feat():initial version

install/boost_1_75_0/include/boost/hof/arg.hpp

@@ -0,0 +1,126 @@
+/*=============================================================================
+    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