Difference between Value type and Reference type

Value Types

A value type instance is an independent instance and holds its data in its own memory allocation. There are a few different value types: Struct, Enum, Tupple.

Struct

Let’s experiment with struct and prove that they’re value types:

Add the following code to your playground:

// 1
struct Car {
    let brand: String
    var model: String
}

// 2
var golf = Car(brand: "Volkswagen", model: "Golf")
// 3
let polo = golf

// 4
golf.model = "Golf 2019"

// 5
print(golf)
print(polo)

In the code above, you will:

• Create a Car struct with brand and model properties.
• Create a new instance of Car named golf.
• Create a copy of the golf instance, named polo.
• Change the golf.model variable to Golf 2019
• Print the 2 different instances. The first print statement prints Car(brand: "Volkswagen", model: "Golf 2019") in the Console. The second one prints Car(brand: "Volkswagen", model: "Golf"). Even if polo is a copy of golf, the instances remain independent with their own unique data copies.

With this simple playground, we’ve confirmed that structs are indeed value types.

Enum

To check that enums are value types, add this code to the playground:

// 1
enum Language {
    case italian
    case english
}

// 2
var italian = Language.italian
// 3
let english = italian

// 4
italian = .english

// 5
print(italian)
print(english)

In the code above, you will:

• Create a Language enum with italian and english cases.
• Create a new instance of Language for the italian language.
• Create a copy of the italian instance, named english.
• Change the italian instance to english.
• Print the two different instances. The first print statement prints english, and the second one prints italian. Even if english is a copy of italian, the instances remain independent.

Tuple

The last value type that we'll explore is tuple. A tuple type is a comma-separated list of zero or more types, enclosed in parentheses. You can access its values using the dot (.) notation followed by the index of the value.

You can also name the elements in a tuple and use the names to access the different values.

Add the following code to the playground:

// 1
var ironMan = ("Tony", "Stark")
// 2
let parent = ironMan

// 3
ironMan.0 = "Alfred"

// 4
print(ironMan)
print(parent)

In the code above, you will:

• Create an ironMan tuple with the strings Tony and Stark.
• Create a copy of the ironMan instance, named parent.
• Change the ironMan.0 index to Alfred.
• Print the 2 different instances. The first print, prints ("Alfred", "Stark") and the second one, prints ("Tony", "Stark"). Again, the instances remain independent.

You can now be certain that structs, enums, and tuples are value types! 

When to Use Value Types

Use value types when comparing instance data with == makes sense.
== checks if every property of the two instances is the same.
With value types you always get a unique, copied instance, and you can be sure that no other part of your app is changing the data under the hood. This is especially helpful in multi-threaded environments where a different thread could alter your data.

Use a value type when you want copies to have an independent state, and the data will be used in code across multiple threads.

In Swift, Array, String, and Dictionary are all value types.

Reference Types

In Swift, reference type instances share a single copy of their data, so that every new instance will point to the same address in memory. A typical example is a class, function, or closure.

To explore these, add a new function to your playground:

func address<T: AnyObject>(of object: T) -> Int {
    return unsafeBitCast(object, to: Int.self)
}

This function prints the address of an object, which will help you check whether you're referencing the same instance or not.

Class

The first reference type that you'll look at is a class.

Add the following code to your playground:

// 1
class Dog: CustomStringConvertible {
    var age: Int
    var weight: Int

    // 2
    var description: String {
        return "Age \(age) - Weight \(weight)"
    }

    // 3
    init(age: Int, weight: Int) {
        self.age = age
        self.weight = weight
    }
}

// 4
let doberman = Dog(age: 1, weight: 70)
// 5
let chihuahua = doberman

// 6
doberman.age = 2
// 7
chihuahua.weight = 10

// 8
print(doberman)
print(chihuahua)

// 9
print(address(of: doberman))
print(address(of: chihuahua))

In the code above, you will:

• Create a new class named Dog, that conforms to CustomStringConvertible to print the custom descriptions of the object.
• Define the custom description of the object.
• Create a new init function. This is needed because, unlike a struct, a class doesn't automatically create an initialization function based on the variables of the object.
• Create a doberman instance of Dog.
• Create a copy of doberman, named chihuahua.
• Change the doberman.age to 2.
• Change the chihuahua.weight to 10.
• Print the description of the two different instances. The first print, prints Age 2 - Weight 10, and the second one prints the same; Age 2 - Weight 10. This is because you're actually referencing the same object.
• Print the address of the two different instances. With these prints, you'll be sure that you're referencing the same address. You'll see that both print statements print the same value.

You can rest assured that a class is a reference type.

Functions and Closures

A closure is used to refer to a function along with the variables from its scope that it encapsulates. Functions are essentially closures that store references to variables in their context.

Take a look at the code below:

let closure = { print("Test") }
func function() -> (){ print("Test") }

closure()
function()

They both do the same thing.

You can find more info about closures in Swift's docs.

When to Use Reference Types

Use a reference type when comparing instance identity with ===makes sense. === checks if two objects share the same memory address.

They’re also useful when you want to create a shared, mutable state.

As a general rule, start by creating your instance as an enum, then move to a struct if you need more customization, and finally move to class when needed.