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Diffstat (limited to 'boost/hana/fwd/back.hpp')
-rw-r--r-- | boost/hana/fwd/back.hpp | 48 |
1 files changed, 48 insertions, 0 deletions
diff --git a/boost/hana/fwd/back.hpp b/boost/hana/fwd/back.hpp new file mode 100644 index 0000000000..197120aff7 --- /dev/null +++ b/boost/hana/fwd/back.hpp @@ -0,0 +1,48 @@ +/*! +@file +Forward declares `boost::hana::back`. + +@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_FWD_BACK_HPP +#define BOOST_HANA_FWD_BACK_HPP + +#include <boost/hana/config.hpp> +#include <boost/hana/core/when.hpp> + + +BOOST_HANA_NAMESPACE_BEGIN + //! Returns the last element of a non-empty and finite iterable. + //! @ingroup group-Iterable + //! + //! Given a non-empty and finite iterable `xs` with a linearization of + //! `[x1, ..., xN]`, `back(xs)` is equal to `xN`. Equivalently, `back(xs)` + //! must be equivalent to `at_c<N-1>(xs)`, and that regardless of the + //! value category of `xs` (`back` must respect the reference semantics + //! of `at`). + //! + //! + //! Example + //! ------- + //! @include example/back.cpp +#ifdef BOOST_HANA_DOXYGEN_INVOKED + constexpr auto back = [](auto&& xs) -> decltype(auto) { + return tag-dispatched; + }; +#else + template <typename It, typename = void> + struct back_impl : back_impl<It, when<true>> { }; + + struct back_t { + template <typename Xs> + constexpr decltype(auto) operator()(Xs&& xs) const; + }; + + constexpr back_t back{}; +#endif +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_FWD_BACK_HPP |