c++ - How can I make message passing between threads in a multithreaded engine less cumbersome?

The C++ engine I'm working on currently is split up into several large threads- Generation (for creating my procedural content), Gameplay (for AI, scripts, simulation), Physics, and Rendering.

The threads communicate with one another through small message objects, which pass from thread to thread. Before stepping, a thread processes all of its incoming messages- updates to transforms, adding and removing objects, etc. Sometimes one thread (Generation) will create something (Art) and pass it to another thread (Rendering) for permanent ownership.

Early in the process and I've noticed a couple things:

1. 
   

   The messaging system is cumbersome. Creating a new message type means subclassing the base Message class, creating a new enum for its type, and writing logic for how the threads should interpret the new type of message. It's a speed-bump for development and is prone to typo-style errors. (Sidenote- working on this has me appreciating how great dynamic languages can be!)

   
   

   Is there a better way to do this? Should I use something like boost::bind to make this automatic? I'm worried that if I do that I lose the ability to say, sort the messages based on type, or something. Not sure if that kind of management will even become necessary.

   
   


2. 
   

   The first point is important because these threads do so much communicating. Creating and passing messages is a big part of making things happen. I'd like to streamline that system, but also be open to other paradigms that might be just as helpful. Are there different multithreaded designs I should think about to help make this easier?

   
   

   For example, there are some resources which are infrequently written, but frequently read from multiple threads. Should I be open to the idea of having shared data, protected by mutexes, that all threads can access?

   
   

This is my first time designing something with multithreading in mind from the ground up. At this early stage I actually think it's going really well (considering) but I'm worried about scaling, and my own efficiency at implementing new stuff.