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