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/*=============================================================================
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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