ceph/src/boost/libs/hana/include/boost/hana/detail/dispatch_if.hpp

   1 /*!
   2 @file
   3 Defines `BOOST_HANA_DISPATCH_IF`.
   4
   5 @copyright Louis Dionne 2013-2016
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_DETAIL_DISPATCH_IF_HPP
  11 #define BOOST_HANA_DETAIL_DISPATCH_IF_HPP
  12
  13 #include <boost/hana/config.hpp>
  14
  15 #include <type_traits>
  16
  17
  18 BOOST_HANA_NAMESPACE_BEGIN
  19     struct deleted_implementation {
  20         template <typename ...T>
  21         static constexpr auto apply(T&& ...) = delete;
  22     };
  23
  24     //! @ingroup group-details
  25     //! Dispatch to the given implementation method only when a condition is
  26     //! satisfied.
  27     //!
  28     //! If the condition is satisfied, this macro is equivalent to the type
  29     //! `IMPL`. Otherwise, it is equivalent to a type with a deleted static
  30     //! function named `apply`. When a tag-dispatching error happens, the
  31     //! condition should be false and the deleted static function `apply`
  32     //! will prevent the compiler from generating too much garbage.
  33     //!
  34     //! @note
  35     //! When `BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS` is defined, the
  36     //! condition is always ignored and this macro expands to the
  37     //! implementation only.
  38     //!
  39     //! @remark
  40     //! This must be implemented as a macro, because we don't want the
  41     //! condition to be evaluated at all when
  42     //! `BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS` is defined.
  43 #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  44     #define BOOST_HANA_DISPATCH_IF(IMPL, ...)                               \
  45         ::std::conditional_t<                                               \
  46             (__VA_ARGS__),                                                  \
  47             IMPL,                                                           \
  48             ::boost::hana::deleted_implementation                           \
  49         >                                                                   \
  50     /**/
  51 #else
  52     #define BOOST_HANA_DISPATCH_IF(IMPL, ...) IMPL
  53 #endif
  54 BOOST_HANA_NAMESPACE_END
  55
  56 #endif // !BOOST_HANA_DETAIL_DISPATCH_IF_HPP