Re: [boost] [type_traits] common_type and SFINAE

Subject: Re: [boost] [type_traits] common_type and SFINAE
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2012-10-05 14:46:48

On Fri, Oct 5, 2012 at 11:36 AM, Matt Calabrese <rivorus_at_[hidden]> wrote:
> On Fri, Oct 5, 2012 at 2:08 PM, Lorenzo Caminiti <lorcaminiti_at_[hidden]>wrote:
>
>> Hello all,
>>
>> One a separate note, why the following SFINAE impl of
>> EqualityComparable returns 0 on std::common_type<int, int>::type?
>>
>>     EqualityComparable<int>::value // 1 -- ok
>>     EqualityComparable<std::common_type<int>::type>::value // 1 -- ok
>>
>>     EqualityComparable<std::common_type<int, int>::type>::value // 0 --
>> why??
>
>
> common_type< int, int >::type would be int&&. In your EqualityComparable
> implementation,  your function parameter types are the argument of
> EqualityComparable directly, which is an r-value reference type. In your
> explicit call to check<_1>, you are passing "a" and "b" directly, but the
> parameter types of check expect r-value references. In order for this to
> work, you'd need to use std::forward.

I see, thanks!

--Lorenzo