals( Point2 )
END OPERATOR

OPERATOR == ( point1 AS ISimplePoint, point2 AS SimplePoint ) AS LOGIC
RETURN point1:Equals( point2 )
END OPERATOR

// OPERATOR == ( point1 AS ISimplePoint, point2 AS ISimplePoint ) AS LOGIC
// RETURN point1:Equals( point2 )
// END OPERATOR

END CLASS
`
The commented lines are not allowed. Now the tests:

`
[Test] ;
METHOD Gleich_TRUE_IP AS VOID STRICT

  LOCAL Point AS ISimplePoint
  Point := SimplePoint{ 1.0, 2.0, 3.0 }

  LOCAL Point2 AS SimplePoint
  Point2 := SimplePoint{ 1.0, 2.0, 3.0 }

  Assert.That( Point == Point2, Is.True )

[Test] ;
METHOD Gleich_TRUE_PI AS VOID STRICT
LOCAL Point AS SimplePoint
Point := SimplePoint{ 1.0, 2.0, 3.0 }

  LOCAL Point2 AS ISimplePoint
  Point2 := SimplePoint{ 1.0, 2.0, 3.0 }

  Assert.That( Point == Point2, Is.True )

[Test] ;
METHOD Gleich_TRUE_PP AS VOID STRICT

  LOCAL Point AS SimplePoint
  Point := SimplePoint{ 1.0, 2.0, 3.0 }

  LOCAL Point2 AS SimplePoint
  Point2 := SimplePoint{ 1.0, 2.0, 3.0 }

  Assert.That( Point == Point2, Is.True )

[Test] ;
METHOD Gleich_TRUE_II AS VOID STRICT

  LOCAL Point AS ISimplePoint  // <---
  Point := SimplePoint{ 1.0, 2.0, 3.0 }

  LOCAL Point2 AS ISimplePoint  // <---
  Point2 := SimplePoint{ 1.0, 2.0, 3.0 }

  Assert.That( Point == Point2, Is.True )

`
The last test fails, because the local variable is declared as interface. And there is no == operator for.

Hope, this was clear.

Wish you a merry christmas and a happy new year

Frank

[RobertvanderHulst]

This will only work in .Net Core and .Net 5. We will add support for this when we have completed the .Net Core support for VS.