/*! @file Forward declares `boost::hana::length`. @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_LENGTH_HPP #define BOOST_HANA_FWD_LENGTH_HPP #include <boost/hana/config.hpp> #include <boost/hana/core/when.hpp> BOOST_HANA_NAMESPACE_BEGIN //! Return the number of elements in a foldable structure. //! @ingroup group-Foldable //! //! Given a `Foldable` `xs`, `length(xs)` must return an object of an //! unsigned integral type, or an `IntegralConstant` holding such an //! object, which represents the number of elements in the structure. //! //! @note //! Since only compile-time `Foldable`s are supported in the library //! right now, `length` must always return an `IntegralConstant`. //! //! //! Example //! ------- //! @include example/length.cpp #ifdef BOOST_HANA_DOXYGEN_INVOKED constexpr auto length = [](auto const& xs) { return tag-dispatched; }; #else template <typename T, typename = void> struct length_impl : length_impl<T, when<true>> { }; struct length_t { template <typename Xs> constexpr auto operator()(Xs const& xs) const; }; constexpr length_t length{}; #endif BOOST_HANA_NAMESPACE_END #endif // !BOOST_HANA_FWD_LENGTH_HPP