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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::if_`. | |
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_IF_HPP | |
11 | #define BOOST_HANA_FWD_IF_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | ||
16 | ||
17 | BOOST_HANA_NAMESPACE_BEGIN | |
18 | //! Conditionally return one of two values based on a condition. | |
19 | //! @ingroup group-Logical | |
20 | //! | |
21 | //! Specifically, `then` is returned iff `cond` is true-valued, and | |
22 | //! `else_` is returned otherwise. Note that some `Logical` models may | |
23 | //! allow `then` and `else_` to have different types, while others may | |
24 | //! require both values to have the same type. | |
25 | //! | |
26 | //! | |
27 | //! @param cond | |
28 | //! The condition determining which of the two values is returned. | |
29 | //! | |
30 | //! @param then | |
31 | //! The value returned when `cond` is true-valued. | |
32 | //! | |
33 | //! @param else_ | |
34 | //! The value returned when `cond` is false-valued. | |
35 | //! | |
36 | //! | |
37 | //! Example | |
38 | //! ------- | |
39 | //! @include example/if.cpp | |
40 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
41 | constexpr auto if_ = [](auto&& cond, auto&& then, auto&& else_) -> decltype(auto) { | |
42 | return tag-dispatched; | |
43 | }; | |
44 | #else | |
45 | template <typename L, typename = void> | |
46 | struct if_impl : if_impl<L, when<true>> { }; | |
47 | ||
48 | struct if_t { | |
49 | template <typename Cond, typename Then, typename Else> | |
50 | constexpr decltype(auto) operator()(Cond&& cond, Then&& then, Else&& else_) const; | |
51 | }; | |
52 | ||
53 | constexpr if_t if_{}; | |
54 | #endif | |
55 | BOOST_HANA_NAMESPACE_END | |
56 | ||
57 | #endif // !BOOST_HANA_FWD_IF_HPP |