1 /*=============================================================================
2 Copyright (c) 2015 Paul Fultz II
3 if_.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_IF_H
9 #define BOOST_HOF_GUARD_IF_H
10
11 /// if
12 /// ==
13 ///
14 /// Description
15 /// -----------
16 ///
17 /// The `if_` function decorator makes the function callable if the boolean
18 /// condition is true. The `if_c` version can be used to give a boolean
19 /// condition directly(instead of relying on an integral constant).
20 ///
21 /// When `if_` is false, the function is not callable. It is a subtitution
22 /// failure to call the function.
23 ///
24 /// Synopsis
25 /// --------
26 ///
27 /// template<class IntegralConstant>
28 /// constexpr auto if_(IntegralConstant);
29 ///
30 /// template<bool B, class F>
31 /// constexpr auto if_c(F);
32 ///
33 /// Requirements
34 /// ------------
35 ///
36 /// IntegralConstant must be:
37 ///
38 /// * IntegralConstant
39 ///
40 /// F must be:
41 ///
42 /// * [ConstInvocable](ConstInvocable)
43 /// * MoveConstructible
44 ///
45 /// Example
46 /// -------
47 ///
48 /// #include <boost/hof.hpp>
49 /// #include <cassert>
50 ///
51 /// struct sum_f
52 /// {
53 /// template<class T>
54 /// int operator()(T x, T y) const
55 /// {
56 /// return boost::hof::first_of(
57 /// boost::hof::if_(std::is_integral<T>())(boost::hof::_ + boost::hof::_),
58 /// boost::hof::always(0)
59 /// )(x, y);
60 /// }
61 /// };
62 ///
63 /// int main() {
64 /// assert(sum_f()(1, 2) == 3);
65 /// assert(sum_f()("", "") == 0);
66 /// }
67 ///
68 /// References
69 /// ----------
70 ///
71 /// * [static_if](static_if)
72 ///
73
74 #include <boost/hof/always.hpp>
75 #include <boost/hof/detail/callable_base.hpp>
76 #include <boost/hof/detail/forward.hpp>
77 #include <boost/hof/detail/delegate.hpp>
78 #include <boost/hof/detail/move.hpp>
79 #include <boost/hof/detail/static_const_var.hpp>
80
81 namespace boost { namespace hof {
82
83 namespace detail {
84
85 template<class C, class...>
86 struct if_depend
87 : C
88 {};
89
90 template<bool Cond, class F>
91 struct if_adaptor : detail::callable_base<F>
92 {
93 BOOST_HOF_INHERIT_CONSTRUCTOR(if_adaptor, detail::callable_base<F>)
94 };
95
96 template<class F>
97 struct if_adaptor<false, F>
98 {
99 template<class... Ts>
if_adaptorboost::hof::detail::if_adaptor100 constexpr if_adaptor(Ts&&...) noexcept
101 {}
102 };
103
104 template<bool Cond>
105 struct make_if_f
106 {
make_if_fboost::hof::detail::make_if_f107 constexpr make_if_f() noexcept
108 {}
109 template<class F>
operator ()boost::hof::detail::make_if_f110 constexpr if_adaptor<Cond, F> operator()(F f) const BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(F, F&&)
111 {
112 return if_adaptor<Cond, F>(static_cast<F&&>(f));
113 }
114 };
115
116 struct if_f
117 {
if_fboost::hof::detail::if_f118 constexpr if_f()
119 {}
120 template<class Cond, bool B=Cond::type::value>
operator ()boost::hof::detail::if_f121 constexpr make_if_f<B> operator()(Cond) const noexcept
122 {
123 return {};
124 }
125 };
126
127 }
128 #if BOOST_HOF_HAS_VARIABLE_TEMPLATES
129 template<bool B>
130 BOOST_HOF_STATIC_CONSTEXPR detail::make_if_f<B> if_c = {};
131 #else
132 template<bool B, class F>
if_c(F f)133 constexpr detail::if_adaptor<B, F> if_c(F f) BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(F, F&&)
134 {
135 return detail::if_adaptor<B, F>(static_cast<F&&>(f));
136 }
137 #endif
138
139 BOOST_HOF_DECLARE_STATIC_VAR(if_, detail::if_f);
140
141 }} // namespace boost::hof
142
143 #endif
144