feat():initial version

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

@@ -0,0 +1,72 @@
+/*=============================================================================
+    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