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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::comparing`. | |
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_FWD_COMPARING_HPP | |
11 | #define BOOST_HANA_FWD_COMPARING_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | ||
15 | ||
16 | BOOST_HANA_NAMESPACE_BEGIN | |
17 | //! Returns a function performing `equal` after applying a transformation | |
18 | //! to both arguments. | |
19 | //! @ingroup group-Comparable | |
20 | //! | |
21 | //! `comparing` creates an equivalence relation based on the result of | |
22 | //! applying a function to some objects, which is especially useful in | |
23 | //! conjunction with algorithms that accept a custom predicate that must | |
24 | //! represent an equivalence relation. | |
25 | //! | |
26 | //! Specifically, `comparing` is such that | |
27 | //! @code | |
28 | //! comparing(f) == equal ^on^ f | |
29 | //! @endcode | |
30 | //! or, equivalently, | |
31 | //! @code | |
32 | //! comparing(f)(x, y) == equal(f(x), f(y)) | |
33 | //! @endcode | |
34 | //! | |
35 | //! @note | |
36 | //! This is not a tag-dispatched method (hence it can't be customized), | |
37 | //! but just a convenience function provided with the `Comparable` concept. | |
38 | //! | |
39 | //! | |
40 | //! Signature | |
41 | //! --------- | |
42 | //! Given a Logical `Bool` and a Comparable `B`, the signature is | |
43 | //! @f$ \mathtt{comparing} : (A \to B) \to (A \times A \to Bool) @f$. | |
44 | //! | |
45 | //! | |
46 | //! Example | |
47 | //! ------- | |
48 | //! @include example/comparing.cpp | |
49 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
50 | constexpr auto comparing = [](auto&& f) { | |
51 | return [perfect-capture](auto&& x, auto&& y) { | |
52 | return equal(f(forwarded(x)), f(forwarded(y))); | |
53 | }; | |
54 | }; | |
55 | #else | |
56 | struct comparing_t { | |
57 | template <typename F> | |
58 | constexpr auto operator()(F&& f) const; | |
59 | }; | |
60 | ||
61 | constexpr comparing_t comparing{}; | |
62 | #endif | |
63 | BOOST_HANA_NAMESPACE_END | |
64 | ||
65 | #endif // !BOOST_HANA_FWD_COMPARING_HPP |