Function S2CellId.childBegin

Iterator-style methods for traversing the immediate children of a cell or all of the children at a given level (greater than or equal to the current level). Note that the end value is exclusive, just like standard STL iterators, and may not even be a valid cell id. You should iterate using code like this:

for(S2CellId c = id.child_begin(); c != id.child_end(); c = c.next()) ...

The convention for advancing the iterator is "c = c.next()" rather than "++c" to avoid possible confusion with incrementing the underlying 64-bit cell id.