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       1             : /*!
       2             : @file
       3             : Defines `boost::hana::detail::ebo`.
       4             : 
       5             : Copyright Louis Dionne 2013-2022
       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_EBO_HPP
      11             : #define BOOST_HANA_DETAIL_EBO_HPP
      12             : 
      13             : #include <boost/hana/config.hpp>
      14             : #include <boost/hana/detail/intrinsics.hpp>
      15             : 
      16             : 
      17             : namespace _hana {
      18             :     //////////////////////////////////////////////////////////////////////////
      19             :     // ebo<K, V>
      20             :     //
      21             :     // Building block to implement the Empty Base Optimization (EBO). We
      22             :     // use a short name and define it in a short namespace to reduce
      23             :     // symbol lengths, since this type is used as a building block for
      24             :     // other widely used types such as `hana::pair`.
      25             :     //
      26             :     // When available, we use compiler intrinsics to reduce the number
      27             :     // of instantiations.
      28             :     //
      29             :     // `ebo` provides a limited set of constructors to reduce instantiations.
      30             :     // Also, the constructors are open-ended and they do not check for the
      31             :     // validity of their arguments, again to reduce compile-time costs.
      32             :     // Users of `ebo` should make sure that they only try to construct an
      33             :     // `ebo` from a compatible value.
      34             :     //
      35             :     // EBOs can be indexed using an arbitrary type. The recommended usage is
      36             :     // to define an integrap constant wrapper for the specific container using
      37             :     // EBO, and then index using that wrapper:
      38             :     //
      39             :     //      template <int> struct idx; // wrapper for tuple
      40             :     //      template <typename ...T>
      41             :     //      struct tuple : ebo<idx<0>, T0>, ebo<idx<1>, T1>, ... { };
      42             :     //
      43             :     // The reason for defining one wrapper per container is to avoid any issues
      44             :     // that can arise when using `ebo_get`, which casts to the base class. If
      45             :     // `tuple` and `pair` are inheritting from `ebo`s with the same indexing
      46             :     // scheme, trying to use `ebo_get` on a tuple of pairs will trigger an
      47             :     // ambiguous base class conversion, since both tuple and pair inherit
      48             :     // from `ebo`s with the same keys.
      49             :     //////////////////////////////////////////////////////////////////////////
      50             :     template <typename K, typename V, bool =
      51             :         BOOST_HANA_TT_IS_EMPTY(V) && !BOOST_HANA_TT_IS_FINAL(V)
      52             :     >
      53             :     struct ebo;
      54             : 
      55             :     // Specialize storage for empty types
      56             :     template <typename K, typename V>
      57             :     struct ebo<K, V, true> : V {
      58             :         constexpr ebo() { }
      59             : 
      60             :         template <typename T>
      61             :         explicit constexpr ebo(T&& t)
      62             :             : V(static_cast<T&&>(t))
      63             :         { }
      64             :     };
      65             : 
      66             :     // Specialize storage for non-empty types
      67             :     template <typename K, typename V>
      68             :     struct ebo<K, V, false> {
      69             :         constexpr ebo() : data_() { }
      70             : 
      71             :         template <typename T>
      72             :         explicit constexpr ebo(T&& t)
      73             :             : data_(static_cast<T&&>(t))
      74             :         { }
      75             : 
      76             :         V data_;
      77             :     };
      78             : 
      79             :     //////////////////////////////////////////////////////////////////////////
      80             :     // ebo_get
      81             :     //////////////////////////////////////////////////////////////////////////
      82             :     template <typename K, typename V>
      83      116527 :     constexpr V const& ebo_get(ebo<K, V, true> const& x)
      84      116527 :     { return x; }
      85             : 
      86             :     template <typename K, typename V>
      87             :     constexpr V& ebo_get(ebo<K, V, true>& x)
      88             :     { return x; }
      89             : 
      90             :     template <typename K, typename V>
      91             :     constexpr V&& ebo_get(ebo<K, V, true>&& x)
      92             :     { return static_cast<V&&>(x); }
      93             : 
      94             : 
      95             :     template <typename K, typename V>
      96             :     constexpr V const& ebo_get(ebo<K, V, false> const& x)
      97             :     { return x.data_; }
      98             : 
      99             :     template <typename K, typename V>
     100             :     constexpr V& ebo_get(ebo<K, V, false>& x)
     101             :     { return x.data_; }
     102             : 
     103             :     template <typename K, typename V>
     104             :     constexpr V&& ebo_get(ebo<K, V, false>&& x)
     105             :     { return static_cast<V&&>(x.data_); }
     106             : } // end namespace _hana
     107             : 
     108             : namespace boost { namespace hana {
     109             :     namespace detail {
     110             :         using ::_hana::ebo;
     111             :         using ::_hana::ebo_get;
     112             :     }
     113             : }} // end namespace boost::hana
     114             : 
     115             : #endif // !BOOST_HANA_DETAIL_EBO_HPP