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

   1 /*!
   2 @file
   3 Defines a replacement for `std::decay`, which is sometimes too slow at
   4 compile-time.
   5
   6 @copyright Louis Dionne 2013-2016
   7 Distributed under the Boost Software License, Version 1.0.
   8 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   9  */
  10
  11 #ifndef BOOST_HANA_DETAIL_DECAY_HPP
  12 #define BOOST_HANA_DETAIL_DECAY_HPP
  13
  14 #include <boost/hana/config.hpp>
  15
  16 #include <type_traits>
  17
  18
  19 BOOST_HANA_NAMESPACE_BEGIN namespace detail {
  20     //! @ingroup group-details
  21     //! Equivalent to `std::decay`, except faster.
  22     //!
  23     //! `std::decay` in libc++ is implemented in a suboptimal way. Since
  24     //! this is used literally everywhere by the `make<...>` functions, it
  25     //! is very important to keep this as efficient as possible.
  26     //!
  27     //! @note
  28     //! `std::decay` is still being used in some places in the library.
  29     //! Indeed, this is a peephole optimization and it would not be wise
  30     //! to clutter the code with our own implementation of `std::decay`,
  31     //! except when this actually makes a difference in compile-times.
  32     template <typename T, typename U = typename std::remove_reference<T>::type>
  33     struct decay {
  34         using type = typename std::remove_cv<U>::type;
  35     };
  36
  37     template <typename T, typename U>
  38     struct decay<T, U[]> { using type = U*; };
  39     template <typename T, typename U, std::size_t N>
  40     struct decay<T, U[N]> { using type = U*; };
  41
  42     template <typename T, typename R, typename ...A>
  43     struct decay<T, R(A...)> { using type = R(*)(A...); };
  44     template <typename T, typename R, typename ...A>
  45     struct decay<T, R(A..., ...)> { using type = R(*)(A..., ...); };
  46 } BOOST_HANA_NAMESPACE_END
  47
  48 #endif // !BOOST_HANA_DETAIL_DECAY_HPP