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boost/hana/functional/always.hpp

/*!
@file
Defines `boost::hana::always`.

@copyright Louis Dionne 2013-2017
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
 */

#ifndef BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
#define BOOST_HANA_FUNCTIONAL_ALWAYS_HPP

#include <boost/hana/config.hpp>
#include <boost/hana/detail/create.hpp>

#include <utility>


namespace boost { namespace hana {
    //! @ingroup group-functional
    //! Return a constant function returning `x` regardless of the
    //! argument(s) it is invoked with.
    //!
    //! Specifically, `always(x)` is a function such that
    //! @code
    //!     always(x)(y...) == x
    //! @endcode
    //! for any `y...`. A copy of `x` is made and it is owned by the
    //! `always(x)` function. When `always(x)` is called, it will return
    //! a reference to the `x` it owns. This reference is valid as long
    //! as `always(x)` is in scope.
    //!
    //!
    //! ### Example
    //! @include example/functional/always.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
    constexpr auto always = [](auto&& x) {
        return [perfect-capture](auto const& ...y) -> decltype(auto) {
            return forwarded(x);
        };
    };
#else
    template <typename T>
    struct _always {
        T val_;

        template <typename ...Args>
        constexpr T const& operator()(Args const& ...) const&
        { return val_; }

        template <typename ...Args>
        constexpr T& operator()(Args const& ...) &
        { return val_; }

        template <typename ...Args>
        constexpr T operator()(Args const& ...) &&
        { return std::move(val_); }
    };

    BOOST_HANA_INLINE_VARIABLE constexpr detail::create<_always> always{};
#endif
}} // end namespace boost::hana

#endif // !BOOST_HANA_FUNCTIONAL_ALWAYS_HPP