Re: [boost] [type_traits] has_equal_to< std::vector<T> > always true_

Subject: Re: [boost] [type_traits] has_equal_to< std::vector<T> > always true_
From: lcaminiti (lorcaminiti_at_[hidden])
Date: 2011-10-21 11:38:45

Vicente Botet wrote:
> 
> Le 20/10/11 17:09, Jeffrey Lee Hellrung, Jr. a écrit :
>> On Thu, Oct 20, 2011 at 7:55 AM, lcaminiti<lorcaminiti@>  wrote:
>>
>>> Hello all,
>>>
>>> has_equal_to<  std::vector<T>  >  always returns true_ because == is
>>> defined for std::vector<T>  even when it is not defined for T. However,
>>> then an attempt to compare for equality std::vector<T>  will generate a
>>> compiler error in this case because there is no == for T.
>>>
> <snip>
>>>
>>> Shall has_equal_to<  std::vector<T>  >  be smarter and return false_ if
>>> T does not have == ? (If not, this case should at least be documented.)
>>>
>>>
>> I'd venture to say this is a deficiency in std::vector's implementation
>> of
>> operator==, which ideally should SFINAE-out bindings of T which do not
>> have
>> an operator==.
> <snip>
>>
>> In my opinion, the upshot is that std::vector's operator== is almost
>> certainly not a special case, so I don't know know if it's worth
>> specializing has_equal_to on it or not.  Maybe, if it's consistently done
>> for all problematic std-defined operators?
>>
> Hi,
> 
> I found the same kind of issues while implementing 
> is_constructible/is_assignable/is_convertible for Boost.Conversion. The 
> option I taken was to specialize these traits for the c++ 
> implementations that were not using SFINAE. This force of course the 
> user to include the specific file containing the specialization.
> 

Would the Boost.TypeTraits authors consider adding the specializations* to
workaround STL not using SFINAE? If not, can these cases be documented?

(*) In this case it'd be nice to name the headers containing the extra
specialization consistently with other Boost libraries like
Boost.Conversion-- Vicente, how did you name the headers containing the
specialization for the STL types?

Thanks.
--Lorenzo

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