[geos-devel] How to destroy PreparedGeometry object?

[Mateusz Loskot]

Martin Davis wrote:
> I would think that there should be a destructor defined for the 
> PreparedGeometry class.  Would this be the appropriate C++ pattern?

Martin,

Not really. The C++ standard says:

"If a class has no user-declared destructor, a destructor is
declared implicitly"

It means, PreparedGeometry class has implicit destructor.
The problem is that it is never called (implicitly) because 
PreparedGeometryFactory constructs PreparedGeometry objects on heap.
So, user has to trigger destructor execution. In most cases, we do in 
C++ this:

PreparedGeometryFactory f;
Geometry* g = ... // create geometry
PreparedGeometry* pg = f.create(g);
... // use prepared geom
delete pg; // request destruction

And I could use this pattern without problems, however it is not bullet 
proof for "DLL Hell" problems on Windows/Visual C++ because "memory 
crosses DLL boundaries". In example above, GEOS DLL binary allocates 
memory for prepared geometry, but client calls "delete" operator on side 
of his program (.exe binary or another .dll). And crash is very probable.

GeometryFactory::destroyGeometry solves this problem because it calls 
delete operator on the GEOS DLL side.

The same pattern is used in GDAL:


"How should I deallocate resources acquainted from GDAL on Windows?"

http://trac.osgeo.org/gdal/wiki/FAQMiscellaneous


Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net
Charter Member of OSGeo, http://osgeo.org