1/*!
2@file
3Defines `boost::hana::detail::index_if`.
4
5Copyright Louis Dionne 2013-2022
6Copyright Jason Rice 2017
7Distributed under the Boost Software License, Version 1.0.
8(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
9 */
10
11#ifndef BOOST_HANA_DETAIL_INDEX_IF_HPP
12#define BOOST_HANA_DETAIL_INDEX_IF_HPP
13
14#include <boost/hana/config.hpp>
15#include <boost/hana/detail/decay.hpp>
16#include <boost/hana/integral_constant.hpp>
17#include <boost/hana/optional.hpp>
18
19#include <cstddef>
20#include <utility>
21
22
23namespace boost { namespace hana { namespace detail {
24 template <std::size_t i, std::size_t N, bool Done>
25 struct index_if_helper;
26
27 template <std::size_t i, std::size_t N>
28 struct index_if_helper<i, N, false> {
29 template <typename Pred, typename X1, typename ...Xs>
30 using f = typename index_if_helper<i + 1, N,
31 static_cast<bool>(detail::decay<decltype(
32 std::declval<Pred>()(std::declval<X1>()))>::type::value)
33 >::template f<Pred, Xs...>;
34 };
35
36 template <std::size_t N>
37 struct index_if_helper<N, N, false> {
38 template <typename ...>
39 using f = hana::optional<>;
40 };
41
42 template <std::size_t i, std::size_t N>
43 struct index_if_helper<i, N, true> {
44 template <typename ...>
45 using f = hana::optional<hana::size_t<i - 1>>;
46 };
47
48 template <typename Pred, typename ...Xs>
49 struct index_if {
50 using type = typename index_if_helper<0, sizeof...(Xs), false>
51 ::template f<Pred, Xs...>;
52 };
53} }} // end namespace boost::hana
54
55#endif // !BOOST_HANA_DETAIL_INDEX_IF_HPP
56

source code of boost/libs/hana/include/boost/hana/detail/index_if.hpp