62 lines
1.8 KiB
C++
62 lines
1.8 KiB
C++
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/*!
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@file
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Defines `boost::hana::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_IF_HPP
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#define BOOST_HANA_IF_HPP
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#include <boost/hana/fwd/if.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_if.hpp>
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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) 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 If = BOOST_HANA_DISPATCH_IF(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::if_(cond, then, else) requires 'cond' to be a Logical");
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#endif
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return If::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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namespace detail {
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template <typename T>
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struct hold {
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T value;
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constexpr T&& operator()() && { return static_cast<T&&>(value); }
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};
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}
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template <typename L, bool condition>
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struct if_impl<L, when<condition>> : default_ {
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template <typename C, typename T, typename E>
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static constexpr auto apply(C&& c, T&& t, E&& e) {
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return hana::eval_if(static_cast<C&&>(c),
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detail::hold<T&&>{static_cast<T&&>(t)},
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detail::hold<E&&>{static_cast<E&&>(e)}
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);
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}
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_IF_HPP
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