Re: [boost] [IdentityType] Const omission undocumented.

Subject: Re: [boost] [IdentityType] Const omission undocumented.
From: Mostafa (mostafa_working_away_at_[hidden])
Date: 2013-12-03 16:39:12

On Tue, 03 Dec 2013 11:23:46 -0800, Krzysztof Czainski  
<1czajnik_at_[hidden]> wrote:

> 2013/12/3 Lorenzo Caminiti <lorcaminiti_at_[hidden]>
>
>> On Sat, Nov 16, 2013 at 11:18 AM, Mathias Gaunard
>> <mathias.gaunard_at_[hidden]> wrote:
>> > On 16/11/2013 01:02, Mostafa wrote:
>> >>
>> >> #include <boost/utility/identity_type.hpp>
>> >> #include <utility>
>> >>
>> >> int main()
>> >> {
>> >>    BOOST_IDENTITY_TYPE(( std::pair<int,int> const )) x;
>> >>    // This compiles, so the const qualification in the
>> >>    // above macro parameter is ignored.
>> >>    x = std::make_pair(0,1);
>> >>
>> >>    return 0;
>> >> }
>> >>
>> >> The omission of const qualification in BOOST_IDENTITY_TYPE types is
>> >> undocumented. This omission maybe deduced from the documented
>> >> implementation, but, as noted, that is not part of the library api  
>> and
>> >> is subject to change. I therefore propose that this behaviour be
>> >> documented as part of the library's public api.
>> >
>> >
>> > That looks like a bug.
>>
>> Is there a way to extract the cv-qualifier from a function parameter
>> type at compile-time?
>>
>> some_mata_function<void (int const)>::type // evaluates to `int const`
>> not just `int`
>>
>> Right now I do the following which looses the cv-qualifier (I also
>> tried to program my own meta-functions but I wan't able to retain the
>> cv-qualifier).
>>
>> boost::function_traits<void (int const)>::arg1_type // evaluates to
>> `int` and not to `int conat` :(
>>
>
> how about (not tested):
> boost::remove_reference< boost::function_traits<void (int
> const&)>::arg1_type >::type

Pointers should be used instead. References will lead to bugs because of  
reference collapsing when add_reference is used. For example:

BOOST_IDENTITY_TYPE(( std::pair<int,int> & )) x;

will be incorrectly deduced as std::pair<int,int>