utl/include/utl/meta/detection.h

/*!
 * \file    detection.h
 * \brief   Detection idiom based on WG21's N4502[1] from Walter E. Brown
 *
 * [1]: www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4502.pdf
 *
 * Copyright (C) 2018-2019 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_detection_h__
#define __utl_meta_detection_h__

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

/*!
 * \ingroup meta
 * \defgroup detection
 * Detection idiom support header.
 */
//! @{

namespace utl {
namespace meta {

   /*!
    * void_t meta-function that maps a sequence of any types to the type void
    */
   //! @{
   #if defined(UTL_WORKAROUND_CWG_1558)
      template<typename... _Ts>
      struct void_ {
         using type = void;
      };
      //! void_t type alias
      template<typename... _Ts>
      using void_t = eval<void_<_Ts...>>;
   #else
      //! void_ type alias
      template <typename...> using void_  = void;
      //! void_t type alias
      template <typename...> using void_t = void;
   #endif
   //! @}

   /*!
    * Not a type to use in detected idiom
    */
   struct nat_ {
      nat_()  = delete;
      ~nat_() = delete;
      nat_(nat_ const&)         = delete;
      void operator = (nat_ const&) = delete;
   };

   //! Detector for detection idiom
   //! @{
   namespace detail {
      template <typename Default,
                typename AlwaysVoid,
                template<typename...> class Op, typename... Args>
      struct detector {
         using detected = false_;
         using type = Default;
      };

      template <typename Default,
                template<typename...> class Op, typename... Args>
      struct detector <Default, void_t<Op<Args...>>, Op, Args...> {
         using detected = true_;
         using type = Op<Args...>;
      };

      //! helper for detected_or_t
      template <typename Default,
                template<typename...> class Op, typename... Args>
      using detected_or = detector<Default, void, Op, Args...>;
   } // namespace detail
   //! @}

   /*!
    * detection interface
    */
   //! @{

   /*!
    *  Checks if Op<Args...> is a valid expression without evaluating it.
    *
    *  \param  Op       a meta-callback function to pass Args...
    *  \param  Args...  types to pass to Op for checking
    *  \return status of the operation [bool_]
    *    \arg  true_    if Op<Args...> is valid expression
    *    \arg  false_   if Op<Args...> is not valid
    *
    * \example
    * \code
    *    // archetypal alias for a copy assignment operation
    *    template< class T >
    *    using copy_assign_t = decltype( declval<T&>() = declval<T const &>() );
    *
    *    template< class T >
    *    using is_copy_assignable = is_detected< T, copy_assign_t >;
    * \endcode
    */
   template <template<typename...> class Op, typename... Args>
   using is_detected = typename detail::detector<nat_, void, Op, Args...>::detected;

   //! Detection predicate
   template< template<typename...> class Op, typename... Args>
   constexpr bool is_detected_v = is_detected<Op, Args...>::value;
   //! @}

   /*!
    * Detection idiom toolkit
    */
   //! @{

   //! evaluates to evaluation of Op<Args...> if it's valid and to nat_ if not
   template <template<typename...> class Op, typename... Args>
   using detected_t = eval <
      detail::detector<nat_, void, Op, Args...>
   >;

   //! evaluates to evaluation of Op<Args...> if it's valid and to \p Default if not
   template <typename Default,
             template<typename...> class Op, typename... Args>
   using detected_or_t = eval <
      detail::detected_or<Default, Op, Args...>
   >;

   //! evaluates to true_ if evaluation of Op<Args...> is \p Expected and to false_ if not
   template <typename Expected,
             template<typename...> class Op, typename... Args >
   using is_detected_exact = same_<Expected, detected_t<Op, Args...> >;

   //! evaluates to true if evaluation of Op<Args...> is \p Expected and to false if not
   template <typename Expected,
             template<typename...> class Op, typename... Args >
   constexpr bool is_detected_exact_v = is_detected_exact< Expected, Op, Args...>::value;

   //! evaluates to true_ if evaluation of Op<Args...> is convertible to \p To
   //! and to false_ if not
   template <typename To,
             template<typename...> class Op, typename... Args >
   using is_detected_convertible = std::is_convertible< detected_t<Op, Args...>, To >;

   //! evaluates to true if evaluation of Op<Args...> is convertible to \p To
   //! and to false if not
   template <typename To,
             template<typename...> class Op, typename... Args >
   constexpr bool is_detected_convertible_v =
      is_detected_convertible<To, Op, Args...>::value;
   //! @}

}}
//!@}

#endif /* __utl_meta_detection_h__ */