Here is all in one -- copied from here
let str = "Hello"
let count = str.length // returns 5 (Int)
extension String {
var length: Int { return countElements(self) } // Swift 1.1
}
extension String {
var length: Int { return count(self) } // Swift 1.2
}
extension String {
var length: Int { return characters.count } // Swift 2.0
}