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/*!
@file
Forward declares `boost::hana::ordering`.
@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_FWD_ORDERING_HPP
#define BOOST_HANA_FWD_ORDERING_HPP
#include <boost/hana/config.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Returns a function performing `less` after applying a transformation
//! to both arguments.
//! @ingroup group-Orderable
//!
//! `ordering` creates a total order based on the result of applying a
//! function to some objects, which is especially useful in conjunction
//! with algorithms that accept a custom predicate that must represent
//! a total order.
//!
//! Specifically, `ordering` is such that
//! @code
//! ordering(f) == less ^on^ f
//! @endcode
//! or, equivalently,
//! @code
//! ordering(f)(x, y) == less(f(x), f(y))
//! @endcode
//!
//! @note
//! This is not a tag-dispatched method (hence it can't be customized),
//! but just a convenience function provided with the `Orderable` concept.
//!
//!
//! Signature
//! ---------
//! Given a Logical `Bool` and an Orderable `B`, the signature is
//! @f$ \mathrm{ordering} : (A \to B) \to (A \times A \to Bool) @f$.
//!
//!
//! Example
//! -------
//! @include example/ordering.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto ordering = [](auto&& f) {
return [perfect-capture](auto&& x, auto&& y) -> decltype(auto) {
return less(f(forwarded(x)), f(forwarded(y)));
};
};
#else
struct ordering_t {
template <typename F>
constexpr auto operator()(F&& f) const;
};
constexpr ordering_t ordering{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_ORDERING_HPP
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