[geos-devel] createEmptyGeometry() creates instance ofGeometryCollection

[Mateusz Łoskot]

Martin Davis wrote:
> Yes.  The spec doesn't cover this, but I chose to use an empty 
> GeometryCollection as a "typeless" empty geometry, for methods which 
> need to return one.  The createEmptyGeometry method is just a 
> convenience method for this convention.

OK, I understant it.

> Anyway, what else could that method return that would make sense 
> (given that it has no type information)?

It depends on the idea behind "empty geometry" or "null geometry" [1].
As I know, OGC Simple Feature Spec. explains that empty geometry
represents empty point set (empty set of coordinates).
According to my understanding:
Point is empty if it contains "uninitialized" coordinates
MultiPoint is empty if it contains no points in its set.
etc.

There is also a distinction between empty and null.
NULL means something undefined, uninitialized - something unsafe.
EMPTY means something well constructor but with empty set of points.
In some case, NULL object can be compared to EMPTY object with true result.

So, I think isEmpty() function should check the state of the coordinates
set of Geometry return appropriate value (true/false).

[1] Saying "null geometry" I think about logical meaning but not the
physical representation of instance of geometry: null reference/null
pointer.

Cheers
-- 
Mateusz Łoskot
http://mateusz.loskot.net