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 Hu Chunming
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/*=============================================================================
Copyright (c) 2014 Paul Fultz II
arg.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_ARGS_H
#define BOOST_HOF_GUARD_FUNCTION_ARGS_H
#include <boost/hof/detail/seq.hpp>
#include <boost/hof/returns.hpp>
#include <boost/hof/detail/static_const_var.hpp>
#include <utility>
/// arg
/// ===
///
/// Description
/// -----------
///
/// The `arg` function returns a function object that returns the Nth argument
/// passed to it. It actually starts at 1, so it is not the zero-based index
/// of the argument.
///
/// Synopsis
/// --------
///
/// template<class IntegralConstant>
/// constexpr auto arg(IntegralConstant);
///
/// template<std::size_t N, class... Ts>
/// constexpr auto arg_c(Ts&&...);
///
///
/// Example
/// -------
///
/// #include <boost/hof.hpp>
/// #include <cassert>
/// using namespace boost::hof;
///
/// int main() {
/// assert(arg(std::integral_constant<int, 3>())(1,2,3,4,5) == 3);
/// }
///
namespace boost { namespace hof {
namespace detail {
template<class T>
struct perfect_ref
{
typedef T type;
typedef typename std::remove_reference<T>::type value_type;
T&& value;
constexpr perfect_ref(value_type& x) noexcept
: value(BOOST_HOF_FORWARD(T)(x))
{}
};
template<std::size_t N>
struct ignore
{
template<class T>
constexpr ignore(T&&...) noexcept
{}
};
template<std::size_t... N>
struct args_at
{
template<class T, class... Ts>
constexpr auto operator()(ignore<N>..., T x, Ts...) const
BOOST_HOF_RETURNS(BOOST_HOF_FORWARD(typename T::type)(x.value));
};
template<std::size_t... N>
constexpr args_at<N...> make_args_at(seq<N...>) noexcept
{
return {};
}
template<std::size_t N, class... Ts>
constexpr auto get_args(Ts&&... xs) BOOST_HOF_RETURNS
(
boost::hof::detail::make_args_at(typename gens<N>::type())(nullptr, BOOST_HOF_RETURNS_CONSTRUCT(perfect_ref<Ts>)(xs)...)
);
template<class T, T N>
struct make_args_f
{
template<class... Ts, class=typename std::enable_if<(N <= sizeof...(Ts))>::type>
constexpr auto operator()(Ts&&... xs) const BOOST_HOF_RETURNS
(
boost::hof::detail::get_args<N>(BOOST_HOF_FORWARD(Ts)(xs)...)
);
};
struct arg_f
{
template<class IntegralConstant>
constexpr make_args_f<std::size_t, IntegralConstant::value> operator()(IntegralConstant) const noexcept
{
return make_args_f<std::size_t, IntegralConstant::value>();
}
};
}
#if BOOST_HOF_HAS_VARIABLE_TEMPLATES
template<std::size_t N>
BOOST_HOF_STATIC_CONSTEXPR detail::make_args_f<std::size_t, N> arg_c = {};
#else
template<std::size_t N, class... Ts>
constexpr auto arg_c(Ts&&... xs) BOOST_HOF_RETURNS
(
boost::hof::detail::get_args<N>(BOOST_HOF_FORWARD(Ts)(xs)...)
);
#endif
BOOST_HOF_DECLARE_STATIC_VAR(arg, detail::arg_f);
}} // namespace boost::hof
#endif
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