boost/hana/length.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::length`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_LENGTH_HPP
0011 #define BOOST_HANA_LENGTH_HPP
0012
0013 #include <boost/hana/fwd/length.hpp>
0014
0015 #include <boost/hana/concept/foldable.hpp>
0016 #include <boost/hana/config.hpp>
0017 #include <boost/hana/core/dispatch.hpp>
0018 #include <boost/hana/integral_constant.hpp>
0019 #include <boost/hana/unpack.hpp>
0020
0021
0022 namespace boost { namespace hana {
0023     //! @cond
0024     template <typename Xs>
0025     constexpr auto length_t::operator()(Xs const& xs) const {
0026         using S = typename hana::tag_of<Xs>::type;
0027         using Length = BOOST_HANA_DISPATCH_IF(length_impl<S>,
0028             hana::Foldable<S>::value
0029         );
0030
0031     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
0032         static_assert(hana::Foldable<S>::value,
0033         "hana::length(xs) requires 'xs' to be Foldable");
0034     #endif
0035
0036         return Length::apply(xs);
0037     }
0038     //! @endcond
0039
0040     namespace detail {
0041         struct argn {
0042             template <typename ...Xs>
0043             constexpr hana::size_t<sizeof...(Xs)> operator()(Xs const& ...) const
0044             { return {}; }
0045         };
0046     }
0047
0048     template <typename T, bool condition>
0049     struct length_impl<T, when<condition>> : default_ {
0050         template <typename Xs>
0051         static constexpr auto apply(Xs const& xs) {
0052             return hana::unpack(xs, detail::argn{});
0053         }
0054     };
0055 }} // end namespace boost::hana
0056
0057 #endif // !BOOST_HANA_LENGTH_HPP