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3rdparty/boost_1_81_0/boost/hana/detail/ebo.hpp 3.8 KB
63e88f80   Hu Chunming   提交三方库
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  /*!
  @file
  Defines `boost::hana::detail::ebo`.
  
  Copyright Louis Dionne 2013-2022
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   */
  
  #ifndef BOOST_HANA_DETAIL_EBO_HPP
  #define BOOST_HANA_DETAIL_EBO_HPP
  
  #include <boost/hana/config.hpp>
  #include <boost/hana/detail/intrinsics.hpp>
  
  
  namespace _hana {
      //////////////////////////////////////////////////////////////////////////
      // ebo<K, V>
      //
      // Building block to implement the Empty Base Optimization (EBO). We
      // use a short name and define it in a short namespace to reduce
      // symbol lengths, since this type is used as a building block for
      // other widely used types such as `hana::pair`.
      //
      // When available, we use compiler intrinsics to reduce the number
      // of instantiations.
      //
      // `ebo` provides a limited set of constructors to reduce instantiations.
      // Also, the constructors are open-ended and they do not check for the
      // validity of their arguments, again to reduce compile-time costs.
      // Users of `ebo` should make sure that they only try to construct an
      // `ebo` from a compatible value.
      //
      // EBOs can be indexed using an arbitrary type. The recommended usage is
      // to define an integrap constant wrapper for the specific container using
      // EBO, and then index using that wrapper:
      //
      //      template <int> struct idx; // wrapper for tuple
      //      template <typename ...T>
      //      struct tuple : ebo<idx<0>, T0>, ebo<idx<1>, T1>, ... { };
      //
      // The reason for defining one wrapper per container is to avoid any issues
      // that can arise when using `ebo_get`, which casts to the base class. If
      // `tuple` and `pair` are inheritting from `ebo`s with the same indexing
      // scheme, trying to use `ebo_get` on a tuple of pairs will trigger an
      // ambiguous base class conversion, since both tuple and pair inherit
      // from `ebo`s with the same keys.
      //////////////////////////////////////////////////////////////////////////
      template <typename K, typename V, bool =
          BOOST_HANA_TT_IS_EMPTY(V) && !BOOST_HANA_TT_IS_FINAL(V)
      >
      struct ebo;
  
      // Specialize storage for empty types
      template <typename K, typename V>
      struct ebo<K, V, true> : V {
          constexpr ebo() { }
  
          template <typename T>
          explicit constexpr ebo(T&& t)
              : V(static_cast<T&&>(t))
          { }
      };
  
      // Specialize storage for non-empty types
      template <typename K, typename V>
      struct ebo<K, V, false> {
          constexpr ebo() : data_() { }
  
          template <typename T>
          explicit constexpr ebo(T&& t)
              : data_(static_cast<T&&>(t))
          { }
  
          V data_;
      };
  
      //////////////////////////////////////////////////////////////////////////
      // ebo_get
      //////////////////////////////////////////////////////////////////////////
      template <typename K, typename V>
      constexpr V const& ebo_get(ebo<K, V, true> const& x)
      { return x; }
  
      template <typename K, typename V>
      constexpr V& ebo_get(ebo<K, V, true>& x)
      { return x; }
  
      template <typename K, typename V>
      constexpr V&& ebo_get(ebo<K, V, true>&& x)
      { return static_cast<V&&>(x); }
  
  
      template <typename K, typename V>
      constexpr V const& ebo_get(ebo<K, V, false> const& x)
      { return x.data_; }
  
      template <typename K, typename V>
      constexpr V& ebo_get(ebo<K, V, false>& x)
      { return x.data_; }
  
      template <typename K, typename V>
      constexpr V&& ebo_get(ebo<K, V, false>&& x)
      { return static_cast<V&&>(x.data_); }
  } // end namespace _hana
  
  namespace boost { namespace hana {
      namespace detail {
          using ::_hana::ebo;
          using ::_hana::ebo_get;
      }
  }} // end namespace boost::hana
  
  #endif // !BOOST_HANA_DETAIL_EBO_HPP