| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::zero`. |
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| 4 | |
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| 5 | Copyright Louis Dionne 2013-2022 |
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| 6 | Distributed under the Boost Software License, Version 1.0. |
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| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) |
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| 8 | */ |
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| 9 | |
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| 10 | #ifndef BOOST_HANA_FWD_ZERO_HPP |
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| 11 | #define BOOST_HANA_FWD_ZERO_HPP |
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| 12 | |
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| 13 | #include <boost/hana/config.hpp> |
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| 14 | #include <boost/hana/core/when.hpp> |
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| 15 | |
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| 16 | |
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| 17 | namespace boost { namespace hana { |
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| 18 | //! Identity of `plus`. |
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| 19 | //! @ingroup group-Monoid |
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| 20 | //! |
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| 21 | //! @tparam M |
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| 22 | //! The tag (a `Monoid`) of the returned identity. |
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| 23 | //! |
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| 24 | //! |
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| 25 | //! Example |
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| 26 | //! ------- |
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| 27 | //! @include example/zero.cpp |
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| 28 | #ifdef BOOST_HANA_DOXYGEN_INVOKED |
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| 29 | template <typename M> |
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| 30 | constexpr auto zero = []() -> decltype(auto) { |
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| 31 | return tag-dispatched; |
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| 32 | }; |
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| 33 | #else |
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| 34 | template <typename M, typename = void> |
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| 35 | struct zero_impl : zero_impl<M, when<true>> { }; |
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| 36 | |
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| 37 | template <typename M> |
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| 38 | struct zero_t { |
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| 39 | constexpr decltype(auto) operator()() const; |
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| 40 | }; |
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| 41 | |
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| 42 | template <typename M> |
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| 43 | BOOST_HANA_INLINE_VARIABLE constexpr zero_t<M> zero{}; |
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| 44 | #endif |
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| 45 | }} // end namespace boost::hana |
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| 46 | |
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| 47 | #endif // !BOOST_HANA_FWD_ZERO_HPP |
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| 48 | |
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