Swift Struct
Swift struct keyword is used to create structures in swift programming.
On a high level, Structures are custom data types that let us store data in the form of properties and Functions(Swift Functions). One of the highlights of Structures is that they are value types and not reference types.
Let’s look at the basic syntax of a struct.
struct structName {
// properties here
}
- A Structure is defined using the keyword struct followed by the name and curly braces.
- Properties are normal variables/constants that are defined in a struct. We’ll look more into Properties in a later tutorial.
- Unlike other languages, Struct in Swift is fully featured. They allow properties, functions, initializers, protocols, etc. Unlike Classes, Structs in Swift doesn’t support inheritance.
- Let’s open up the playground and play around with structures in it.
Swift Struct Example
Here’s an example of a Struct definition with properties.
import Foundation
struct Rectangle {
var width = 0
var height = 0
}
An instance of the above struct can be created in the following way.
import Foundation
struct Rectangle {
var width = 0
var height = 0
}
var rectangle = Rectangle()
To access or modify the properties of a structure we use the dot operator on the instance as shown below.
var rectangle = Rectangle() rectangle.width = 10 rectangle.height = 15 print(rectangle.width) // prints 10 print(rectangle.height) //prints 15
If the instance was initialized as let, changing the properties won’t be possible.
let rectangle = Rectangle() rectangle.width = 10 //Error: Cannot assign to property: 'rectangle' is a 'let' constant rectangle.height = 15 //Error: Cannot assign to property: 'rectangle' is a 'let' constant print(rectangle.width) // prints 10 print(rectangle.height) //prints 15
Defining functions inside a Structure
import Foundation
struct Rectangle {
var width = 0
var height = 0
func area(width :Int, height: Int) -> String {
return "Area is \(width*height)"
}
}
var rectangle = Rectangle()
rectangle.width = 10
rectangle.height = 15
rectangle.area(width: rectangle.width, height: rectangle.height) //prints 150
Modifying struct properties inside functions
To modify a property inside a function we need to mark the function with the keyword mutating.
import Foundation
struct Rectangle {
var width = 0
var height = 0
mutating func printModifiedWidth() -> String {
width = width*2
return "Modified width is \(width)"
}
}
var rectangle = Rectangle()
rectangle.width = 10
rectangle.printModifiedWidth() //prints Modified width is 20
[quote font=”helvetica”]Note: Properties inside a structure can’t be changed inside a simple function. Declaring function as mutating inside struct allows us to alter properties in Structures.[/quote]
Swift Struct Initialisers
The default initializer takes 2 forms. Either an empty initializer() or the memberwise initializer that lists the structure’s properties inside its parenthesis so that you can assign new values to them.
A memberwise initializer for the above structure would look as follows:
var rectangle = Rectangle(width: 10, height: 10)
A custom initializer can be defined as follows:
import Foundation
struct Rectangle {
var width = 0
var height = 0
init(width: Int, height: Int) {
self.width = width*2
self.height = height*2
}
}
var rectangle = Rectangle(width: 10, height: 10)
print(rectangle.width) //prints 20
print(rectangle.height) //prints 20
Initialisers Delegation is also possible as shown below.
import Foundation
struct Rectangle {
var width = 0
var height = 0
init(width: Int, height: Int) {
self.width = width
self.height = height
}
init(width: Int) {
self.init(width: width, height: width)
}
}
var square = Rectangle(width: 10)
print(square.width) //prints 10
print(square.height) //prints 10
Swift Struct Static Functions
Static Functions can be called without creating an instance of Structure as shown below:
import Foundation
struct Rectangle {
var width = 0
var height = 0
static func printString() -> String {
return "Hello how you're doing"
}
}
Rectangle.printString() //prints "Hello how you're doing"
Swift Struct Computed Properties
Properties can be computed dynamically using the following syntax.
import Foundation
struct Rectangle {
var width = 0
var height = 0
var area: Int {
get {
return width*height
}
set {
area = 0
}
}
}
var rectangle = Rectangle()
print(rectangle.area) //prints 0
rectangle.width = 20
rectangle.height = 20
print(rectangle.area) //prints 400
- set is used to give an initial value to the property.
- get is the one that actually returns the computed property.
- Using the computed properties the area property would be automatically calculated when the width or height is altered.
Swift Structures are passed by values and not by reference
import Foundation
struct Rectangle {
var width = 0
var height = 0
var area: Int {
get {
return width*height
}
set {
area = 0
}
}
}
var rectangle = Rectangle()
rectangle.width = 20
rectangle.height = 20
var rect = rectangle
rect.width = 30
print(rectangle.width) //prints 20
print(rect.width) //prints 30
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