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+/*!
+@file
+Defines `boost::hana::drop_front_exactly`.
+
+@copyright Louis Dionne 2013-2016
+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_DROP_FRONT_EXACTLY_HPP
+#define BOOST_HANA_DROP_FRONT_EXACTLY_HPP
+
+#include <boost/hana/fwd/drop_front_exactly.hpp>
+
+#include <boost/hana/bool.hpp>
+#include <boost/hana/concept/integral_constant.hpp>
+#include <boost/hana/concept/iterable.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/dispatch.hpp>
+#include <boost/hana/drop_front.hpp>
+#include <boost/hana/integral_constant.hpp>
+#include <boost/hana/is_empty.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+ //! @cond
+ template <typename Xs, typename N>
+ constexpr auto drop_front_exactly_t::operator()(Xs&& xs, N const& n) const {
+ using It = typename hana::tag_of<Xs>::type;
+ using DropFrontExactly = BOOST_HANA_DISPATCH_IF(drop_front_exactly_impl<It>,
+ hana::Iterable<It>::value &&
+ hana::IntegralConstant<N>::value
+ );
+
+ #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+ static_assert(hana::Iterable<It>::value,
+ "hana::drop_front_exactly(xs, n) requires 'xs' to be an Iterable");
+
+ static_assert(hana::IntegralConstant<N>::value,
+ "hana::drop_front_exactly(xs, n) requires 'n' to be an IntegralConstant");
+ #endif
+
+ static_assert(N::value >= 0,
+ "hana::drop_front_exactly(xs, n) requires 'n' to be non-negative");
+
+ return DropFrontExactly::apply(static_cast<Xs&&>(xs), n);
+ }
+
+ template <typename Xs>
+ constexpr auto drop_front_exactly_t::operator()(Xs&& xs) const {
+ return (*this)(static_cast<Xs&&>(xs), hana::size_c<1>);
+ }
+ //! @endcond
+
+ namespace detail {
+ template <typename Xs, typename N>
+ constexpr void check_dfe_overflow(Xs const& xs, N const&, hana::true_) {
+ constexpr bool n_overflew_length = decltype(
+ hana::is_empty(hana::drop_front(xs, hana::size_c<N::value - 1>))
+ )::value;
+ static_assert(!n_overflew_length,
+ "hana::drop_front_exactly(xs, n) requires 'n' to be less than or "
+ "equal to the number of elements in 'xs'");
+ }
+
+ template <typename Xs, typename N>
+ constexpr void check_dfe_overflow(Xs const&, N const&, hana::false_) { }
+ }
+
+ template <typename It, bool condition>
+ struct drop_front_exactly_impl<It, when<condition>> : default_ {
+ template <typename Xs, typename N>
+ static constexpr auto apply(Xs&& xs, N const& n) {
+ auto result = hana::drop_front(static_cast<Xs&&>(xs), n);
+ constexpr bool check_for_overflow =
+ decltype(hana::is_empty(result))::value && N::value != 0;
+
+ detail::check_dfe_overflow(xs, n, hana::bool_c<check_for_overflow>);
+
+ return result; // NRVO applied
+ }
+ };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_DROP_FRONT_EXACTLY_HPP