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always.hpp
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/*============================================================================= Copyright (c) 2014 Paul Fultz II always.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_FUNCTION_ALWAYS_H #define BOOST_HOF_GUARD_FUNCTION_ALWAYS_H #include <boost/hof/detail/delegate.hpp> #include <boost/hof/detail/unwrap.hpp> #include <boost/hof/detail/static_const_var.hpp> /// always /// ====== /// /// Description /// ----------- /// /// The `always` function returns a function object that will always return /// the value given to it, no matter what parameters are passed to the /// function object. The nullary version(i.e. `always(void)`) will return /// `void`. On compilers, that don't support constexpr functions returning /// `void`, a private empty type is returned instead. This return type is /// specified as `BOOST_HOF_ALWAYS_VOID_RETURN`. /// /// Synopsis /// -------- /// /// template<class T> /// constexpr auto always(T value); /// /// template<class T> /// constexpr auto always(void); /// /// /// Semantics /// --------- /// /// assert(always(x)(xs...) == x); /// /// Requirements /// ------------ /// /// T must be: /// /// * CopyConstructible /// /// Example /// ------- /// /// #include <boost/hof.hpp> /// #include <algorithm> /// #include <cassert> /// using namespace boost::hof; /// /// int main() { /// int ten = 10; /// assert( always(ten)(1,2,3,4,5) == 10 ); /// } /// /// // Count all /// template<class Iterator, class T> /// auto count(Iterator first, Iterator last) /// { /// return std::count_if(first, last, always(true)); /// } /// #ifndef BOOST_HOF_NO_CONSTEXPR_VOID #if defined(__clang__) && BOOST_HOF_HAS_RELAXED_CONSTEXPR #define BOOST_HOF_NO_CONSTEXPR_VOID 0 #else #define BOOST_HOF_NO_CONSTEXPR_VOID 1 #endif #endif namespace boost { namespace hof { namespace always_detail { template<class T, class=void> struct always_base { T x; BOOST_HOF_DELEGATE_CONSTRUCTOR(always_base, T, x) typedef typename detail::unwrap_reference<T>::type result_type; template<class... As> constexpr result_type operator()(As&&...) const noexcept(std::is_reference<result_type>::value || BOOST_HOF_IS_NOTHROW_COPY_CONSTRUCTIBLE(result_type)) { return this->x; } }; template<class T> struct always_base<T, typename std::enable_if<!BOOST_HOF_IS_EMPTY(T)>::type> { T x; constexpr always_base(T xp) noexcept(BOOST_HOF_IS_NOTHROW_COPY_CONSTRUCTIBLE(T)) : x(xp) {} typedef typename detail::unwrap_reference<T>::type result_type; template<class... As> constexpr result_type operator()(As&&...) const noexcept(std::is_reference<result_type>::value || BOOST_HOF_IS_NOTHROW_COPY_CONSTRUCTIBLE(result_type)) { return this->x; } }; #if BOOST_HOF_NO_CONSTEXPR_VOID #define BOOST_HOF_ALWAYS_VOID_RETURN boost::hof::always_detail::always_base<void>::void_ #else #define BOOST_HOF_ALWAYS_VOID_RETURN void #endif template<> struct always_base<void> { constexpr always_base() noexcept {} struct void_ {}; template<class... As> constexpr BOOST_HOF_ALWAYS_VOID_RETURN operator()(As&&...) const noexcept { #if BOOST_HOF_NO_CONSTEXPR_VOID return void_(); #endif } }; struct always_f { template<class T> constexpr always_detail::always_base<T> operator()(T x) const noexcept(BOOST_HOF_IS_NOTHROW_COPY_CONSTRUCTIBLE(T)) { return always_detail::always_base<T>(x); } constexpr always_detail::always_base<void> operator()() const noexcept { return always_detail::always_base<void>(); } }; struct always_ref_f { template<class T> constexpr always_detail::always_base<T&> operator()(T& x) const noexcept { return always_detail::always_base<T&>(x); } }; } BOOST_HOF_DECLARE_STATIC_VAR(always, always_detail::always_f); BOOST_HOF_DECLARE_STATIC_VAR(always_ref, always_detail::always_ref_f); }} // namespace boost::hof #endif