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Subject: Re: [boost] [type_traits] common_type and SFINAE
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-10-06 15:46:04

On Thu, Oct 4, 2012 at 8:56 PM, Jeffrey Lee Hellrung, Jr. <
jeffrey.hellrung_at_[hidden]> wrote:

> On Thu, Oct 4, 2012 at 9:15 AM, Vicente J. Botet Escriba <
> vicente.botet_at_[hidden]> wrote:
>> Le 04/10/12 15:02, Jeffrey Lee Hellrung, Jr. a écrit :
>>> On Thu, Oct 4, 2012 at 12:28 AM, Lorenzo Caminiti <lorcaminiti_at_[hidden]
>>> >wrote:
>>> Hello all,
>>>> common_type<T, U>::type is defined in a way that gives a hard compiler
>>>> error if T and U do not have a common type (this is also how
>>>> common_type is implemented in C++11).
>>>> An alternative design (I'm not sure if implementable) would have been
>>>> to not define the type member in case T and U have no common type.
>>>> This way I could use SFINAE to detect if common_type<T, U>::type does
>>>> not exist and therefore if T and U have not common type -- so to
>>>> implement N3351's concept Common<T, U>.
>>>> Is there a way to implement this alternative design?
>>>> Do you also need this in C++03?
>>> Even if Lorenzo doesn't needs it, it will be great to have it in Boost.
>> Jeffrey, do you have something that is working already?
> I don't yet, but this should be fairly straightforward based on the
> present C++03 implementation. I guess I can try to get this in this weekend?

Never mind, I think I had misremembered what the implementation looks like;
I don't see a straightforward way to effect this in the C++03
implementation :(

- Jeff

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