Boost C++ Libraries

boost/hana/detail/nested_to_fwd.hpp

/*!
@file
Forward declares `boost::hana::detail::nested_to`.

Copyright Louis Dionne 2013-2022
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_DETAIL_NESTED_TO_FWD_HPP
#define BOOST_HANA_DETAIL_NESTED_TO_FWD_HPP

#include <boost/hana/config.hpp>


namespace boost { namespace hana { namespace detail {
    template <typename Algorithm>
    struct nested_to_t {
        template <typename X>
        constexpr decltype(auto) operator()(X&& x) const;
    };

    //! @ingroup group-details
    //! Provides a `.to` static constexpr function object.
    //!
    //! When creating a binary function object of type `Algo` whose signature
    //! is `Object x Object -> Return`, `nested_to<Algo>` can be used as a base
    //! class of `Algo`. Doing so will provide a static constexpr member called
    //! `to`, which has the following signature:
    //! @code
    //!     Object -> Object -> Return
    //! @endcode
    //!
    //! Note that the function object `Algo` must be default-constructible,
    //! since the algorithm will be called as `Algo{}(arguments...)`.
    //!
    //! @note
    //! This function object is especially useful because it takes care of
    //! avoiding ODR violations caused by the nested static constexpr member.
    template <typename Algorithm>
    struct nested_to { static constexpr nested_to_t<Algorithm> to{}; };

    template <typename Algorithm>
    constexpr nested_to_t<Algorithm> nested_to<Algorithm>::to;
} }} // end namespace boost::hana

#endif // !BOOST_HANA_DETAIL_NESTED_TO_FWD_HPP