feat():initial version

install/boost_1_75_0/include/boost/hof/if.hpp

@@ -0,0 +1,143 @@
+/*=============================================================================
+    Copyright (c) 2015 Paul Fultz II
+    if_.h
+    Distributed under the Boost Software License, Version 1.0. (See accompanying
+    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+==============================================================================*/
+
+#ifndef BOOST_HOF_GUARD_IF_H
+#define BOOST_HOF_GUARD_IF_H
+
+/// if
+/// ==
+/// 
+/// Description
+/// -----------
+/// 
+/// The `if_` function decorator makes the function callable if the boolean
+/// condition is true. The `if_c` version can be used to give a boolean
+/// condition directly(instead of relying on an integral constant).
+/// 
+/// When `if_` is false, the function is not callable. It is a subtitution
+/// failure to call the function.
+/// 
+/// Synopsis
+/// --------
+/// 
+///     template<class IntegralConstant>
+///     constexpr auto if_(IntegralConstant);
+/// 
+///     template<bool B, class F>
+///     constexpr auto if_c(F);
+/// 
+/// Requirements
+/// ------------
+/// 
+/// IntegralConstant must be:
+/// 
+/// * IntegralConstant
+/// 
+/// F must be:
+/// 
+/// * [ConstInvocable](ConstInvocable)
+/// * MoveConstructible
+/// 
+/// Example
+/// -------
+/// 
+///     #include <boost/hof.hpp>
+///     #include <cassert>
+/// 
+///     struct sum_f
+///     {
+///         template<class T>
+///         int operator()(T x, T y) const
+///         {
+///             return boost::hof::first_of(
+///                 boost::hof::if_(std::is_integral<T>())(boost::hof::_ + boost::hof::_),
+///                 boost::hof::always(0)
+///             )(x, y);
+///         }
+///     };
+/// 
+///     int main() {
+///         assert(sum_f()(1, 2) == 3);
+///         assert(sum_f()("", "") == 0);
+///     }
+/// 
+/// References
+/// ----------
+/// 
+/// * [static_if](static_if)
+/// 
+
+#include <boost/hof/always.hpp>
+#include <boost/hof/detail/callable_base.hpp>
+#include <boost/hof/detail/forward.hpp>
+#include <boost/hof/detail/delegate.hpp>
+#include <boost/hof/detail/move.hpp>
+#include <boost/hof/detail/static_const_var.hpp>
+
+namespace boost { namespace hof {
+
+namespace detail {
+
+template<class C, class...>
+struct if_depend
+: C
+{};
+
+template<bool Cond, class F>
+struct if_adaptor : detail::callable_base<F>
+{
+    BOOST_HOF_INHERIT_CONSTRUCTOR(if_adaptor, detail::callable_base<F>)
+};
+
+template<class F>
+struct if_adaptor<false, F>
+{
+    template<class... Ts>
+    constexpr if_adaptor(Ts&&...) noexcept
+    {}
+};
+
+template<bool Cond>
+struct make_if_f
+{
+    constexpr make_if_f() noexcept
+    {}
+    template<class F>
+    constexpr if_adaptor<Cond, F> operator()(F f) const BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(F, F&&)
+    {
+        return if_adaptor<Cond, F>(static_cast<F&&>(f));
+    }
+};
+
+struct if_f
+{
+    constexpr if_f()
+    {}
+    template<class Cond, bool B=Cond::type::value>
+    constexpr make_if_f<B> operator()(Cond) const noexcept
+    {
+        return {};
+    }
+};
+
+}
+#if BOOST_HOF_HAS_VARIABLE_TEMPLATES
+template<bool B>
+BOOST_HOF_STATIC_CONSTEXPR detail::make_if_f<B> if_c = {};
+#else
+template<bool B, class F>
+constexpr detail::if_adaptor<B, F> if_c(F f) BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(F, F&&)
+{
+    return detail::if_adaptor<B, F>(static_cast<F&&>(f));
+}
+#endif
+
+BOOST_HOF_DECLARE_STATIC_VAR(if_, detail::if_f);
+
+}} // namespace boost::hof
+
+#endif