整理

include/boost/core/identity.hpp

@@ -0,0 +1,61 @@
+/*
+Copyright 2021-2023 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+*/
+#ifndef BOOST_CORE_IDENTITY_HPP
+#define BOOST_CORE_IDENTITY_HPP
+
+#include <boost/config.hpp>
+#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
+#include <utility>
+#endif
+
+namespace boost {
+
+struct identity {
+    typedef void is_transparent;
+
+#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
+    template<class T>
+    BOOST_CONSTEXPR T&& operator()(T&& value) const BOOST_NOEXCEPT {
+        return std::forward<T>(value);
+    }
+#else
+    template<class T>
+    BOOST_CONSTEXPR const T& operator()(const T& value) const BOOST_NOEXCEPT {
+        return value;
+    }
+
+    template<class T>
+    BOOST_CONSTEXPR T& operator()(T& value) const BOOST_NOEXCEPT {
+        return value;
+    }
+#endif
+
+    template<class>
+    struct result { };
+
+    template<class T>
+    struct result<identity(T&)> {
+        typedef T& type;
+    };
+
+#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
+    template<class T>
+    struct result<identity(T)> {
+        typedef T&& type;
+    };
+
+    template<class T>
+    struct result<identity(T&&)> {
+        typedef T&& type;
+    };
+#endif
+};
+
+} /* boost */
+
+#endif