1 /*=============================================================================
2 Copyright (c) 2014 Paul Fultz II
3 arg.h
4 Distributed under the Boost Software License, Version 1.0. (See accompanying
5 file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6 ==============================================================================*/
7
8 #ifndef BOOST_HOF_GUARD_FUNCTION_ARGS_H
9 #define BOOST_HOF_GUARD_FUNCTION_ARGS_H
10
11 #include <boost/hof/detail/seq.hpp>
12 #include <boost/hof/returns.hpp>
13 #include <boost/hof/detail/static_const_var.hpp>
14 #include <utility>
15
16 /// arg
17 /// ===
18 ///
19 /// Description
20 /// -----------
21 ///
22 /// The `arg` function returns a function object that returns the Nth argument
23 /// passed to it. It actually starts at 1, so it is not the zero-based index
24 /// of the argument.
25 ///
26 /// Synopsis
27 /// --------
28 ///
29 /// template<class IntegralConstant>
30 /// constexpr auto arg(IntegralConstant);
31 ///
32 /// template<std::size_t N, class... Ts>
33 /// constexpr auto arg_c(Ts&&...);
34 ///
35 ///
36 /// Example
37 /// -------
38 ///
39 /// #include <boost/hof.hpp>
40 /// #include <cassert>
41 /// using namespace boost::hof;
42 ///
43 /// int main() {
44 /// assert(arg(std::integral_constant<int, 3>())(1,2,3,4,5) == 3);
45 /// }
46 ///
47
48 namespace boost { namespace hof {
49
50 namespace detail {
51
52 template<class T>
53 struct perfect_ref
54 {
55 typedef T type;
56 typedef typename std::remove_reference<T>::type value_type;
57 T&& value;
perfect_refboost::hof::detail::perfect_ref58 constexpr perfect_ref(value_type& x) noexcept
59 : value(BOOST_HOF_FORWARD(T)(x))
60 {}
61 };
62
63 template<std::size_t N>
64 struct ignore
65 {
66 template<class T>
ignoreboost::hof::detail::ignore67 constexpr ignore(T&&...) noexcept
68 {}
69 };
70
71 template<std::size_t... N>
72 struct args_at
73 {
74 template<class T, class... Ts>
75 constexpr auto operator()(ignore<N>..., T x, Ts...) const
76 BOOST_HOF_RETURNS(BOOST_HOF_FORWARD(typename T::type)(x.value));
77 };
78
79 template<std::size_t... N>
make_args_at(seq<N...>)80 constexpr args_at<N...> make_args_at(seq<N...>) noexcept
81 {
82 return {};
83 }
84
85 template<std::size_t N, class... Ts>
86 constexpr auto get_args(Ts&&... xs) BOOST_HOF_RETURNS
87 (
88 boost::hof::detail::make_args_at(typename gens<N>::type())(nullptr, BOOST_HOF_RETURNS_CONSTRUCT(perfect_ref<Ts>)(xs)...)
89 );
90
91 template<class T, T N>
92 struct make_args_f
93 {
94 template<class... Ts, class=typename std::enable_if<(N <= sizeof...(Ts))>::type>
95 constexpr auto operator()(Ts&&... xs) const BOOST_HOF_RETURNS
96 (
97 boost::hof::detail::get_args<N>(BOOST_HOF_FORWARD(Ts)(xs)...)
98 );
99 };
100
101 struct arg_f
102 {
103 template<class IntegralConstant>
operator ()boost::hof::detail::arg_f104 constexpr make_args_f<std::size_t, IntegralConstant::value> operator()(IntegralConstant) const noexcept
105 {
106 return make_args_f<std::size_t, IntegralConstant::value>();
107 }
108 };
109
110 }
111 #if BOOST_HOF_HAS_VARIABLE_TEMPLATES
112 template<std::size_t N>
113 BOOST_HOF_STATIC_CONSTEXPR detail::make_args_f<std::size_t, N> arg_c = {};
114 #else
115 template<std::size_t N, class... Ts>
116 constexpr auto arg_c(Ts&&... xs) BOOST_HOF_RETURNS
117 (
118 boost::hof::detail::get_args<N>(BOOST_HOF_FORWARD(Ts)(xs)...)
119 );
120 #endif
121
122 BOOST_HOF_DECLARE_STATIC_VAR(arg, detail::arg_f);
123
124 }} // namespace boost::hof
125
126 #endif
127