Add hub tests and example

test/test_hub_empty_sort.cpp

@@ -0,0 +1,32 @@
+/* Copyright 2026 Joaquin M Lopez Munoz.
+ * 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)
+ */
+
+#include <boost/config.hpp>
+
+#if BOOST_CXX_VERSION < 201103L
+
+int main() { return 0; }
+
+#else
+
+#include <boost/container/hub.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+
+int main()
+{
+  /* Warning reported in 
+   * https://lists.boost.org/archives/list/boost@lists.boost.org/
+   * message/ZTL2D2UMIHAAC6ZLHLH4Y3XETTQXAHBZ/ .
+   */
+  boost::container::hub<int> h{};
+  h.sort();
+  BOOST_TEST(h.empty());
+
+  return boost::report_errors();
+}
+
+#endif