Declare tun factory and return its instance in new_tun_factory method

OpenVPN Adapter/OpenVPNClient.mm

@@ -10,6 +10,7 @@
 
 #import <Foundation/Foundation.h>
 
+#import "TUNFactory.h"
 #import "OpenVPNAdapter+Internal.h"
 #import "OpenVPNClient.h"
 
@@ -18,7 +19,7 @@ OpenVPNClient::OpenVPNClient(void *adapter) : ClientAPI::OpenVPNClient() {
 }
 
 TunClientFactory* OpenVPNClient::new_tun_factory(const ExternalTun::Config& conf, const OptionList& opt) {
-    return nullptr;
+    return new TUNFactory();
 }
 
 bool OpenVPNClient::tun_builder_new() {

OpenVPN Adapter/TUNFactory.h

@@ -0,0 +1,18 @@
+//
+//  TUNFactory.h
+//  OpenVPN Adapter
+//
+//  Created by Sergey Abramchuk on 15.04.17.
+//
+//
+
+#import <openvpn/tun/client/tunbase.hpp>
+
+using namespace openvpn;
+
+class TUNFactory: public TunClientFactory {
+public:
+    virtual TunClient::Ptr new_tun_client_obj(openvpn_io::io_context& io_context,
+                                              TunClientParent& parent,
+                                              TransportClient* transcli) override;
+};

OpenVPN Adapter/TUNFactory.mm

@@ -0,0 +1,18 @@
+//
+//  TUNFactory.m
+//  OpenVPN Adapter
+//
+//  Created by Sergey Abramchuk on 15.04.17.
+//
+//
+
+#import "TUNFactory.h"
+
+// !!! tuncli.hpp may be used as example of implementation
+
+TunClient::Ptr TUNFactory::new_tun_client_obj(openvpn_io::io_context& io_context,
+                                              TunClientParent& parent,
+                                              TransportClient* transcli)
+{
+    return nullptr;
+}