Re: [boost] [type_traits] Big changes in develop

Subject: Re: [boost] [type_traits] Big changes in develop
From: John Maddock (jz.maddock_at_[hidden])
Date: 2015-05-30 03:31:15

On 29/05/2015 21:18, Peter Dimov wrote:

>> And while you're on a roll.... can you see how to fix 
>> https://svn.boost.org/trac/boost/ticket/7544? common_type should have 
>> member ::type only when such a common type exists.
>
> On C++11 or on C++03?
>
> On C++11, it's as easy as
>
> template<class...> struct common_type
> {
> };
>
> template<class T> struct common_type: decay<T>
> {
> };
>
> template<class T1, class T2, class... T,
>    class R = decltype(declval<bool>()? declval<T1>(): declval<T2>())>
>
> struct common_type<T1, T2, T...>: common_type<typename decay<R>::type, 
> T...>
> {
> };
>
> On C++03... I don't know, is there a point? It needs expression SFINAE 
> and default template parameters for function templates and will 
> probably not compile on most non-C++11 compilers anyway. Might be 
> useful in principle on clang/gcc in C++03 mode, but is this an 
> important target?
>
Probably not, I suspect C++11 only support is fine.  The above is really 
neat BTW, I tried something similar in VC++ and couldn't get it to work :(

John.