OP: Smacking a toddler is , imo, popintless from a discipline pov.
They are too young to be able to project what they feel on to somone else, that is a clear developmental stage which they simply haven't reached. They learn by copying. So you won't teach a young child 'do as you wouldn't be done by' by smacking, and are more likely to teach them 'if someone does something you don't like, hit them!'. Toddlers are also programmed to take pysical knocks in their stride and not be deterred: how else would they learn to walk? So although they won't like being smacked, it may well not deter them next time.
Bloss: you see, I think talk of punishing small children for not sharing toys is starting fom the wrong premise. They have no idea that they are hurting someone else by snatching, or not sharing (again, no theory of the mins, yet), they are deeply territorial, and jealous, and think that THEY will suffer if they don't have the toy. They are simply not able to cope, developmentally, with deferred gratification. So, distraction, etc. OR it is possible, with some children, to actively teach them to 'take turns', and learn that they will get a toy back, and quickly, and that they will survive the experience of having not been in possession. You just play 'quick turns' with them, "your turn, your turn, now your turn" . Not all children will be ammenable, but I know some who have been.
As it happens, I don't think smacking is always abusive or the worst thing that can happen or damaging, (often it is, and always when extreme, but not always or inevitably, I think) but I don't see that it is necessary, or beneficial.