This is indeed a "lovely" topic for debate. I'm bound to say that, I'm currently doing a final-year project for college on the topic.
I've came up with a solution, which I feel is the best balance between C++-y style Event classes, and C-style simple code. Simple is great for
RT situations: its easy to see if something is RT or not.

In essence it boils down to this:
-Create an "EventBase" class            with pure virtual type() and size() functions
-Derive actual events like "EventPlay"  overriding type() and size()
-Create a ringbuffer                    checkout the example, it uses the JACK ringbuffer
-Create events *on the stack*           ie EventPlay eventPlay();
-Write events into the ringbuffer using jack_ringbuffer_write();

The last two steps are key, as they enfore the event gets *copied* into the ringbuffer, and the original goes out
of scope (and gets destroyed).

This solution is very workable, on the condition that only fundamental C datatypes are passed trough the Event class.
Things will get nasty if you pass std::shared_ptr<> objects trough it, I've made that mistake already :)

The following code shows communication from the GUI thread to the RT thread. Create another ringbuffer and functions
for communicating in the other direction.

https://github.com/harryhaaren/realtimeAudioThreading

HTH, -Harry