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+/*!
+@file
+Defines `boost::hana::eval_if`.
+
+@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_EVAL_IF_HPP
+#define BOOST_HANA_EVAL_IF_HPP
+
+#include <boost/hana/fwd/eval_if.hpp>
+
+#include <boost/hana/bool.hpp>
+#include <boost/hana/concept/constant.hpp>
+#include <boost/hana/concept/logical.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/dispatch.hpp>
+#include <boost/hana/eval.hpp>
+#include <boost/hana/if.hpp>
+
+#include <type_traits>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+ //! @cond
+ template <typename Cond, typename Then, typename Else>
+ constexpr decltype(auto) eval_if_t::operator()(Cond&& cond, Then&& then, Else&& else_) const {
+ using Bool = typename hana::tag_of<Cond>::type;
+ using EvalIf = BOOST_HANA_DISPATCH_IF(eval_if_impl<Bool>,
+ hana::Logical<Bool>::value
+ );
+
+ #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+ static_assert(hana::Logical<Bool>::value,
+ "hana::eval_if(cond, then, else) requires 'cond' to be a Logical");
+ #endif
+
+ return EvalIf::apply(static_cast<Cond&&>(cond),
+ static_cast<Then&&>(then),
+ static_cast<Else&&>(else_));
+ }
+ //! @endcond
+
+ template <typename L, bool condition>
+ struct eval_if_impl<L, when<condition>> : default_ {
+ template <typename ...Args>
+ static constexpr auto apply(Args&& ...) = delete;
+ };
+
+ //////////////////////////////////////////////////////////////////////////
+ // Model for arithmetic data types
+ //////////////////////////////////////////////////////////////////////////
+ template <typename L>
+ struct eval_if_impl<L, when<std::is_arithmetic<L>::value>> {
+ template <typename Cond, typename T, typename E>
+ static constexpr auto apply(Cond const& cond, T&& t, E&& e) {
+ return cond ? hana::eval(static_cast<T&&>(t))
+ : hana::eval(static_cast<E&&>(e));
+ }
+ };
+
+ //////////////////////////////////////////////////////////////////////////
+ // Model for Constants over a Logical
+ //////////////////////////////////////////////////////////////////////////
+ template <typename C>
+ struct eval_if_impl<C, when<
+ hana::Constant<C>::value &&
+ Logical<typename C::value_type>::value
+ >> {
+ template <typename Then, typename Else>
+ static constexpr decltype(auto)
+ eval_if_helper(hana::true_, Then&& t, Else&&)
+ { return hana::eval(static_cast<Then&&>(t)); }
+
+ template <typename Then, typename Else>
+ static constexpr decltype(auto)
+ eval_if_helper(hana::false_, Then&&, Else&& e)
+ { return hana::eval(static_cast<Else&&>(e)); }
+
+ template <typename Cond, typename Then, typename Else>
+ static constexpr decltype(auto) apply(Cond const&, Then&& t, Else&& e) {
+ constexpr auto cond = hana::value<Cond>();
+ constexpr bool truth_value = hana::if_(cond, true, false);
+ return eval_if_helper(hana::bool_c<truth_value>,
+ static_cast<Then&&>(t),
+ static_cast<Else&&>(e));
+ }
+ };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_EVAL_IF_HPP