49 lines
1.5 KiB
C++
49 lines
1.5 KiB
C++
|
/*!
|
||
|
|
@file
|
||
|
|
Forward declares `boost::hana::second`.
|
||
|
|
|
||
|
|
@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_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
|