C++ - calling derived function from abstract base pointer

Question

Hello, I have been trying to create a TCP Server model based on inheritance, with varying success. These servers are managed by a singleton whose task it is to shut these servers down and other simple maintenance functions:

class TCPServer {
public:
    TCPServer();
    ~TCPServer();

    void Bind(TCPDaemon *daemon) {
        if(!daemon->IsRunning()) {
            throw TCPBindException("Daemon is inactive");
        }

        // if the port is not taken, bind this daemon to it
        if(this->servers.count(daemon->port())==0) {
            this->servers[daemon->port()]=daemon;
            ...
        } else {
            throw TCPBindException("Port is taken");
        }
    }

    void Shutdown() {
        MASON::UINT16 i;
        for(i=0;i<this->servers.size();i++) {
            this->Shutdown((*this->servers.begin()).first);
        }
    }

    void Shutdown(unsigned short port)  {
        if(this->servers.count(port)) {

            if(this->servers[port]->IsRunning()) {
                this->servers[port]->Stop();
            }

            delete this->servers[port];
            this->servers.erase(port);

        }
    }

private:
    std::map<unsigned short, TCPDaemon*> servers;

};

The TCPDaemon class's Stop() function is a pure virtual. My problem is that when the Shutdown() function is called, it is attempting to call this pure virtual instead of the derived class' version. How can I force it to do the right thing?

Thanks in advance

[edit] sorry I did not include the TCPDaemon code before, it derives from a TCPSocket class (which I have checked to be 100% working and is fairly self-explanatory). Here it is:

class TCPDaemon: public TCPSocket {
public:
    TCPDaemon(unsigned short port) {
        this->_enabled=false;
        this->_host.ipaddr(INADDR_ANY);
        this->_host.port(port);
        this->paused=false;

        struct sockaddr_in opts=this->_host.Compile();

        #ifdef PLATFORM_WINDOWS
            WSADATA wsaData;
            if(WSAStartup(0x0202, &wsaData)) {
                throw TCPDaemonException("Failed to start WSA");
            }
        #endif

        this->raw_socket=socket(AF_INET, SOCK_STREAM, 0);
        if(this->raw_socket<=0) {
            throw TCPDaemonException("Failed to create socket");
        }

        if(int status=bind(this->raw_socket, (sockaddr*)&opts, sizeof(sockaddr))) {
            printf("error [%i]\r\n", status);
            throw TCPDaemonException("Failed to bind to port");
        }

        if(listen(this->raw_socket, 5)) {
            throw TCPDaemonException("Failed to listen on port");
        }

        this->_enabled=true;

    }

    virtual ~TCPDaemon() {
        this->Shutdown();
    }

    virtual void Start()=0;
    virtual void Run(TCPSocket*)=0;
    virtual void Stop()=0;

    unsigned short port() {
        return this->host().port();
    }

    bool IsRunning() {
        return this->_enabled;
    }

    TCPSocket *Accept() {
        SOCKET client;
        struct sockaddr client_addr;
        int len=sizeof(client_addr);
        client=accept(this->raw_socket, &client_addr, &len);

        return new TCPSocket(client, &client_addr);
    }

    void Shutdown() {

    }

private:
    bool _enabled;
    bool paused;

};

and here is a sample derived server and its method of creation:

   class EchoServer: public TCPDaemon {
    public:
        EchoServer(MASON::UINT16 port): TCPDaemon(port) {
        }

        ~EchoServer() {}

        virtual void Start() {

        }

        virtual void Run(TCPSocket *client) {
            printf("RUN\r\n");
            Accessor<TCPSocket> acc_client=client;
            acc_client->Write(Accessor<Blob> (new Blob(std::string("hello!"))));
            acc_client->Disconnect();
        }

        virtual void Stop() {

        }

    };

myTCPServer->Bind(new EchoServer(8008));

[edit+1] I think what the problem boils down to is this (i could easily be wrong): I have a std::map of the base class, TCPDaemon, which has a pure virtual/abstract function Stop(). It appears that when I call Stop() through one of the entries in the map, it is attempting to call TCPDaemon::Stop(), as opposed to the overriding function EchoServer::Stop(). Could this be the issue? If so, how do I resolve it?

Answer 1

If you say that it calls pure virtual method instead of derived class' version, that means that your derived class calls it. E.g.

class t1
{
 public:
virtual ~t1(){};
virtual void foo()=0        
  {
    std::cout << "pure virtual";
};
};

class t2:public t1
{
public :

virtual void foo() 
    {
    t1::foo();
    std::cout << "derived class ";
};
};

As far as I remember only derived class' object can call base class' pure virtual function (surely, if it's implemented in base class)

Answer 2

Check the syntax of what you are declaring:

class TCPDaemon
{
    virtual void stop() = 0;
};

class MyDaemon : public TCPDaemon
{
    virtual void stop()
    {
        //Do stuff here.
    }
};

That's the best I can do without more code.

EDIT:

Ok, so it seems like you are using abstract functions. Next question is this: what is the error you are getting? I can say for certain that it is not attempting to call Stop() from TCPDeamon. That would be impossible, as it is not even implemented.

Answer 3

I'm not sure if this is the problem you're seeing, but there's certainly a problem in the following function.

void Shutdown() {
    MASON::UINT16 i;
    for(i=0;i<this->servers.size();i++) {
        this->Shutdown((*this->servers.begin()).first);
    }
}

After each loop iterator, i will increase by one AND size will decrease, so you'll only ShutDown half your servers. Maybe keeping some open is the cause of further errors.

You could do either:

void Shutdown() {
    MASON::UINT16 i;
    std::size_t nrServers = this->servers.size();
    for(i=0;i<nrServers;i++) {
        this->Shutdown((*this->servers.begin()).first);
    }
}

or, which I would prefer as it better shows the intention of the code:

void Shutdown() {
    while (!this->servers.empty()) {
        this->Shutdown((*this->servers.begin()).first);
    }
}

Answer 4

I finally worked it out in the end, thanks to the input I have received. The problem was with the delete call in TCPServer::Shutdown(unsigned short), which was causing a memory access violation in a completely different part of the code... A rather noobie mistake, I shall be wrapping smart pointers around everything ASAP.

Thanks for all your feedback!