/*! @file Forward declares `boost::hana::one`. @copyright Louis Dionne 2013-2017 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_ONE_HPP #define BOOST_HANA_FWD_ONE_HPP #include <boost/hana/config.hpp> #include <boost/hana/core/when.hpp> namespace boost { namespace hana { //! Identity of the `Ring` multiplication. //! @ingroup group-Ring //! //! @tparam R //! The tag (must be a model of `Ring`) of the returned identity. //! //! //! Example //! ------- //! @include example/one.cpp #ifdef BOOST_HANA_DOXYGEN_INVOKED template <typename R> constexpr auto one = []() -> decltype(auto) { return tag-dispatched; }; #else template <typename R, typename = void> struct one_impl : one_impl<R, when<true>> { }; template <typename R> struct one_t { constexpr decltype(auto) operator()() const; }; template <typename R> BOOST_HANA_INLINE_VARIABLE constexpr one_t<R> one{}; #endif }} // end namespace boost::hana #endif // !BOOST_HANA_FWD_ONE_HPP