Smacking is an easy way out and shows you have lost control. The problem is very few people know how to exert control. You have to start at an early age, do not rush every time your baby or child demands attention.  Assess if immediate action is required or not.  All persons looking after a child need to be consistent  in their approach.  When giving instructions or commands say it as though you mean it.
To smack someone smaller and younger than you is ruling by fear. It is also teaching your child, that it is right to hit someone else.
In teaching, teachers need to be able to teach and also exert control. You can always spot the teacher who can not control, they are the ones, who have an unruly class.
Follow the example of the teacher, whose class is not unruly.
I have never hit a child in my life, but I find that when I give an instruction, the child obeys. It may be that my voice instills fear, I do not know.
A smack is always a form of assault, try smacking an adult.