[C++-sig] shared_ptr and register_ptr_to_python

[Jim Bosch]

On 10/10/2010 03:58 AM, Marek Denis wrote:
> On 10.10.2010 03:47, Jim Bosch wrote:
>>
>> Pretty much everything you need should automatically work, whether you
>> use shared_ptr or raw pointers with call policies. If you've invoked
>> register_ptr_to_python on all the classes you're using,
>
> That's what I couldn't get while reading Boost tutorial (the example
> code didn't work either :( ).
> Let's say I have three classes:
>
> class A {};
> class B : public A {};
> class C : public B {};
>
> should I later just put:
> register_ptr_to_python< A >();
> register_ptr_to_python< B >();
> register_ptr_to_python< C >();
> ?

Actually, you want something like this:

register_ptr_to_python< boost::shared_ptr<A> >();

If you want to use raw pointers, I don't think you need 
register_ptr_to_python (see below).

> Here is the small code snippet:
>
> +v
> boost::python::call_method<void>(callback,"addToQueue",c);
> -v
>
> where callback is a PyObject* pointer,and 'c' is a pointer to the object.
> Whenever I call this instruction, I can see that the 'c' object is being
> copied (I just put some cout << "debug" << endl in the definition of the
> Copy Constructor), so I assumed that Python code doesn't operate on the
> object that was created in C++ module. Instead, it just operates on the
> copy.
>

I'm actually kind of surprised about this - in other circumstances 
(particularly when returning values in wrapped functions by pointer), 
Boost.Python doesn't copy pointers or references unless you explicitly 
ask it to.  One thing that may be worth trying: when you wrap C Python, 
mark it as noncopyable:

class_<C,bases<B>,boost::noncopyable>("C")

I'd be curious to see what happens with your call_method code snippet 
with that in place.

In any case, if you're willing to use shared_ptr everywhere instead of 
raw pointers, and switch your register_ptr_to_python calls as I've shown 
above, I think that will also stop any copying that's going on here. 
There should be a way to do it with raw pointers too, but I don't see it 
yet (and it may not exist).

Jim