Class 2

What's the different between exclamation mark and question mark after the vars? they are same, but exclamation mark means the var will automatical unwrap.

@IBOutlet weak var display: UILabel!
@IBOutlet weak var display: UILabel?

Class 3

Same way to instance array and dictionary

var opStack = Array<Op>()
var opStack = [Op]()

var knowOps = Dictionary<String, Op>()
var knowOps = [String:Op]()

In Swift Array and Dictionary is struct, not class. When passed to argument, they are copied. Copied isn't copy actually, untill the value has be changed.

Class 4

lazy

only var can lazy initialized, let can't.

init method

There is a free init for class and struct

Designated init:

initialize all properties introduced by you class, before calling a superclass's init (different with Obj-C and C++) Convenience init: Must call designated init

AnyObject

cast with as

check with is

var b: AnyObject
var a = b as? UIButton 

if b is UIButton {

}

String.Index

Unicode characters, can use Int to index a string

Class 5

The two method to hierarchy in code

addSubview(aView: UIView) // sent to a aVies's superview
removeFromSuperView()    // this is send to the view you want to remove