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/*!
@file
Forward declares `boost::hana::detail::nested_than`.
@copyright Louis Dionne 2013-2016
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_THAN_FWD_HPP
#define BOOST_HANA_DETAIL_NESTED_THAN_FWD_HPP
#include <boost/hana/config.hpp>
BOOST_HANA_NAMESPACE_BEGIN namespace detail {
template <typename Algorithm>
struct nested_than_t {
template <typename X>
constexpr decltype(auto) operator()(X&& x) const;
};
//! @ingroup group-details
//! Provides a `.than` static constexpr function object.
//!
//! When creating a binary function object of type `Algo` whose signature
//! is `A x B -> Return`, `nested_than<Algo>` can be used as a base class
//! of `Algo`. Doing so will provide a static constexpr member called
//! `than`, which has the following signature:
//! @code
//! B -> A -> Return
//! @endcode
//!
//! Note that the function object `Algo` must be default-constructible,
//! since it 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_than { static constexpr nested_than_t<Algorithm> than{}; };
template <typename Algorithm>
constexpr nested_than_t<Algorithm> nested_than<Algorithm>::than;
} BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_DETAIL_NESTED_THAN_FWD_HPP
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