[Gdal-dev] References and scripting language bindings

[Kevin Ruland]

Ari,

You understand perfectly.

The solution you propose is also correct.  But it needs to be done "all
the way down" to support longer chains of calls:

f = ogr.Open('data').GetLayerByIndex(0).GetFeature(0)

It seems that everybody down the stack will need to know the root
datasource.

This does pose some interesting problems though, right now,
GetLayerByIndex() does not return an object owned by the script (ie, not
a %newobject).  It will need to return a new object which has some
control over a dataset pointer.  etc.

It will be a little piece of work.

Also, I'm hearing there is a similar situation in the FeatureDefn &
FieldDefn objects.

Kevin

Ari Jolma wrote:

> Hi, this problem was discussed on IRC yesterday but I'm slow to
> understand, so I try to describe the problem again and think about a
> solution.
>
> The problem is that if you do something like
>
> layer = datasource.Open('data').GetLayerByIndex(0)
> print layer.Name
>
> this will cause a problem if the scripting language has deleted the
> datasource (and the layer with it). This is  because it was just a
> temporary variable. I first thought that layer should be undefined on
> the 2nd line. But this is analogous to the case if you create a
> reference to a variable and there scripting languages typically do not
> delete the variable until the referencing variable is not used anymore
> (it goes out of scope for example).
>
> The datasource class (and many other classes) in OGR has similar
> reference count system. Thus I think we should use them in the
> scripting language bindings. I.e. do not destroy a datasource object
> if it has references to it. In GetLayerByIndex increment the reference
> count. In a layer object make a note which datasource object it needs
> and decrement the count of that when the layer object it destroyed.
>
> Is this difficult to implement in swig? The problem seems to be where
> to put the link to the parent (e.g. the datasource handle for the
> layer object)? It has to be done while still in the C wrapper.
>
> I don't know how all this works in Java which has a different kind of
> garbage collection system.
>
> Ari
>
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