Strings and collection indices

The idea

Swift String is a collection of Character values, each representing an extended grapheme cluster. Integer subscripts are not its character-access interface. startIndex identifies the first position, endIndex is the position after the last character, and index(after:) advances through valid positions. The first property returns an optional, naturally representing the empty-string case without a forced access.

Different views expose different units: utf8 contains encoded bytes, while the String collection contains characters. Do not substitute a byte count for a character count in a user-facing requirement. A Substring slice can share the original storage; convert it to String when you want a standalone stored value. Check boundaries before forming a range and avoid advancing past endIndex in an empty or exhausted collection.

let text="AéB"
let second=text.index(after:text.startIndex)
print(text[second])
print(text.count)
print(text.utf8.count)

é
3
4

The character view contains three characters, while UTF-8 needs two bytes for the accented letter.