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+/*=============================================================================
+ Copyright (c) 2012 Paul Fultz II
+ identity.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_IDENTITY_H
+#define BOOST_HOF_GUARD_FUNCTION_IDENTITY_H
+
+/// identity
+/// ========
+///
+/// Description
+/// -----------
+///
+/// The `identity` function is an unary function object that returns whats given to it.
+///
+/// Semantics
+/// ---------
+///
+/// assert(identity(x) == x);
+///
+/// Synopsis
+/// --------
+///
+/// template<class T>
+/// constexpr T identity(T&& x);
+///
+
+#include <utility>
+#include <initializer_list>
+#include <boost/hof/detail/forward.hpp>
+#include <boost/hof/detail/static_const_var.hpp>
+
+namespace boost { namespace hof { namespace identity_detail {
+
+struct identity_base
+{
+ template<class T>
+ constexpr T operator()(T&& x) const
+ noexcept(std::is_reference<T>::value || BOOST_HOF_IS_NOTHROW_MOVE_CONSTRUCTIBLE(T))
+ {
+ return BOOST_HOF_FORWARD(T)(x);
+ }
+
+ template<class T>
+ constexpr std::initializer_list<T>& operator()(std::initializer_list<T>& x) const noexcept
+ {
+ return x;
+ }
+
+ template<class T>
+ constexpr const std::initializer_list<T>& operator()(const std::initializer_list<T>& x) const noexcept
+ {
+ return x;
+ }
+
+ template<class T>
+ constexpr std::initializer_list<T> operator()(std::initializer_list<T>&& x) const noexcept(noexcept(std::initializer_list<T>(std::move(x))))
+ {
+ return BOOST_HOF_FORWARD(std::initializer_list<T>)(x);
+ }
+};
+
+}
+
+BOOST_HOF_DECLARE_STATIC_VAR(identity, identity_detail::identity_base);
+
+}} // namespace boost::hof
+
+#endif