Generator inside a class prevent __del__ ??

[Duncan Booth]

Rob Nikander <rnikaREMOVEnder at adelphia.net> wrote in 
news:kNKdnYsIEuHbdRjdRVn-uA at adelphia.com:

> I checked out the documentation for that gc.garbage list and it says 
> that the collector can't free objects in cycles if the cyles have 
> objects that have __del__ methods.  So it puts them in this list.
> 
> I wonder what other garbage collectors do in this situation?  Anyone 
> know?  Java?
> 

Most garbage collectors will do peculiar things if you have destructors or 
finalizers in the objects. The problem is that if two objects with 
finalizers reference each other there is no correct order to release the 
objects that will guarantee that the other object still exists, so the 
system either has to choose an arbitrary order, or refuse to call the 
finalizers.

The .Net garbage collector is typical. Objects may have finalizers, and 
these finalizers are called as part of the garbage collection. The system 
guarantees that any finalizer is called exactly 0 or more times --- usually 
it is called once when the object is garbage collected, but if the object 
is never collected it may not be called at all, and if the object 
resurrects itself (e.g. during the finalizer it assigns itself to a global 
variable) the finalizer could be called more than once.

A separate thread pool is used for finalizers, so your finalizer could be 
called while a user thread is executing a method on the object, and two 
objects which refer to each other could have their finalizers called in any 
order, or even simultaneously on separate threads. Effectively, this makes 
finalizers useless in all but the most obscure situations.

When resources need to be released you should try to do it explicitly. In 
.Net this is handled by the Dispose() method, and the finalizer can then 
either try calling Dispose() if it has not yet been called, or could try 
logging an error although even that may be problematic from a finalizer.