Well, geometrically I agree with you, but practically speaking I suspect it simplifies calling code if you return an expected type (where such a type exists).
This also allows you to have nice invariants such as "buffer always returns a Polygonal geometry". Remember: floating point <> real numbers, and Geometry <> geometry Martin Davis, Senior Technical Architect Vivid Solutions Inc. www.vividsolutions.com Suite #1A-2328 Government Street Victoria, B.C. V8T 5G5 Phone: (250) 385 6040 - Local 308 Fax: (250) 385 6046 > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of > [EMAIL PROTECTED] > Sent: March 30, 2006 11:59 PM > To: GEOS Development List > Subject: Re: [geos-devel] createEmptyGeometry() > createsinstance ofGeometryCollection > > > On Fri, Mar 31, 2006 at 12:04:39AM +0200, Mateusz £oskot wrote: > > 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. > > But what's defined in an EMPTY set of points ? > It's the points themself giving semantic of a geometry type. > What distinguish an EMPTY point from an EMPTY line from an > EMPTY collection ? It's the exactly same topological space: > the NO space at all. How can you say that NOTHING is a point > or a line or an area ? > > --strk; > _______________________________________________ > geos-devel mailing list > geos-devel@geos.refractions.net > http://geos.refractions.net/mailman/listinfo/geos-devel > _______________________________________________ geos-devel mailing list geos-devel@geos.refractions.net http://geos.refractions.net/mailman/listinfo/geos-devel
