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/*!
@file
Forward declares `boost::hana::second`.
@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_FWD_SECOND_HPP
#define BOOST_HANA_FWD_SECOND_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Returns the second element of a pair.
//! @ingroup group-Product
//!
//! Note that if the `Product` actually stores the elements it contains,
//! `hana::second` is required to return a lvalue reference, a lvalue
//! reference to const or a rvalue reference to the second element, where
//! the type of reference must match that of the pair passed to `second`.
//! If the `Product` does not store the elements it contains (i.e. it
//! generates them on demand), this requirement is dropped.
//!
//! Example
//! -------
//! @include example/second.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto second = [](auto&& product) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename P, typename = void>
struct second_impl : second_impl<P, when<true>> { };
struct second_t {
template <typename Pair>
constexpr decltype(auto) operator()(Pair&& pair) const;
};
constexpr second_t second{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_SECOND_HPP
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