Added thirdparty: boost library

thirdparty/include/boost/any/detail/placeholder.hpp

@@ -0,0 +1,33 @@
+// Copyright Antony Polukhin, 2021-2023.
+//
+// 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_ANY_ANYS_DETAIL_PLACEHOLDER_HPP
+#define BOOST_ANY_ANYS_DETAIL_PLACEHOLDER_HPP
+
+#include <boost/config.hpp>
+#ifdef BOOST_HAS_PRAGMA_ONCE
+# pragma once
+#endif
+
+#include <boost/type_index.hpp>
+
+/// @cond
+namespace boost {
+namespace anys {
+namespace detail {
+
+class BOOST_SYMBOL_VISIBLE placeholder {
+public:
+    virtual ~placeholder() {}
+    virtual const boost::typeindex::type_info& type() const noexcept = 0;
+};
+
+} // namespace detail
+} // namespace anys
+} // namespace boost
+/// @endcond
+
+#endif  // #ifndef BOOST_ANY_ANYS_DETAIL_PLACEHOLDER_HPP