utl/include/utl/meta/sfinae.h

/*!
 * \file    use.h
 * \brief   Template meta-programming SFINAE helpers
 *
 * Copyright (C) 2018 Christos Choutouridis
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation, either version 3
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
#ifndef __utl_meta_use_h__
#define __utl_meta_use_h__

#include <utl/impl/impl.h>
#include <utl/meta/logical.h>


/*!
 * \ingroup meta
 * \defgroup use
 * conditional use support header. This is a SFINAE helper
 */
//! @{
namespace utl {

   /*!
    * void_t
    * Utility meta-function that maps a sequence of any types to the type void
    * \note The idea is:
    *    template <typename ...>
    *    using void_t = void;
    *
    * Until CWG 1558 (a C++14 defect), unused parameters in alias templates were not
    * guaranteed to ensure SFINAE and could be ignored, so earlier compilers require
    * a more complex definition of void_t, such as the following implementation
    * https://en.cppreference.com
    */
   //! @{
   template<typename... _Ts>
   struct void_t_impl {
      typedef void type;
   };
   //! The actual void_t type alias
   template<typename... _Ts>
   using void_t = typename void_t_impl<_Ts...>::type;
   //! @}


   //! If same type resolves to _Ret, else SFINAE
   //! @{
   template <typename _T1, typename _T2, typename _Ret =_T1>
   struct use_if_same_ {
      using type = if_t <
                      same_<_T1, _T2>::value,
                      _Ret
                   >;
   };
   template <typename _T1, typename _T2, typename _Ret =_T1>
   using use_if_same_t = typename use_if_same_<_T1, _T2, _Ret>::type;
   //! @}


   //! If not same type resolves to _Ret, else SFINAE
   //! @{
   template <typename _T1, typename _T2, typename _Ret =_T1>
   struct use_if_not_same_ {
      using type = if_t <
                      !same_<_T1, _T2>::value,
                      _Ret
                   >;
   };
   template <typename _T1, typename _T2, typename _Ret =_T1>
   using use_if_not_same_t = typename use_if_not_same_<_T1, _T2, _Ret>::type;
   //! @}


   //! If any type (_T1 or _T2) type resolves to _Ret, else to SFINAE
   //! @{
   template <typename _T1, typename _T2, typename _Ret =_T1>
   struct use_if_any_ {
      using type = if_t <
                      or_<_T1, _T2>::value,
                      _Ret
                   >;
   };
   template <typename _T1, typename _T2, typename _Ret =_T1>
   using use_if_any_t = typename use_if_any_<_T1, _T2, _Ret>::type;
   //! @}


   //! If both type (_T1 and _T2) type resolves to _Ret, else to SFINAE
   //! @{
   template <typename _T1, typename _T2, typename _Ret =_T1>
   struct use_if_both_ {
      using type = if_t <
                      and_<_T1, _T2>::value,
                      _Ret
                   >;
   };
   template <typename _T1, typename _T2, typename _Ret =_T1>
   using use_if_both_t = typename use_if_both_<_T1, _T2, _Ret>::type;
   //! @}

   //! Publicly recognized alias template for if_t
   template<bool _C, typename _Tp = void>
   using enable_if_t = typename if_<_C, _Tp>::type;

   //! Uniform use_ style alias template for if_t
   //!@{
   template<bool _C, typename _Tp = void>
   using use_if_t = typename if_<_C, _Tp>::type;
   //!@{
}

//! @}
#endif /* __utl_meta_use_h__ */