utl/include/utl/meta/sfinae.h

/*!
 * \file    sfinae.h
 * \brief   Template meta-programming SFINAE helpers
 */
#ifndef __utl_meta_sfinae_h__
#define __utl_meta_sfinae_h__

#include <utl/core/impl.h>
#include <type_traits>

/*!
 * \ingroup meta
 * \defgroup sfinae SFINAE
 * conditional use support header.
 */
//! @{
namespace utl {
namespace meta {

   //! \name enable_if from STL
   //! @{

   //! enable_if, imported from stl
   template <bool If, typename _Tp = void>   using enable_if = std::enable_if<If, _Tp>;

   //! alias template for enable_if
   template<bool If, typename _Tp = void>    using enable_if_t = eval< enable_if<If, _Tp> >;

   //! @}

   //! \name when implementation
   //! @{
   namespace details {
//      template <typename... T>
//      struct dev_null { using type = dev_null; };  //< Same as typelist

      template <bool If>  struct when_ { };
      template <>         struct when_<true> { using type = void; };
   }
   //! Tool to enable a partial specialization only if a boolean condition is true.
   //! Well formed only if \p If is true
   template <bool If>
   using when = eval< details::when_<If> >;

//   //! Well formed only if all of \p Ifs are \c true
//   template <bool ...Ifs>
//   using when_all = details::dev_null<
//         when<Ifs>...
//   >;

   //! @}

   //! If same type resolves to _Ret, else SFINAE
   template <typename _T1, typename _T2, typename _Ret =_T1>
   using use_if_same_t = enable_if_t<
      same_<_T1, _T2>::value, _Ret
   >;
   //! If not same type resolves to _Ret, else SFINAE
   template <typename _T1, typename _T2, typename _Ret =_T1>
   using use_if_not_same_t = enable_if_t<
      !same_<_T1, _T2>::value, _Ret
   >;
   //! If any type (_T1 or _T2) type resolves to _Ret, else to SFINAE
   template <typename T1, typename... Ts>
   using use_if_any_t = enable_if_t<
      or_<T1, Ts...>::value, T1
   >;

   //! If both type (_T1 and _T2) type resolves to _Ret, else to SFINAE
   template <typename T1, typename... Ts>
   using use_if_all_t = enable_if_t<
      and_<T1, Ts...>::value, T1
   >;



}}

//! @}

#endif /* __utl_meta_sfinae_h__ */