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/*!
@file
Defines `boost::hana::eval_if`.
@copyright Louis Dionne 2013-2017
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_<truth_value>{},
static_cast<Then&&>(t),
static_cast<Else&&>(e));
}
};
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_EVAL_IF_HPP
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