Question

I’m trying to implement a Polymorphic Queue.
Here is my trial:

QQueue <Request *> requests;

while(...)
    {
        QString line = QString::fromUtf8(client->readLine()).trimmed();

        if(...)){                      
            Request *request=new Request();
            request->tcpMessage=line.toUtf8();
            request->decodeFromTcpMessage(); //this initialize variables in request using tcpMessage
            if(request->requestType==REQUEST_LOGIN){
                LoginRequest loginRequest;
                request=&loginRequest;
                request->tcpMessage=line.toUtf8();
                request->decodeFromTcpMessage();
                requests.enqueue(request);
            }
            //Here pointers in "requests" do not point to objects I created above, and I noticed that their destructors are also called.
            LoginRequest *loginRequest2=dynamic_cast<LoginRequest *>(requests.dequeue());   
            loginRequest2->decodeFromTcpMessage();
        }
    }

Unfortunately, I could not manage to make work Polymorphic Queue with this code because of the reason I mentioned in second comment.I guess, I need to use smart-pointers, but how?
I’m open to any improvement of my code or a new implementation of polymorphic queue.

Thanks.

Answer 1

There are 2 problems in your source:

• you claim memory by the Request *request=new Request();, which gets abandoned by the later request=&loginRequest; assignment (and is no longer deletable)
• the LoginRequest loginRequest; variable gets destructed when the execution leaves the {} block where the variable is defined, resulting in a dangling pointer in
request

I would suggest to remove the Request *request=new Request(); line, and later in the if(...){ block assign the concrete LoginRequest object by

LoginRequest* loginRequest = new LoginRequest();
/* fill the request */
requests.enqueue(loginRequest);

You can get rid of the queued objects by deleting them manually when they got poped out of the queue (after they are processed), or by using a container-safe smartpointer in the queue (boost::shared_ptr is fine, maybe QT has also one of them, std::auto_ptr IS NOT container-safe).

PITFALL Also make sure that the destructor of Request is virtual, since you cannot delete objects by a pointer to its base classe when there is no virtual destructor in the base class(c++ can call the base class destructor with the derived class instance in this case, leading in undefined behavior like memory leaks or crashes)