What is the difference between "let" and "static let"?

Question

class Foo {
    let fooValue = 1
}

print(Foo.fooValue) // not work


class Bar {
    static let barValue = 1
}

print(Bar.barValue) // work; print "1"

Why? I expected that Foo example to work, because the value of fooValue is constant, value and memory address known in compilation time. But I need use keyword static to work.

Answer 1

fooValue is an instance property. There's one separate fooValue per every instance (object) of the Foo class.

barValue is a static property. There's one shared barValue that belongs to the class.

Here's a more concrete example. Suppose I had this class:

class Human {
    static let numberOfLimbs = 4
    let name: String
}

What would you expect to happen if I asked you what the name of a Human is? I.e. Human.name. Well, you wouldn't be able to answer me, because there's no one single name of all humans. Each human would have a separate name. You could however, tell me the number of limbs humans have, (Human.numberOfLimbs), which is (almost) always 4.