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 type alias,
      11              :  * or if not present, assign a default type.
      12              :  *
      13              :  * \warning Please use defaults responsibly.  In many cases it is
      14              :  * better to require the type alias 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_TYPE_ALIAS_OR_DEFAULT(ALIAS_NAME)                           \
      19              :   template <typename CheckingType, typename Default, typename = std::void_t<>> \
      20              :   struct get_##ALIAS_NAME##_or_default {                                       \
      21              :     using type = Default;                                                      \
      22              :   };                                                                           \
      23              :   template <typename CheckingType, typename Default>                           \
      24              :   struct get_##ALIAS_NAME##_or_default<                                        \
      25              :       CheckingType, Default, std::void_t<typename CheckingType::ALIAS_NAME>> { \
      26              :     using type = typename CheckingType::ALIAS_NAME;                            \
      27              :   };                                                                           \
      28              :   template <typename CheckingType, typename Default>                           \
      29              :   using get_##ALIAS_NAME##_or_default_t =                                      \
      30              :       typename get_##ALIAS_NAME##_or_default<CheckingType, Default>::type;