Appendix C. A message-passing framework and complete ATM example

Back in chapter 4, I presented an example of sending messages between threads using a message-passing framework, using a simple implementation of the code in an ATM as an example. What follows is the complete code for this example, including the message-passing framework.

Listing C.1 shows the message queue. It stores a list of messages as pointers to a base class; the specific message type is handled with a template class derived from that base class. Pushing an entry constructs an appropriate instance of the wrapper class and stores a pointer to it; popping an entry returns that pointer. Because the message_base class doesn’t have any member functions, the popping thread will need to cast the pointer to a suitable wrapped_message<T> pointer before it can access the stored message.

Sending messages is handled through an instance of the sender class shown in listing C.2. This is just a thin wrapper around a message queue that only allows messages to be pushed. Copying instances of sender just copies the pointer to the queue rather than the queue itself.