/*============================================================================= 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