Asynchronous single-threaded event handler

For demonstration purposes only

AsyncEventHandler implementation
Dialect: C++20 (uses <semaphore>)

Properties:

• 1 thread, sleeps when idle
• 1 externally provided event handler function for all events
• 1 externally provided event parameter buffer of user-defined size
• 4 handler function parameters individual to every event
• 1 externally provided queue of events of user-defined size
• Binds event ID number to a set of parameters for the handler
• Events triggered using event number
• Supports negative event numbers (for example, for error handling events)
• Activate/deactivate handler execution on per-event and global level
• Diverse error codes in case something goes wrong
• Trivially destructible
• Buffer memory and a thread object are provided externally (doesn't manage object lifetime of anything)
• You can change internal pointers at runtime if you're into that sort of thing
• It actually seems to work

Includes a test function with detailed explanation and example of setup, use and error handling.