Re: [boost] enable_shared_from_this2

Subject: Re: [boost] enable_shared_from_this2
From: Frank Mori Hess (frank.hess_at_[hidden])
Date: 2009-10-27 13:36:51

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On Monday 26 October 2009, Berserker wrote:
> The "patched" code:
>
> void init_weak_once() const
> {
>     // Reinitialization support
>     if( weak_this_.expired() )
>     {
>        // shared_from_this in costructors support
> shared_this_.reset(static_cast
> <T*>(const_cast<enable_shared_from_this2>(this)),
> detail::esft2_deleter_wrapper());
>        weak_this_ = shared_this_;
>     }
> }
>
> I know that the static_cast/const_cast is "problematic" on VC6 (and
> maybe other compilers) but it could be solved using an offset from
> enable_shared_from_this2 * to T *.
> I've attached a testcase that shows the problems with the current
> enable_shared_from_this2 implementation (with the proposed patch it works).
> Any feedback about this?

IIRC, the original version of enable_shared_from_this that supported use in 
constructors did a static_cast to the derived type.  But it failed some of 
the tests, so it was changed to a dynamic_cast.  The version currently in svn 
looks broken, like it was from a later version which relied on a 
shared_from_this() free function like:

template<typename T>
shared_ptr<T> shared_from_this(T *);

which used shared_ptr aliasing to create and return a shared_ptr which pointed 
at the same object as was passed in as an argument (so 
enable_shared_from_this didn't have to do a cast or store a pointer to the 
derived type).  But I don't see any such free function in 
enable_shared_from_this2.hpp.

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