Re: [Boost-users] A boost::python problem

From: lxcypp lxc (lxcypp_at_[hidden])
Date: 2008-08-04 03:02:15

Thank you for your replay.

You mean:
when Python call C++ function , it will pass a RAW pointer (NOT a
shared_ptr) to C++, and when pointer implict convert to a shared_ptr<T>, we
got a totally different shared_ptr to the pointer, so that the
_internal_weak_this will be reinitialized .

Is that right?

2008/8/4 Peter Dimov <pdimov_at_[hidden]>

> I can at least explain why the problem occurs, but I'm not sure if I can
> suggest a solution at this time.
>
> The problem is that inside X::test there are two separate shared_ptr
> instances pointing to the same A object. One has been created in main(), and
> another has been created by Boost.Python for the x.test( a ) call in t1.py.
>
> It so happens that the current behavior of enable_shared_from_this is for
> the second instance to override the first one; that is, _internal_weak_this
> is reinitialized from the 'pObj' argument when it's created.
>
> Once 'pObj' goes out of scope, the weak_ptr expires, and shared_from_this
> starts throwing bad_weak_ptr.
>
> By using a raw pointer argument, you avoid this reinitialization.
>
> We've been thinking of changing the behavior of enable_shared_from_this in
> such cases, making it ignore subsequent shared_ptr instances to the same
> object and having it stick to the first one. This, however, will not happen
> for the upcoming 1.36 release of Boost.
>
> You can in principle achieve the same result by patching your local copy of
> Boost to make weak_ptr::_internal_assign only initialize when
> this->expired() is true, but this has never been tested.
>
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