drogon/community

Hey Guys
Just a simple question!
this is a test for me!
I wanna bind a callback to specific port!
in other words, I need to make conditions on port numbers to send related response to client that sends request to specific port!
as you can see, we can just add some listeners having ip , port!
But addListener method has no any callback args!

drogon::app().addListener("0.0.0.0",20001);
drogon::app().addListener("0.0.0.0",20002);
drogon::app().addListener("0.0.0.0",20003);

drogon::HttpAppFramework::instance().registerHandler("/index",
                        [=](const drogon::HttpRequestPtr &req , 
                            std::function<void (const drogon::HttpResponsePtr &)> &&callback)
                        {
                            if (req ->port == 20001)
                            {
                                auto resp = drogon::HttpResponse::newFileResponse("/path/to/img01.jpg", "", drogon::ContentType::CT_IMAGE_JPG);
                                callback(resp);
                            }
                            else if (req ->port == 20002)
                            {
                                auto resp = drogon::HttpResponse::newFileResponse("/path/to/img02.jpg", "", drogon::ContentType::CT_IMAGE_JPG);
                                callback(resp);
                            }
                            else if (req ->port == 20003)
                            {
                                auto resp = drogon::HttpResponse::newFileResponse("/path/to/img03.jpg", "", drogon::ContentType::CT_IMAGE_JPG);
                                callback(resp);
                            }
                        });

in the top example, req->port has error because req has no any port member!
so I don't know how can I set conditions or bind callbacks on ports!
All The Best,
Hadi