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Editing: is_unpackable.hpp
/*============================================================================= Copyright (c) 2016 Paul Fultz II is_unpackable.hpp Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) ==============================================================================*/ #ifndef BOOST_HOF_GUARD_IS_UNPACKABLE_HPP #define BOOST_HOF_GUARD_IS_UNPACKABLE_HPP /// is_unpackable /// ============= /// /// This is a trait that can be used to detect whether the type can be called /// with `unpack`. /// /// Synopsis /// -------- /// /// template<class T> /// struct is_unpackable; /// /// Example /// ------- /// /// #include <boost/hof.hpp> /// #include <cassert> /// /// int main() { /// static_assert(boost::hof::is_unpackable<std::tuple<int>>::value, "Failed"); /// } /// #include <boost/hof/unpack_sequence.hpp> #include <boost/hof/is_invocable.hpp> #include <boost/hof/always.hpp> #include <boost/hof/detail/static_const_var.hpp> #include <boost/hof/detail/unpack_tuple.hpp> namespace boost { namespace hof { namespace detail { struct unpack_impl_f { template<class F, class Sequence> constexpr auto operator()(F&& f, Sequence&& s) const BOOST_HOF_RETURNS ( boost::hof::unpack_sequence<typename std::remove_cv<typename std::remove_reference<Sequence>::type>::type>:: apply(BOOST_HOF_FORWARD(F)(f), BOOST_HOF_FORWARD(Sequence)(s)) ); }; BOOST_HOF_DECLARE_STATIC_VAR(unpack_impl, unpack_impl_f); #if BOOST_HOF_CHECK_UNPACK_SEQUENCE struct private_unpack_type {}; template<class Sequence> struct unpack_impl_result { static_assert(boost::hof::is_invocable<unpack_impl_f, decltype(boost::hof::always(private_unpack_type())), Sequence>::value, "Unpack is invalid for this sequence. The function used to unpack this sequence is not callable." ); typedef decltype(boost::hof::detail::unpack_impl(boost::hof::always(private_unpack_type()), std::declval<Sequence>())) type; }; template<class Sequence> struct is_proper_sequence : std::is_same< private_unpack_type, typename unpack_impl_result<Sequence>::type > {}; #endif template<class Sequence, class=void> struct is_unpackable_impl : std::true_type { #if BOOST_HOF_CHECK_UNPACK_SEQUENCE static_assert(is_proper_sequence<Sequence>::value, "Unpack is invalid for this sequence. The function used to unpack this sequence does not invoke the function." ); #endif }; template<class Sequence> struct is_unpackable_impl<Sequence, typename detail::holder< typename unpack_sequence<Sequence>::not_unpackable >::type> : std::false_type {}; } template<class Sequence> struct is_unpackable : detail::is_unpackable_impl< typename std::remove_cv<typename std::remove_reference<Sequence>::type>::type > { #if BOOST_HOF_CHECK_UNPACK_SEQUENCE typedef detail::is_unpackable_impl< typename std::remove_cv<typename std::remove_reference<Sequence>::type>::type > base; typedef std::conditional<base::value, detail::is_proper_sequence<Sequence>, std::true_type> check; static_assert(check::type::value, "Unpack is invalid for this sequence. The function used to unpack this sequence does not invoke the function." ); #endif }; }} // namespace boost::hof #endif
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