[gdal-dev] how to union(merge) polygons using ogr?

[梁天辰]

Hi all,
I need to union(make OGRGeometry wkbPolygon and that has no interiorRings) polygons in the following 2 situations:
1. OGRFeature::getGeometryType() returns wkbPolygon and OGRPolygon::getNumInteriorRings() returns an int that is bigger than 0.
2.  OGRFeature::getGeometryType() returns wkbMultiPolygon. 
Neither OGRGeometry::Union nor UnionCascaded returns what I need.
Union just removes the interiorRing ;UnionCascaded returns nullptr.
Am I using these functions the right way or Do I have to use other member functions?
Thanks.
Regards,
梁天辰
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