Re: [Boost-users] [shared_ptr] Multiple enable_shared_from_this in inheritance tree

Subject: Re: [Boost-users] [shared_ptr] Multiple enable_shared_from_this in inheritance tree
From: Edward Diener (eldiener_at_[hidden])
Date: 2012-03-26 17:54:21

On 3/26/2012 4:39 AM, ivan.lelann_at_[hidden] wrote:
>
>
> ----- Mail original -----
>> De: "Edward Diener"<eldiener_at_[hidden]>
>> À: boost-users_at_[hidden]
>> Envoyé: Dimanche 25 Mars 2012 21:37:55
>> Objet: [Boost-users] [shared_ptr] Multiple enable_shared_from_this in	inheritance tree
>>
>> There was a Code Project article
>> (http://www.codeproject.com/Articles/286304/Solution-for-multiple-enable_shared_from_this-in-i)
>> about a problem using multiple enable_shared_from_this derivations in
>> an
>> inheritance tree.
>>
>> Does this problem actually exist in the current Boost implementation
>> ?
>
>
> I've been trapped by this two years ago, and now under 1.48 code below throws under VS2010 when calling B::shared_from_this().
>
> // BEGIN
> #include<boost/smart_ptr/enable_shared_from_this.hpp>
> #include<boost/make_shared.hpp>
>
> struct A : boost::enable_shared_from_this<A>
> {};
>
> struct B : boost::enable_shared_from_this<B>
> {};
>
> struct C : A, B
> {};
>
> int main()
> {
> 	boost::make_shared<C>()->A::shared_from_this();
> 	boost::make_shared<C>()->B::shared_from_this();
>
> 	return 0;
> }
> // END
>
> weak_this_ is null so you have a bad_weak_ptr exception in shared_count::shared_count( weak_count const&  r ).
> Root problem seems to be that template function sp_enable_shared_from_this is only called for first matching specialization.
>
> Following code does not throw :  (*DISCLAIMER* this could make your computer explode, and is probably utterly wrong anyway)
>
> 	boost::shared_ptr<C>  c = boost::make_shared<C>();
> 	c->A::shared_from_this();
> 	boost::detail::sp_enable_shared_from_this (&c, c.get(), static_cast<boost::enable_shared_from_this<B>*>  (c.get()));
> 	c->B::shared_from_this();
>

Thanks !

I am not actually deriving from 2 classes, each one of which is derived 
from enable_shared_from_this<> as in your example.

My case was initially that I had a base class deriving from 
enable_shared_from_this<> and another class deriving from that base 
class and its own enable_shared_from_this<>. I changed so that the base 
class's derivation serves for the derived class and wrote a 
shared_from_this() member function for the derived class which casts the 
result of the base class's shared_from_this() to a shared_ptr<Derived>.

That appears to work fine.

My main point is that if having more than one enable_shared_from_this<> 
in the inheritance hierarchy normally fails, this should be documented 
with the proper information on how to workaround the problem.