Re: [Boost-users] How shared_from_this() work when it inheritedfrom more than one time

Subject: Re: [Boost-users] How shared_from_this() work when it inheritedfrom more than one time
From: Björn Karlsson (Bjorn.Karlsson_at_[hidden])
Date: 2009-08-10 03:04:45

Hello R.P,

Inheriting from enable_shared_from_this more than once is not a good idea.

> From: boost-users-bounces_at_[hidden] [mailto:boost-users- 
> bounces_at_[hidden]] On Behalf Of R. P. Janaka
> Sent: Monday, August 10, 2009 6:54 AM
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] How shared_from_this() work when it 
> inherited from more than one time
> 
> But in my case there is no option like this (Class A is a base library 
> class). So I have to do this.
> 

Don't let B inherit from enable_shared_from_this. You've already done that in A. So your question then becomes "how do I retrieve a shared_ptr<B> from this?". Here's how:

  boost::shared_dynamic_cast<B, A>(shared_from_this());

In the example you've provided, it would also be safe to use boost::shared_static_cast.
To conclude: You should only inherit from enable_shared_from_this in A, and B should only inherit from A. 
 
> I did the above to avoid the ambiguous call error So how can I specify 
> that i am calling the shared_from_this() in class B....?

You did. But the problem is that you have two weak_ptrs -- one weak_ptr<A> and one weak_ptr<B> -- and only one of them will be initialized correctly when you create the smart pointer. (If you're interested in debugging your case to really understand what happens, have a look at the function
sp_enable_shared_from_this() in shared_ptr.hpp.) 

Does that help?

Cheers,
Bjorn Karlsson