On 3/26/2012 4:39 AM, ivan.lelann@free.fr wrote:
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De: "Edward Diener"<eldiener@tropicsoft.com> À: boost-users@lists.boost.org 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_shar...) 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.