what are the unspoken rules around approaching women in the streets?

Suppose I am drinking in a bar. Asking a bar tender for her number doesnt't seem too out there(maybe in the bar scene this is expected). But what about a girl at the supermarket? A barista at a cafe? A lifeguard on the beach?

Are these forbidden territories? Can one traverse the lands and end up with the richness of the soil? I don't know.

I'm sure you lads have some stories....