Question

I’m using a library that has callbacks like this:

void onReceive (Lacewing::Server &Server, Lacewing::Server::Client &Client,
                char * Data, int Size) {
  /* callback body */
}

Server.onReceive (onReceive); /* to register the handler */

I would like to be able to wrap this in a class that can decide what to do when it receives a packet (observer pattern).

How can I do this with C style callbacks? The library does not define an interface to inherit from.

Thanks

Answer 1

Since you’re using liblacewing, every class has a void * Tag member provided for user data:

/* class method */

void MyServer::onReceive (Lacewing::Server &Server, Lacewing::Server::Client &Client,
            char * Data, int Size)
{
     /* callback body - this is inside the class */
}


/* global function wraps the class method */

void onReceive (Lacewing::Server &Server, Lacewing::Server::Client &Client,
            char * Data, int Size)
{
    ((MyServer *) Server.Tag)->onReceive (Server, Client, Data, Size);
}

then:

Server.Tag = myServerInstance; /* set the class instance pointer */
Server.onReceive (::onReceive); /* register the global function */