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- /*!
- @file
- Forward declares `boost::hana::zero`.
- @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_ZERO_HPP
- #define BOOST_HANA_FWD_ZERO_HPP
- #include <boost/hana/config.hpp>
- #include <boost/hana/core/when.hpp>
- BOOST_HANA_NAMESPACE_BEGIN
- //! Identity of `plus`.
- //! @ingroup group-Monoid
- //!
- //! @tparam M
- //! The tag (a `Monoid`) of the returned identity.
- //!
- //!
- //! Example
- //! -------
- //! @include example/zero.cpp
- #ifdef BOOST_HANA_DOXYGEN_INVOKED
- template <typename M>
- constexpr auto zero = []() -> decltype(auto) {
- return tag-dispatched;
- };
- #else
- template <typename M, typename = void>
- struct zero_impl : zero_impl<M, when<true>> { };
- template <typename M>
- struct zero_t;
- template <typename M>
- constexpr zero_t<M> zero{};
- #endif
- BOOST_HANA_NAMESPACE_END
- #endif // !BOOST_HANA_FWD_ZERO_HPP
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