Value vs. Reference Types

Question 1

let p3 = Point(x: 4, y: 2) → p3 is created with a struct of variables x and y. Can we change the x or y for p3?

Answer 1

We cannot change the x and y of p3 despite them being variables because p3 is a constant

Question 2

var p4 = AnotherPoint(x: 3, y: 2) is created with a struct of constants x and y. Can we change the x or y for p4?

Answer 2

We can’t change the x or y for p4 either because x and y are constants.
BUT you can assign p4 to a different AnotherPoint by doing p4 = AnotherPoint(x: 43, y: 6)

Question 3

Why does our function need mutating? ‘

struct Point {
var x: Double
var y: Double

mutating func moveLeft(steps: Double) {
    x -= steps
} }

Answer 3

without mutating, this will not work because we have to remember that value types are immutable.

Question 4

When dealing with classes, what happens to the anotherRobot.model for the example below?

var someRobot = Robot(model: "T1999")
var anotherRobot = someRobot

someRobot.model = “T2000”

Answer 4

Both of these become T2000 because these are both reference types. they are pulling this information from memory

Question 5

class Robot {
    var model: String

init(model:String) {
    self.model = model
} }

If we create an instance of Robot that is a constant, can we change the model of it? See below

let thirdRobot = Robot(model: "T3000")
thirdRobot.model = "T4000"

Answer 5

This does change the model to T4000 because the variable doesn’t actually contain the actual object but a reference of the object in memory.

Question 6

How does the mutating keyword change the behavior of a struct’s instance method?

Answer 6

It indicates that the method will modify the struct’s values allowing the compiler to create a copy of the struct

Question 7

TF: Assigning a value type to a variable means you can change the value assigned to the variable, but not the value itself.

Answer 7

True

Question 8

TF: When we assign an instance of a reference type to a constant, the thing that’s constant isn’t the object, it’s the reference to the object.

Answer 8

True

Question 9

TF: We can modify properties in a reference type even when it is assigned to a constant

Answer 9

True

Question 10

In reference types, final class implements the same behavior as which keyword?

Answer 10

Static

Question 11

How do we prevent a reference type from being subclassed?

Answer 11

Use final keyword

Question 12

TF: Modifying a reference type contained within a value type creates a new copy of the value type

Answer 12

False - Since the reference type is directly modified in memory, the value type is not mutated or copied

Question 13

How does the class keyword differ from static when creating type methods?

Answer 13

Indicated that the method can be overridden in subclasses.

Question 14

If we have a class, final class Calculator {}, can I subclass it?

Answer 14

No, final classes cannot be changed.

Question 15

If we have a method in a class, and we want to alter it in a subclass, what do we have to type before changing the method?

Answer 15

Override, because you are changing it.

Question 16

We have a Struct called Robot and a Class called Robot. We assigned a constant let thirdRobot = Robot(model: “T3000”). We then change the mode, thirdRobot.model = “T4000”. Can we do this for structs and classes?

Answer 16

Structs → We CANNOT because it is a value type.

Classes → You CAN because reference types allow it.

Question 17

How different is it to write a type method for structs vs classes?

Answer 17

For structs, you just type static in front of the function

For classes, you can type static, final OR class in front of the function. If you put class, you can override that function in a subclass. With a static function, you cannot.

Question 18

What is a method inside of another method called?

Answer 18

Helper method