94 lines
3.4 KiB
C++
94 lines
3.4 KiB
C++
/*!
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@file
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Defines `boost::hana::eval_if`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_EVAL_IF_HPP
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#define BOOST_HANA_EVAL_IF_HPP
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#include <boost/hana/fwd/eval_if.hpp>
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#include <boost/hana/bool.hpp>
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#include <boost/hana/concept/constant.hpp>
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#include <boost/hana/concept/logical.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/dispatch.hpp>
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#include <boost/hana/eval.hpp>
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#include <boost/hana/if.hpp>
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#include <type_traits>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @cond
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template <typename Cond, typename Then, typename Else>
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constexpr decltype(auto) eval_if_t::operator()(Cond&& cond, Then&& then, Else&& else_) const {
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using Bool = typename hana::tag_of<Cond>::type;
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using EvalIf = BOOST_HANA_DISPATCH_IF(eval_if_impl<Bool>,
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hana::Logical<Bool>::value
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);
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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static_assert(hana::Logical<Bool>::value,
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"hana::eval_if(cond, then, else) requires 'cond' to be a Logical");
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#endif
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return EvalIf::apply(static_cast<Cond&&>(cond),
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static_cast<Then&&>(then),
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static_cast<Else&&>(else_));
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}
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//! @endcond
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template <typename L, bool condition>
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struct eval_if_impl<L, when<condition>> : default_ {
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template <typename ...Args>
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static constexpr auto apply(Args&& ...) = delete;
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};
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//////////////////////////////////////////////////////////////////////////
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// Model for arithmetic data types
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//////////////////////////////////////////////////////////////////////////
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template <typename L>
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struct eval_if_impl<L, when<std::is_arithmetic<L>::value>> {
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template <typename Cond, typename T, typename E>
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static constexpr auto apply(Cond const& cond, T&& t, E&& e) {
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return cond ? hana::eval(static_cast<T&&>(t))
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: hana::eval(static_cast<E&&>(e));
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}
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};
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//////////////////////////////////////////////////////////////////////////
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// Model for Constants over a Logical
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//////////////////////////////////////////////////////////////////////////
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template <typename C>
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struct eval_if_impl<C, when<
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hana::Constant<C>::value &&
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Logical<typename C::value_type>::value
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>> {
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template <typename Then, typename Else>
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static constexpr decltype(auto)
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eval_if_helper(hana::true_, Then&& t, Else&&)
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{ return hana::eval(static_cast<Then&&>(t)); }
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template <typename Then, typename Else>
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static constexpr decltype(auto)
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eval_if_helper(hana::false_, Then&&, Else&& e)
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{ return hana::eval(static_cast<Else&&>(e)); }
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template <typename Cond, typename Then, typename Else>
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static constexpr decltype(auto) apply(Cond const&, Then&& t, Else&& e) {
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constexpr auto cond = hana::value<Cond>();
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constexpr bool truth_value = hana::if_(cond, true, false);
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return eval_if_helper(hana::bool_c<truth_value>,
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static_cast<Then&&>(t),
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static_cast<Else&&>(e));
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}
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_EVAL_IF_HPP
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