[C++-sig] [Boost.Python] How to adjust Python reference counts for hybrid objects?

[Jim Bosch]

On 11/14/2011 09:54 PM, Per Knudsgaard wrote:
> I have gotten (3) to work, but I am wondering about two things.
> Since I couldn't get the constructor to work, I went for a modifier
> that returns the object.

Sorry no one responded to your questions earlier. I havne't looked too
closely yet, but I thought I'd point out that you have basically written 
a big workaround for the fact that you're using intrusive_ptr instead of 
shared_ptr. Pretty much all of the things you're trying to do work 
out-of-the-box with shared_ptr, and you don't have to worry about the 
reference count yourself at all.

Unfortunately, similar support for intrusive_ptr isn't really present in 
Boost.Python, but that's because intrusive_ptr isn't nearly as flexible.

>
> So, two questions:
>
> *         Is this a legal/recommended way to do a modifier?

Legal, yes.  I think.  If switching to shared_ptr is a possibility, I'd 
consider that the recommended way to deal with the entire problem. 
Without it, I think you're in rather uncharted territory.

> *         If I only do a single INCREF, then the object is destroyed
> while executing the NewObject().share() call.  Why do I need two
> INCREFs?

I'm a bit stumped by this, too, but I haven't looked as closely as I 
might have if you didn't seem to have a satisfactory solution or if I 
didn't think shared_ptr was really the way to go here.  Part of the 
explanation might be the fact that Boost.Python doesn't hold a reference 
to what you get out of get_owner(), so you don't have one either, and 
that might do funny things when the only Python reference is a temporary 
object you're calling the share() method on.  But I still would have 
expected that to be safe.

HTH

Jim