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