Re: [boost] [type_erasure] Review started (July 18-27, 2012), rebind_any bad_cast

Subject: Re: [boost] [type_erasure] Review started (July 18-27, 2012), rebind_any bad_cast
From: Larry Evans (cppljevans_at_[hidden])
Date: 2012-07-25 12:36:19

On 07/25/12 10:57, Larry Evans wrote:
> On 07/18/12 00:13, Lorenzo Caminiti wrote:
>> Hello all,
>>
>> *** The review of Steven Watanabe's proposed Boost.TypeErasure library
>> begins on July 18, 2012 and ends on July 27, 2012. ***
>>
> The attached produces runtime error:
> 
> ./rebind_any.exe
> creating b_self
> any(const U&)
> creating b_copy
> terminate called after throwing an instance of
> 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::type_erasure::bad_any_cast>
>> '
>   what():  std::bad_cast
> make: *** [run] Aborted
> 
> Yet, the type returned by rebind_any was used in the any_cast; hence,
> it must be a good cast, AFAICT.
> 
> What am I doing wrong?
> 
OOPS.  I should have realized that the AnyB arg to rebind_any has no
information about what's the actual type stored in the b_self
because AnyB was defined before b_self was created.

Sorry for noise.

However, the doc:

http://steven_watanabe.users.sourceforge.net/type_erasure/libs/type_erasure/doc/html/boost/type_erasure/rebind_any.html

was a bit confusing since it says:

  A metafunction that returns any type corresponding to a placeholder.
  If T is not a placeholder, returns T unchanged.

Since the T was _self, which is a placeholder, I assumed it would
return the type corresponding to _self; however, since that's
unknown until the actual Any is created, it must return something
else.  I actually demangled the type returned and it was:

b_type=boost::type_erasure::any<boost::mpl::vector<boost::type_erasure::copy_constructible<boost::type_erasure::_self>,
boost::type_erasure::typeid_<boost::type_erasure::_self>,
boost::type_erasure::addable<boost::type_erasure::_self,
boost::type_erasure::_self, boost::type_erasure::_self>, mpl_::na,
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na,
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na,
mpl_::na, mpl_::na>, boost::type_erasure::_self>

which looks like it's just the Any template arg to rebind_any, which
leaves me wondering what's the purpose of rebind_any.

-regards,
Larry