1 /*!
2 @file
3 Defines `boost::hana::always`.
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_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 
19 BOOST_HANA_NAMESPACE_BEGIN
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
__anonfde165180102(auto&& x) 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     constexpr detail::create<_always> always{};
61 #endif
62 BOOST_HANA_NAMESPACE_END
63 
64 #endif // !BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
65