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Diffstat (limited to 'boost/hana/fwd/second.hpp')
-rw-r--r-- | boost/hana/fwd/second.hpp | 48 |
1 files changed, 48 insertions, 0 deletions
diff --git a/boost/hana/fwd/second.hpp b/boost/hana/fwd/second.hpp new file mode 100644 index 0000000000..edbbaa36fe --- /dev/null +++ b/boost/hana/fwd/second.hpp @@ -0,0 +1,48 @@ +/*! +@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 |