[gdal-dev] Adding multipolygons to an existing geometry collection

Even Rouault even.rouault at mines-paris.org
Thu Mar 25 14:16:31 EDT 2010

> What's the "Java dirty trick" that the code talks about?


Do you really want to hear the explanation ;-) ? OK, here it is.

This is related to subtelities of SWIG and Java garbage collection.

Imagine that DISOWN is still used for the input parameter of 

Consider the following code :

geom1 = ogr.CreateGeometryFromWKT(xxxx);
geom2 = ogr.ForceToMultiPolygon(geom1);
geom2 = null;
/* big memory allocation */

If nothing special is done, during the big memory allocation, the garbage 
collector will collect the geom2 object. During its finalization, this will 
call the destructor of the underlying C++ object (let's call it geom2_cpp). 
But since the call of ForceToMultiPolygon(), geom1_cpp is owned by geom2_cpp, 
thus deleting geom2_cpp will cause geom1_cpp to become invalid. And 
foo(geom1) would operate on garbage.

This can be fixed (I have already done this for Feature.SetGeometryDirectly()) 
by adding special code in ogr_java.i so that geom1 (the Java object) holds a 
Java reference to geom2 (the Java object). So the 2 objects are bound 
together and garbage collection of one cannot occur if the other one is still 
in scope...

Calling Clone() and separating the lifetime of the 2 objects avoids the above 

Best regards,


> Ari
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