Once when I was 11 - a girl was having a go at me in the school playground with her friends and went to rake her claws (she had inch long nails that she filed into points) across my face. I knew that was what was coming as she did it frequently to others.
As I'd grown up being regularly battered, I knew on some level that she wasn't actually going to achieve anything other than possibly scarring me for life, but had clearly never been on the receiving end of retaliation/self defence from her usual victims, as she was completely unguarded on her left side. Instead of punching or pushing her (as that would have put my face even closer to damage), I did a full on, back and shoulder into it, bodyweight swing and got her perfectly across the cheek.
The shock in her eyes and the bright red flushing on her cheek just before she ran off howling was quite something. I didn't even get questioned about it by the teachers, so I guess either somebody saw it all happen or they were told by the other kids why I'd done it.
She came back to school after the weekend with her nails all cut down.
I've been slapped and it stings, but as long as your ear isn't caught with full force, it can't actually hurt you. My automatic response is rage, not upset. Being roundhouse kicked so that it lands like a slap really, really stings (it was a training accident), but even that just made me angry - because I knew it was an accident, I walked stormed off and practically took the doors off the gym as I went to get myself under control.
I would never slap a man, as that would be completely ineffective and end up with me in hospital - the only way to deal with somebody that much bigger and stronger is to be far more aggressive if you can't run away immediately - to give you space to run for your life. So I've sent a few flying in my time.
In short, don't slap if you're in danger. Evade and run.