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Subject: Re: [boost] [type_erasure]any(U,static_binding) stores wrong type
From: Larry Evans (cppljevans_at_[hidden])
Date: 2012-07-29 13:07:17

On 07/29/12 10:06, Larry Evans wrote:
> 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?
with the any diffs shown in 1st attachment,
the output of 2nd attachment is:

creating a_fst from a_ctor_arg

Showing that the expected type, i.e. ph_erasee<_a>, is created.
The any change makes sure to create the expected type,
mpl::at<Map,T>::type with help of new template function:

 make_storage<Constructee, U&...>

which constructs, a Constructee.

I think this would avoid some nasty suprises with
the wrong arg type, U, is accidentally passed to the
any<U,static_binding>. If U is a valid CTOR arg to
the expected type, no error would be issued, but
type_cast would return a pointer to the expected type
even though the actual type created was a U.




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