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+/*=============================================================================
+ 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