Changed interpretation of grapheme clusters

An additional big advancement is the way String interprets grapheme clusters. Conformity of Unicode 9 gives resolution to this.

The use of extended grapheme clusters for character values in Swift 4 means that concatenation and modification of Strings may cause no affect on a resulting String's character count.

For example, if you append a COMBINING ACUTE ACCENT (U+0301) to the end of the String initialized to "cafe", the resulting String will have a character count of 4, and the fourth character will be "e", not e':

var word = "cafe"
print("total chars in \(word) is \(word.count)")

It prints "total chars in cafe is 4":

word += "\u{301}"    // COMBINING ACUTE ACCENT, U+0301
print("totalchars in \(word) is \(word.count)")

It prints "total chars in cafe? is 4", whereas the count would increase by 1 to reflect 5 as a result of print statement earlier.

Similar to Dictionaries, the total number of modifications made to String API can be summed up by the following image: