/*! @file Defines `boost::hana::detail::index_if`. Copyright Louis Dionne 2013-2022 Copyright Jason Rice 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_DETAIL_INDEX_IF_HPP #define BOOST_HANA_DETAIL_INDEX_IF_HPP #include <boost/hana/config.hpp> #include <boost/hana/detail/decay.hpp> #include <boost/hana/integral_constant.hpp> #include <boost/hana/optional.hpp> #include <cstddef> #include <utility> namespace boost { namespace hana { namespace detail { template <std::size_t i, std::size_t N, bool Done> struct index_if_helper; template <std::size_t i, std::size_t N> struct index_if_helper<i, N, false> { template <typename Pred, typename X1, typename ...Xs> using f = typename index_if_helper<i + 1, N, static_cast<bool>(detail::decay<decltype( std::declval<Pred>()(std::declval<X1>()))>::type::value) >::template f<Pred, Xs...>; }; template <std::size_t N> struct index_if_helper<N, N, false> { template <typename ...> using f = hana::optional<>; }; template <std::size_t i, std::size_t N> struct index_if_helper<i, N, true> { template <typename ...> using f = hana::optional<hana::size_t<i - 1>>; }; template <typename Pred, typename ...Xs> struct index_if { using type = typename index_if_helper<0, sizeof...(Xs), false> ::template f<Pred, Xs...>; }; } }} // end namespace boost::hana #endif // !BOOST_HANA_DETAIL_INDEX_IF_HPP