How come I didn't have to instantiate the NSCharacterSet object first before using it?

Question

I'm following a tutorial learning about delegates and protocols while using the UITextField object. In the tutorial I noticed that I didn't have to instantiate the NSCharacterSet object first by typing

let letterCharacters = NSCharacterSet()

The tutorial's code worked by reaching directly for the letters variable of NSCharacterSet which is mind blowing. I assumed objects always needed to be instantiated first before using them or making reference to them. Here is the full function that works:

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        let existingTextHasDecimalSeperator = textField.text?.range(of: ".")
        let replacementTextHasDecimalSeperator = string.range(of: ".")
        let letterCharacters = NSCharacterSet.letters
        let foundCharacter = string.rangeOfCharacter(from: letterCharacters)


        if existingTextHasDecimalSeperator != nil && replacementTextHasDecimalSeperator != nil {
            return false
        } else if foundCharacter != nil {
            return false
        } else {
            return true
        }
    }

Why am I allowed to use NSCharacterSet.letters directly instead of creating a NSCharacterSet object first?

Answer 1

From the Apple documentation:

class var letters: CharacterSet

A character set containing the characters in Unicode General Category L* & M*.

The key piece here is the class part of that definition. If it is a class variable, than you do not need to instantiate the object with NSCharacterSet().

A quick primer on variable types.

• A class variable can be accessed simply by calling the variable with the class name CharacterSet.letters for example
• An instance variable needs to have its class instantiated first before it can be accessed, characterSet.inverted would be an example of this

Typically, a class variable would never change so it has no need to maintain state. An instance variable likely will.