Re: [boost] [type_erasure]any(U,static_binding) stores wrong type

Subject: Re: [boost] [type_erasure]any(U,static_binding) stores wrong type
From: Larry Evans (cppljevans_at_[hidden])
Date: 2012-07-29 11:06:00

On 07/29/12 09:48, Larry Evans wrote:
> On 07/29/12 08:06, Larry Evans wrote:
>> When the attached is run, it produces output:
>>
>> ./static_binding.exe
>> std::is_copy_constructible<ph_erasee_type>=0
>> 53:ph_ctor_arg<boost::type_erasure::_a>::CTOR.default:my_instance=0
>> creating a_fst from a_ctor_arg
>> 64:ph_ctor_arg<boost::type_erasure::_a>::CTOR.copy:my_instance=1:(void*)this=0x66ab50
>> :*fst_cast=ph_erasee<boost::type_erasure::_a>.my_instance=0:(void*)fst_cast=0x66ab50
>> dynamic_cast<ph_ctor_arg_type*>(value_base_ptr)=0x66ab50
>> dynamic_cast<ph_erasee_type*>(value_base_ptr)=0
>>
>> Indicating that a ph_ctor_arg_type is being stored in a_fst instead
>> of the expected ph_erasee_type.
>>
> OOPS, reading more closely the requires from:
> 
> it says:
> 
> 
>   U is a model of Concept.
> 
> 
> and obviously ph_ctor_arg_type is not because it occurs nowhere in
> ConceptA; however, it would be clearer if the requirement were:
> 
>   U is a suitable argument to the copy constructor for
>   mpl::map<Map, Tag>::type, where Tag is the 2nd template
>   arg to the any to be constructed.
> 
OOPS again, because a ph_ctor_arg_type *is* a suitable arg to
ph_erasee_type.  Maybe the requirement should just be:

  U is same as mpl::at<Map, Tag>::type where Tag is 2nd
  template arg to the any being constructed
  and U is a model of Concept.

I think that would cover all the cases; however, it's
still worrisome that no diagnostic is provided when
U is something other than mpl::at<Map,Tag>::type.

Am I missing something?

-regards,
Larry