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Diffstat (limited to 'boost/hana/fwd/plus.hpp')
-rw-r--r-- | boost/hana/fwd/plus.hpp | 63 |
1 files changed, 63 insertions, 0 deletions
diff --git a/boost/hana/fwd/plus.hpp b/boost/hana/fwd/plus.hpp new file mode 100644 index 0000000000..e1a6e058f2 --- /dev/null +++ b/boost/hana/fwd/plus.hpp @@ -0,0 +1,63 @@ +/*! +@file +Forward declares `boost::hana::plus`. + +@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_PLUS_HPP +#define BOOST_HANA_FWD_PLUS_HPP + +#include <boost/hana/config.hpp> +#include <boost/hana/core/when.hpp> + + +BOOST_HANA_NAMESPACE_BEGIN + //! Associative binary operation on a `Monoid`. + //! @ingroup group-Monoid + //! + //! @param x, y + //! Two objects to combine with the `Monoid`'s binary operation. + //! + //! + //! Cross-type version of the method + //! -------------------------------- + //! The `plus` method is "overloaded" to handle distinct data types + //! with certain properties. Specifically, `plus` is defined for + //! _distinct_ data types `A` and `B` such that + //! 1. `A` and `B` share a common data type `C`, as determined by the + //! `common` metafunction + //! 2. `A`, `B` and `C` are all `Monoid`s when taken individually + //! 3. `to<C> : A -> B` and `to<C> : B -> C` are `Monoid`-embeddings, as + //! determined by the `is_embedding` metafunction. + //! + //! The definition of `plus` for data types satisfying the above + //! properties is obtained by setting + //! @code + //! plus(x, y) = plus(to<C>(x), to<C>(y)) + //! @endcode + //! + //! + //! Example + //! ------- + //! @include example/plus.cpp +#ifdef BOOST_HANA_DOXYGEN_INVOKED + constexpr auto plus = [](auto&& x, auto&& y) -> decltype(auto) { + return tag-dispatched; + }; +#else + template <typename T, typename U, typename = void> + struct plus_impl : plus_impl<T, U, when<true>> { }; + + struct plus_t { + template <typename X, typename Y> + constexpr decltype(auto) operator()(X&& x, Y&& y) const; + }; + + constexpr plus_t plus{}; +#endif +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_FWD_PLUS_HPP |