vectors __getitem__ and ref counting

[SwigAtSF]

• Artifact_id: 1707016
• Opened: 2007-04-25 02:00:16 +0200
• Submitter: J Robert Ray <jrray@users.sf.net>
• Assignee: Marcelo Matus <marcelomatus@users.sf.net>

I'm on swig-1.3.31.

The way vectors are wrapped, getting an item out of a vector will return an object that is effectively a wrapped pointer, but its validity depends on the lifespan of the vector.

v = vector() item = v[3] del v

segfault

print item.foo

I didn't notice this being documented anywhere.

Note that this is the kind of code I was getting a crash on:

def get(): return vector()

segfault

print get()[3].foo

Python optimistically decrefs the result of get() after calling getitem but before getattr. It took me a while to understand this and realize why it was crashing.

I think the bug in all of this is that getitem does not increase the reference count of the vector instance it is a member of, and python may choose to delete the vector while objects that refer to its memory still exist.

Ideally the object wrapping the result of v[3] would hold a reference to v to prevent this dangling pointer problem.