hana/fwd/length.hpp

0001 /*!
0002 @file
0003 Forward declares `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_FWD_LENGTH_HPP
0011 #define BOOST_HANA_FWD_LENGTH_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Return the number of elements in a foldable structure.
0019     //! @ingroup group-Foldable
0020     //!
0021     //! Given a `Foldable` `xs`, `length(xs)` must return an object of an
0022     //! unsigned integral type, or an `IntegralConstant` holding such an
0023     //! object, which represents the number of elements in the structure.
0024     //!
0025     //! @note
0026     //! Since only compile-time `Foldable`s are supported in the library
0027     //! right now, `length` must always return an `IntegralConstant`.
0028     //!
0029     //!
0030     //! Example
0031     //! -------
0032     //! @include example/length.cpp
0033 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0034     constexpr auto length = [](auto const& xs) {
0035         return tag-dispatched;
0036     };
0037 #else
0038     template <typename T, typename = void>
0039     struct length_impl : length_impl<T, when<true>> { };
0040
0041     struct length_t {
0042         template <typename Xs>
0043         constexpr auto operator()(Xs const& xs) const;
0044     };
0045
0046     BOOST_HANA_INLINE_VARIABLE constexpr length_t length{};
0047 #endif
0048 }} // end namespace boost::hana
0049
0050 #endif // !BOOST_HANA_FWD_LENGTH_HPP