Add C++ filter example

The filter example uses the C++ template which helps new filter creation
by showing the minimal implementation required for a filter.

examples/examplefiltersession.hh

@@ -0,0 +1,44 @@
+#pragma once
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include <maxscale/cppdefs.hh>
+#include <maxscale/filter.hh>
+
+class ExampleFilter;
+
+class ExampleFilterSession : public maxscale::FilterSession
+{
+    // Prevent copy-constructor and assignment operator usage
+    ExampleFilterSession(const ExampleFilterSession&);
+    ExampleFilterSession& operator = (const ExampleFilterSession&);
+
+public:
+    ~ExampleFilterSession();
+
+    // Called when a client session has been closed
+    void close();
+
+    // Create a new filter session
+    static ExampleFilterSession* create(MXS_SESSION* pSession, const ExampleFilter* pFilter);
+
+    // Handle a query from the client
+    int routeQuery(GWBUF* pPacket);
+
+    // Handle a reply from server
+    int clientReply(GWBUF* pPacket);
+
+private:
+    // Used in the create function
+    ExampleFilterSession(MXS_SESSION* pSession);
+};