utl/include/utl/meta/detect.h

/*!
 * \file    utl/meta/detect.h
 * \brief   Template meta-programming void 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_detect_h__
#define __utl_meta_detect_h__

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

/*!
 * \ingroup meta
 * \defgroup detect
 * Implementation of the detection idiom
 */
//! @{

namespace utl {

   //! detection idiom (negative case).
   template <typename _Def,
             typename _AlwaysVoid,
             template<typename...> class _Op,
             typename... _Args>
   struct detector_ {
      using value_ = false_;
      using type = _Def;
   };

   //! detection idiom (positive case).
   template <typename _Def,
             template<typename...> class _Op,
             typename... _Args>
   struct detector_ <_Def, void_t<_Op<_Args...>>, _Op, _Args...> {
      using value_ = true_;
      using type = _Op<_Args...>;
   };

   // Detect whether _Op<_Args...> is a valid type, use _Def if not.
   template <typename _Def,
             template<typename...> class _Op,
             typename... _Args>
   using detected_or_ = detector_ <_Def, void, _Op, _Args...>;

   // template alias
   template <typename _Def,
             template<typename...> class _Op,
             typename... _Args>
   using detected_or_t = typename detected_or_<_Def, _Op, _Args...>::type;



}
//! @}
#endif /* __utl_meta_detect_h__ */