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1 | /*! | |
2 | @file | |
3 | Forward declares `boost::hana::detail::nested_than`. | |
4 | ||
5 | @copyright Louis Dionne 2013-2016 | |
6 | Distributed under the Boost Software License, Version 1.0. | |
7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) | |
8 | */ | |
9 | ||
10 | #ifndef BOOST_HANA_DETAIL_NESTED_THAN_FWD_HPP | |
11 | #define BOOST_HANA_DETAIL_NESTED_THAN_FWD_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | ||
15 | ||
16 | BOOST_HANA_NAMESPACE_BEGIN namespace detail { | |
17 | template <typename Algorithm> | |
18 | struct nested_than_t { | |
19 | template <typename X> | |
20 | constexpr decltype(auto) operator()(X&& x) const; | |
21 | }; | |
22 | ||
23 | //! @ingroup group-details | |
24 | //! Provides a `.than` static constexpr function object. | |
25 | //! | |
26 | //! When creating a binary function object of type `Algo` whose signature | |
27 | //! is `A x B -> Return`, `nested_than<Algo>` can be used as a base class | |
28 | //! of `Algo`. Doing so will provide a static constexpr member called | |
29 | //! `than`, which has the following signature: | |
30 | //! @code | |
31 | //! B -> A -> Return | |
32 | //! @endcode | |
33 | //! | |
34 | //! Note that the function object `Algo` must be default-constructible, | |
35 | //! since it will be called as `Algo{}(arguments...)`. | |
36 | //! | |
37 | //! @note | |
38 | //! This function object is especially useful because it takes care of | |
39 | //! avoiding ODR violations caused by the nested static constexpr member. | |
40 | template <typename Algorithm> | |
41 | struct nested_than { static constexpr nested_than_t<Algorithm> than{}; }; | |
42 | ||
43 | template <typename Algorithm> | |
44 | constexpr nested_than_t<Algorithm> nested_than<Algorithm>::than; | |
45 | } BOOST_HANA_NAMESPACE_END | |
46 | ||
47 | #endif // !BOOST_HANA_DETAIL_NESTED_THAN_FWD_HPP |