[C++-sig] intrusive pointer comparison in python

[Alex Mohr]

> Here's a better example:
> 
> class foo;
> typedef boost::intrusive_ptr<foo> fooHandle;
> 
> // personal refcounting implementation
> class foo: public reference_counted {}; 
> };
> 
> class foocontainer {
> public:
> 	foocontainer():fooobj(foo::getfoo()) {}
> 	fooHandle getfoo() {return fooobj;}
> private:
> 	fooHandle fooobj;
> };
> 
> BOOST_PYTHON_MODULE(test_intrusive) {
> 	class_<foo,fooHandle>("foo");
> 	class_<foocontainer>("foocontainer")
> 		.def("getfoo",&foocontainer::getfoo);
> }
> 
> Now if I go into python shell:
>>> x=foocontainer()
>>> test1=x.getfoo()
>>> test2=x.getfoo()
>>> test1==test2
> False

This is due to the fact that boost::python doesn't really handle what I 
call the "object identity" problem.  In this case, you expect test1 and 
test2 to be the same python object because they "represent" the same C++ 
object.  They're not the same python object though -- boost python 
creates a new python wrapper each time a fooHandle is converted to-python.

There's no simple way to solve this problem in boost::python as-is.  It 
would be great if boost::python could have some support for doing this, 
because it's obviously an important thing.

BTW: I should mention that boost::python does handle this correctly for 
objects that are exposed to python polymorphically (deriving from 
wrapper<T>).  But then only for polymorphic wrapper objects 
(boost::python holds the python object in wrapper<T>).

Incidentally, you can make your 'test1 == test2' line work by adding the 
equality operator to your wrapper, and have it compare the pointers. 
But you can't make 'test1 is test2' work.

Alex