[Gdal-dev] GetGeometryRef in scripts

[Frank Warmerdam]

On 9/20/05, Kevin Ruland <kruland at ku.edu> wrote:
> The current swig bindings have some short falls but I do believe they
> are fairly useable provided the developer follow the guidelines of the
> underlying C++ library.  The rules in a nutshell are:
> 
> 1) you must maintain a handle to the Datasource as long as you are using
> objects from it (with some exceptions such as XXX).  If the Datsource is
> destroyed, developer pays for all damages.
> 2) Geometries returned by GetGeometryRef() become stale when its parent
> dies.
> 3)  FieldDefn must have their parent FeatureDefn alive.
> 
> I might have these wrong - the whole situation is rather confusing to me.

Kevin, 

I believe your summary is correct. 
 
> The only thing which is certainly a problem are bindings for gc
> languages.  Even if the developer maintains good habits, the gc might
> still collect the trash in the wrong order.

I'm not sure why it matters what order things are destroyed in as
long as you hold a reference to things till you are done with them. 

>  There are a couple of ways
> to mitigate this:  We could adopt a "no-fail" destructor strategy.  That
> is ensure that under no circumstances will destructors being called in
> the wrong order cause troubles.  Quite frankly I don't know how this can
> be done.   

Perhaps you could clarify where we have problems with gc 
languages.  I *feel* this isn't a problem, because language
variables that reference objects should not need to do anything
at all for objects they don't own.  So, for instance, a reference
to a geometry that is owned by some other object does not need
to do any work at all in it's destructor other than check it's
"owned" flag.  

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent