std::tie creates a tuple of references, useful for unpacking tuples into existing variables or for comparisons. Unpacking example: int a, b; tie(a, b) = make_pair(1, 2); Now a = 1 and b = 2.
This works with existing variables, unlike structured bindings which declare new ones. Comparison example: tie(a1, b1) < tie(a2, b2) compares lexicographically. First compare a1 vs a2, then b1 vs b2 if equal.
This is cleaner than writing the comparison manually. Use std::ignore to skip values you don't need: tie(x, ignore, z) = someTuple; This extracts only the first and third elements.