From: daniel.kruegler@googlemail.com   
      
   [3rd attempt after 2 days]   
      
   On 2012-11-16 10:13, Zhihao Yuan wrote:   
     > Non-throw swapable is quite useful, and it can be more useful if the   
     > concept can be tested.  If we already have an object of type T, I   
     > think the following one is enough:   
     >   
     > template    
     > constexpr bool is_nothrow_swapable(T& t) {   
     >     using std::swap;   
     >     return noexcept(swap(t, t)) or   
     >         (std::is_nothrow_move_constructible::value and   
     >          std::is_nothrow_move_assignable::value);   
     > }   
     >   
     > However, I want it to be able to work on just a type, to work as an   
     > integral_constant.  Is that doable?  Comments on how to implement an   
     > is_swapable are also welcome.  Thanks.   
      
   I'm not sure whether I understood your question correctly, because you   
   mention (a) the ability to work on just a type and (b) you refer to   
   is_swapable (instead of is_nothrow_swapable). I guess that (b) is just a   
   thinko on your side. What about a variation of your constexpr function   
   so that it does not depend on any argument anymore:   
      
   template    
   constexpr bool is_nothrow_swappable() {   
       using std::swap;   
       return noexcept(swap(std::declval(), std::declval())) or   
             (std::is_nothrow_move_constructible::value and   
              std::is_nothrow_move_assignable::value);   
   }   
      
   template   
   struct is_nothrow_swappable_trait :   
       std::integral_constant()>   
   {   
   };   
      
   ?   
      
   Further, I think that the additional parts   
      
      or   
           (std::is_nothrow_move_constructible::value and   
            std::is_nothrow_move_assignable::value)   
      
   are not really helpful, because they should automatically be deduced   
   correctly from   
      
   #include    
   #include  // std::swap templates   
      
   template    
   constexpr bool is_nothrow_swappable() {   
       using std::swap;   
       return noexcept(swap(std::declval(), std::declval()));   
   }   
      
   template   
   struct is_nothrow_swappable_trait :   
       std::integral_constant()>   
   {   
   };   
      
   HTH & Greetings from Bremen,   
      
   Daniel Krügler   
      
      
   --   
         [ See http://www.gotw.ca/resources/clcm.htm for info about ]   
         [ comp.lang.c++.moderated.    First time posters: Do this! ]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)