ceph/src/boost/boost/hof/if.hpp

   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>
 100     constexpr if_adaptor(Ts&&...) noexcept
 101     {}
 102 };
 103
 104 template<bool Cond>
 105 struct make_if_f
 106 {
 107     constexpr make_if_f() noexcept
 108     {}
 109     template<class F>
 110     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 {
 118     constexpr if_f()
 119     {}
 120     template<class Cond, bool B=Cond::type::value>
 121     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>
 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