1 /*!
2 @file
3 Defines `boost::hana::flip`.
4 
5 @copyright Louis Dionne 2013-2017
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_FUNCTIONAL_FLIP_HPP
11 #define BOOST_HANA_FUNCTIONAL_FLIP_HPP
12 
13 #include <boost/hana/config.hpp>
14 #include <boost/hana/detail/create.hpp>
15 
16 #include <utility>
17 
18 
19 BOOST_HANA_NAMESPACE_BEGIN
20     //! @ingroup group-functional
21     //! Invoke a function with its two first arguments reversed.
22     //!
23     //! Specifically, `flip(f)` is a function such that
24     //! @code
25     //!     flip(f)(x, y, z...) == f(y, x, z...)
26     //! @endcode
27     //!
28     //! ### Example
29     //! @include example/functional/flip.cpp
30 #ifdef BOOST_HANA_DOXYGEN_INVOKED
__anon0aef5bd60102(auto&& f) 31     constexpr auto flip = [](auto&& f) {
32         return [perfect-capture](auto&& x, auto&& y, auto&& ...z) -> decltype(auto) {
33             return forwarded(f)(forwarded(y), forwarded(x), forwarded(z)...);
34         };
35     };
36 #else
37     template <typename F>
38     struct flip_t {
39         F f;
40 
41         template <typename X, typename Y, typename ...Z>
42         constexpr decltype(auto) operator()(X&& x, Y&& y, Z&& ...z) const& {
43             return f(
44                 static_cast<Y&&>(y),
45                 static_cast<X&&>(x),
46                 static_cast<Z&&>(z)...
47             );
48         }
49 
50         template <typename X, typename Y, typename ...Z>
51         constexpr decltype(auto) operator()(X&& x, Y&& y, Z&& ...z) & {
52             return f(
53                 static_cast<Y&&>(y),
54                 static_cast<X&&>(x),
55                 static_cast<Z&&>(z)...
56             );
57         }
58 
59         template <typename X, typename Y, typename ...Z>
60         constexpr decltype(auto) operator()(X&& x, Y&& y, Z&& ...z) && {
61             return std::move(f)(
62                 static_cast<Y&&>(y),
63                 static_cast<X&&>(x),
64                 static_cast<Z&&>(z)...
65             );
66         }
67     };
68 
69     constexpr detail::create<flip_t> flip{};
70 #endif
71 BOOST_HANA_NAMESPACE_END
72 
73 #endif // !BOOST_HANA_FUNCTIONAL_FLIP_HPP
74