Notice he said code units, not code points. Code units are fixed size. So for example, in Java, you have 16-bit code units, and have constant time indexing into them.

So you could use a BreakIterator [1] to iterate through grapheme clusters. The iterator returns the index to the cluster though, not the cluster itself. The index is in terms of code units, not in terms of grapheme clusters.

[1] https://docs.oracle.com/javase/6/docs/api/java/text/BreakIte...