[geos-devel] createEmptyGeometry() createsinstance ofGeometryCollection

[Martin Davis]

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: geos-devel-bounces at geos.refractions.net 
> [mailto:geos-devel-bounces at geos.refractions.net] On Behalf Of 
> strk at refractions.net
> 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 at geos.refractions.net 
> http://geos.refractions.net/mailman/listinfo/geos-devel
>