ceph/src/boost/boost/hana/detail/ebo.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::detail::ebo`.
   4
   5 @copyright Louis Dionne 2013-2016
   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     constexpr V const& ebo_get(ebo<K, V, true> const& x)
  84     { 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 BOOST_HANA_NAMESPACE_BEGIN
 109     namespace detail {
 110         using ::_hana::ebo;
 111         using ::_hana::ebo_get;
 112     }
 113 BOOST_HANA_NAMESPACE_END
 114
 115 #endif // !BOOST_HANA_DETAIL_EBO_HPP