ceph/src/Beast/include/beast/core/detail/empty_base_optimization.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_EMPTY_BASE_OPTIMIZATION_HPP
   9 #define BEAST_DETAIL_EMPTY_BASE_OPTIMIZATION_HPP
  10
  11 #include <type_traits>
  12 #include <utility>
  13
  14 namespace beast {
  15 namespace detail {
  16
  17 template<class T>
  18 struct empty_base_optimization_decide
  19     : std::integral_constant <bool,
  20         std::is_empty <T>::value
  21 #ifdef __clang__
  22         && !__is_final(T)
  23 #endif
  24     >
  25 {
  26 };
  27
  28 template<
  29     class T,
  30     int UniqueID = 0,
  31     bool ShouldDeriveFrom =
  32         empty_base_optimization_decide<T>::value
  33 >
  34 class empty_base_optimization : private T
  35 {
  36 public:
  37     empty_base_optimization() = default;
  38
  39     empty_base_optimization(T const& t)
  40         : T (t)
  41     {}
  42
  43     empty_base_optimization(T&& t)
  44         : T (std::move (t))
  45     {}
  46
  47     T& member() noexcept
  48     {
  49         return *this;
  50     }
  51
  52     T const& member() const noexcept
  53     {
  54         return *this;
  55     }
  56 };
  57
  58 //------------------------------------------------------------------------------
  59
  60 template<
  61     class T,
  62     int UniqueID
  63 >
  64 class empty_base_optimization <T, UniqueID, false>
  65 {
  66 public:
  67     empty_base_optimization() = default;
  68
  69     empty_base_optimization(T const& t)
  70         : m_t (t)
  71     {}
  72
  73     empty_base_optimization(T&& t)
  74         : m_t (std::move (t))
  75     {}
  76
  77     T& member() noexcept
  78     {
  79         return m_t;
  80     }
  81
  82     T const& member() const noexcept
  83     {
  84         return m_t;
  85     }
  86
  87 private:
  88     T m_t;
  89 };
  90
  91 } // detail
  92 } // beast
  93
  94 #endif