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
48namespace boost { namespace hof {
49
50namespace detail {
51
52template<class T>
53struct perfect_ref
54{
55 typedef T type;
56 typedef typename std::remove_reference<T>::type value_type;
57 T&& value;
58 constexpr perfect_ref(value_type& x) noexcept
59 : value(BOOST_HOF_FORWARD(T)(x))
60 {}
61};
62
63template<std::size_t N>
64struct ignore
65{
66 template<class T>
67 constexpr ignore(T&&...) noexcept
68 {}
69};
70
71template<std::size_t... N>
72struct 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
79template<std::size_t... N>
80constexpr args_at<N...> make_args_at(seq<N...>) noexcept
81{
82 return {};
83}
84
85template<std::size_t N, class... Ts>
86constexpr 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
91template<class T, T N>
92struct 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
101struct arg_f
102{
103 template<class IntegralConstant>
104 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
112template<std::size_t N>
113BOOST_HOF_STATIC_CONSTEXPR detail::make_args_f<std::size_t, N> arg_c = {};
114#else
115template<std::size_t N, class... Ts>
116constexpr 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
122BOOST_HOF_DECLARE_STATIC_VAR(arg, detail::arg_f);
123
124}} // namespace boost::hof
125
126#endif
127

source code of boost/libs/hof/include/boost/hof/arg.hpp