Event-driven simulation class
I am working through some of the exercises in The C++ Programming Language by Bjarne Stroustrup. I am confused by problem 11 at the end of Chapter 12:
(*5) Design and implement a library for writing event-driven simulations. Hint: <task.h>. ... An object of class task should be able to save its state and to have that state restored so that it can operate as a coroutine. Specific tasks can be defined as objects of classes derived from task. The program to be executed by a task might be defined as a virtual function. ... There should be a scheduler implementing a concept of virtual time. ... The tasks will need to communicate. Design a class queue for that. ...
I am not sure exactly what this is asking for. Is a task a separate thread? (As far as I know it is not possible to create a new thread without system calls, and since this is a book about C++ I do not believe that is the intent.) Without interrupts, how is it possible to start and stop a running function? I assume this would involve busy waiting (which is to say, continually loop and check a condition) although I cannot see how that could be applied to a function that might not terminate for some time (if it contains an infinite loop, for example).
EDIT: Please see my post below with more information.
Answer
Here's my understanding of an "event-driven simulation":
- A controller handles an event queue, scheduling events to occur at certain times, then executing the top event on the queue.
- Events ocur instantaneously at the scheduled time. For example, a "move" event would update the position and state of an entity in the simulation such that the state vector is valid at the current simulation time. A "sense" event would have to make sure all entities' states are at the current time, then use some mathematical model to evaluate how well the current entity can sense the other entities. (Think robots moving around on a board.)
- Thus time progresses discontinuously, jumping from event to event. Contrast this with a time-driven simulation, where time moves in discrete steps and all entities' states are updated every time step (a la most Simulink models).
- Events can then occur at their natural rate. It usually doesn't make sense to recompute all data at the finest rate in the simulation.
Most production event-driven simulations run in a single thread. They can be complex by their very nature, so trying to synchronize a multi-threaded simulation tends to add exponential layers of complexity. With that said, there's a standard for multi-process military simulations called Distributive Interactive Simulation (DIS) that uses predefined TCP messages to transmit data between processes.
EDIT: It's important to define a difference between modeling and simulation. A model is a mathematical representation of a system or process. A simulation is built from one or more models that are executed over a period of time. Again, an event driven simulation hops from event to event, while a time driven simulation proceeds at a constant time step.