ceph/src/Beast/include/beast/core/detail/get_lowest_layer.hpp

   1 //
   2 // Copyright (c) 2013-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
   3 //
   4 // Distributed under the Boost Software License, Version 1.0. (See accompanying
   5 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   6 //
   7
   8 #ifndef BEAST_DETAIL_GET_LOWEST_LAYER_HPP
   9 #define BEAST_DETAIL_GET_LOWEST_LAYER_HPP
  10
  11 #include <type_traits>
  12
  13 namespace beast {
  14 namespace detail {
  15
  16 template<class T>
  17 class has_lowest_layer
  18 {
  19     template<class U, class R =
  20         typename U::lowest_layer_type>
  21     static std::true_type check(int);
  22     template<class>
  23     static std::false_type check(...);
  24     using type = decltype(check<T>(0));
  25 public:
  26     static bool constexpr value = type::value;
  27 };
  28
  29 template<class T, bool B>
  30 struct maybe_get_lowest_layer
  31 {
  32     using type = T;
  33 };
  34
  35 template<class T>
  36 struct maybe_get_lowest_layer<T, true>
  37 {
  38     using type = typename T::lowest_layer_type;
  39 };
  40
  41 // If T has a nested type lowest_layer_type,
  42 // returns that, else returns T.
  43 template<class T>
  44 struct get_lowest_layer
  45 {
  46     using type = typename maybe_get_lowest_layer<T,
  47         has_lowest_layer<T>::value>::type;
  48 };
  49
  50 } // detail
  51 } // beast
  52
  53 #endif