Re: compile-time cost of alias templates

[Jason Merrill <jason at redhat dot com>]

On 12/18/2011 11:00 AM, Jonathan Wakely wrote:
> Currently in<functional>  we have this helper class to constrain the
> std::bind function template via SFINAE:
>
>    template<typename _Tp>
>      class __is_socketlike
>      {
>        typedef typename decay<_Tp>::type _Tp2;
>      public:
>        static const bool value =
>          is_integral<_Tp2>::value || is_enum<_Tp2>::value;
>      };
>
> I was going to replace the static member with:
>        static const bool value = __or_<is_integral<_Tp2>, is_enum<_Tp2>>::value;
>
> But now that we have alias templates I'm wondering if it would be
> better to write it as:
>
>    template<typename _Tp>
>      using __is_socketlike
>        = __or_<is_integral<typename decay<_Tp>::type>,
>                is_enum<typename decay<_Tp>::type>>;
>
>
> Does the implementation in G++ mean that using an alias template
> avoids instantiating the separate __is_socketlike class template, or
> is there just as much cost in instantiating an alias template as a
> class template?

Instantiating an alias is simpler, aliases are basically transparent. 
However, this change would mean that any mangling exposure of 
__is_socketlike would change.  But that may not be an issue since it's 
an internal type.

Jason