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authorDongHun Kwak <dh0128.kwak@samsung.com>2017-09-13 11:08:07 +0900
committerDongHun Kwak <dh0128.kwak@samsung.com>2017-09-13 11:09:00 +0900
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Imported Upstream version 1.64.0upstream/1.64.0
Change-Id: Id9212edd016dd55f21172c427aa7894d1d24148b Signed-off-by: DongHun Kwak <dh0128.kwak@samsung.com>
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+/*!
+@file
+Defines `boost::hana::detail::ebo`.
+
+@copyright Louis Dionne 2013-2016
+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
+
+BOOST_HANA_NAMESPACE_BEGIN
+ namespace detail {
+ using ::_hana::ebo;
+ using ::_hana::ebo_get;
+ }
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_DETAIL_EBO_HPP