Line data    Source code

       1            0 : // Distributed under the MIT License.
       2              : // See LICENSE.txt for details.
       3              : 
       4              : #pragma once
       5              : 
       6              : #include <type_traits>
       7              : 
       8              : /*!
       9              :  * \ingroup TypeTraitsGroup
      10              :  * \brief Generate a metafunction that will retrieve the specified
      11              :  * `static constexpr` member variable, or, if not present, a default.
      12              :  *
      13              :  * \warning Please use defaults responsibly.  In many cases it is
      14              :  * better to require the static member variable is present so the
      15              :  * functionality isn't hidden.  Also make sure to document the
      16              :  * compile-time interface, e.g. with a \ref protocols "protocol".
      17              :  */
      18            1 : #define CREATE_GET_STATIC_MEMBER_VARIABLE_OR_DEFAULT(VARIABLE_NAME)          \
      19              :   template <typename CheckingType, auto Default, typename = std::void_t<>>   \
      20              :   struct get_##VARIABLE_NAME##_or_default {                                  \
      21              :     static constexpr auto value = Default;                                   \
      22              :   };                                                                         \
      23              :   template <typename CheckingType, auto Default>                             \
      24              :   struct get_##VARIABLE_NAME##_or_default<                                   \
      25              :       CheckingType, Default,                                                 \
      26              :       std::void_t<decltype(CheckingType::VARIABLE_NAME)>> {                  \
      27              :     static_assert(                                                           \
      28              :         std::is_same_v<std::decay_t<decltype(Default)>,                      \
      29              :                        std::decay_t<decltype(CheckingType::VARIABLE_NAME)>>, \
      30              :         "Types of default and found value are not the same.");               \
      31              :     static constexpr auto value = CheckingType::VARIABLE_NAME;               \
      32              :   };                                                                         \
      33              :   template <typename CheckingType, auto Default>                             \
      34              :   constexpr auto get_##VARIABLE_NAME##_or_default_v =                        \
      35              :       get_##VARIABLE_NAME##_or_default<CheckingType, Default>::value;