[carlopi]

Not sure whether they classify as obscure, but I haven't see cited already:

- Dominator tree (https://en.wikipedia.org/wiki/Dominator_(graph_theory))

- Single-Connected-Components-Graph

- Deterministic data structures (eg. a set that acts deterministic to the fact that addresses might be somehow randomly assigned, very useful for ensuring reproducibility)

Already cited, but it's clearly among the most elegant:

- union-find (!!!!)

and as a bonus one that is easily overlooked:

-std::deque, that when restricted to push_back() or push_front() guarantees not to ever move objects around.