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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
|
