Added a new boost/core/functor.hpp header.

The feader defines a new functor class template that can be used to wrap
raw functions into a function object class. This is useful, for example,
for integrating std::unique_ptr and unique_resource with custom deleters
implemented as raw functions (e.g. in C libraries).

include/boost/core/functor.hpp

@@ -0,0 +1,34 @@
+/*
+ *             Copyright Andrey Semashev 2024.
+ * 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)
+ */
+/*!
+ * \file   functor.hpp
+ * \author Andrey Semashev
+ * \date   2024-01-23
+ *
+ * This header contains a \c functor implementation. This is a function object
+ * that invokes a function that is specified as its template parameter.
+ */
+
+#ifndef BOOST_CORE_FUNCTOR_HPP
+#define BOOST_CORE_FUNCTOR_HPP
+
+namespace boost::core {
+
+//! A function object that invokes a function specified as its template parameter
+template< auto Function >
+struct functor
+{
+    template< typename... Args >
+    auto operator() (Args&&... args) const noexcept(noexcept(Function(static_cast< Args&& >(args)...))) -> decltype(Function(static_cast< Args&& >(args)...))
+    {
+        return Function(static_cast< Args&& >(args)...);
+    }
+};
+
+} // namespace boost::core
+
+#endif // BOOST_CORE_FUNCTOR_HPP