1/*!
2@file
3Defines `boost::hana::always`.
4
5Copyright Louis Dionne 2013-2022
6Distributed 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_ALWAYS_HPP
11#define BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
12
13#include <boost/hana/config.hpp>
14#include <boost/hana/detail/create.hpp>
15
16#include <utility>
17
18
19namespace boost { namespace hana {
20 //! @ingroup group-functional
21 //! Return a constant function returning `x` regardless of the
22 //! argument(s) it is invoked with.
23 //!
24 //! Specifically, `always(x)` is a function such that
25 //! @code
26 //! always(x)(y...) == x
27 //! @endcode
28 //! for any `y...`. A copy of `x` is made and it is owned by the
29 //! `always(x)` function. When `always(x)` is called, it will return
30 //! a reference to the `x` it owns. This reference is valid as long
31 //! as `always(x)` is in scope.
32 //!
33 //!
34 //! ### Example
35 //! @include example/functional/always.cpp
36#ifdef BOOST_HANA_DOXYGEN_INVOKED
37 constexpr auto always = [](auto&& x) {
38 return [perfect-capture](auto const& ...y) -> decltype(auto) {
39 return forwarded(x);
40 };
41 };
42#else
43 template <typename T>
44 struct _always {
45 T val_;
46
47 template <typename ...Args>
48 constexpr T const& operator()(Args const& ...) const&
49 { return val_; }
50
51 template <typename ...Args>
52 constexpr T& operator()(Args const& ...) &
53 { return val_; }
54
55 template <typename ...Args>
56 constexpr T operator()(Args const& ...) &&
57 { return std::move(val_); }
58 };
59
60 BOOST_HANA_INLINE_VARIABLE constexpr detail::create<_always> always{};
61#endif
62}} // end namespace boost::hana
63
64#endif // !BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
65

source code of boost/libs/hana/include/boost/hana/functional/always.hpp