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

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De: "Edward Diener" À: boost-users@lists.boost.org Envoyé: Mercredi 28 Mars 2012 01:56:24 Objet: Re: [Boost-users] [shared_ptr] Multiple enable_shared_from_this in inheritance tree

On 3/27/2012 5:08 AM, ivan.lelann@free.fr wrote:

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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.

I agree.

I wondered how current C++11 standard libraries handled this. With VS2010, replacing "boost" with "std" made both "shared_from_this()" calls throw. (with two esft bases the compiler chooses a dummy specialization)

I think it is quite simply that there can not be more than one enable_shared_from_this derivation in a hierarchy. Unfortunately one is left to discover this for himself.

IIRC, with g++ (4.6) using its C++11's std instead of boost, I got a compile-time error. Something like "... is ambiguous". I'll try to check that as it is quite self-documenting. I failed to see how one can properly implement enable_shared_from_this without extra compiler support. BTW, you can throw even without multiple esft. With boost or VS2010's std: struct A { virtual void foo() {};}; struct B : A, boost::enable_shared_from_this <B> {}; int main() { A * pa = new B; boost::shared_ptr <A> spa (pa); // dummy sp_enable_shared_from_this called here as A do not inherit esft // shared_ptr <A> spa (new B) would have been ok boost::dynamic_pointer_cast <B> (spa)->shared_from_this(); // shared_from_this() throws bad_weak_ptr return 0; } Regards, Ivan