Swift Initializers

An initializer is a special method that is called when an instance of a class, structure, or enumeration is created. It is used to set the initial values of the instance's properties and perform any other setup necessary for the instance to work properly. Swift provides several types of initializers that you can use to create and initialize your objects.

Default Initializer

Swift automatically provides a default initializer for any class or structure that does not have any other initializers defined. This initializer has no parameters and sets all of the instance's properties to their default values. You can call this initializer by using the type's name followed by empty parentheses.

1class Person {
2    var name: String = ""
3    var age: Int = 0
4}
5
6let William = Person()

Parameterized Initializer

A parameterized initializer is a custom initializer that allows you to set the initial values of the instance's properties when you create an object. You define a parameterized initializer by adding one or more parameters to the initializer's signature, and then assigning the parameter values to the instance's properties.

1class Person {
2    var name: String
3    var age: Int
4    
5    init(name: String, age: Int) {
6        self.name = name
7        self.age = age
8    }
9}
10
11let William = Person(name: "William", age: 22)

Initializer Overloading

You can define multiple initializers for a class or structure by using initializer overloading. This allows you to provide different ways to create objects based on the input parameters.

1class Person {
2    var name: String
3    var age: Int
4    
5    init(name: String, age: Int) {
6        self.name = name
7        self.age = age
8    }
9    
10    convenience init(name: String) {
11        self.init(name: name, age: 0)
12    }
13}
14
15let William = Person(name: "William", age: 22)
16let bob = Person(name: "Bob")

In the example above, we have defined two initializers for the Person class. The first initializer takes both a name and an age parameter, while the second initializer only takes a name parameter. The second initializer is a convenience initializer, which is a shorthand way of calling the primary initializer with default values.

Convenience Initializer

A convenience initializer is a custom initializer that provides a convenient way to create an object with default values. It must call a designated initializer to set the instance's properties.

1class Person {
2    var name: String
3    var age: Int
4    
5    init(name: String, age: Int) {
6        self.name = name
7        self.age = age
8    }
9    
10    convenience init(name: String) {
11        self.init(name: name, age: 0)
12    }
13}
14
15let bob = Person(name: "Bob")

In the example above, the Person class has a convenience initializer that takes a name parameter and sets the instance's age property to 0. This initializer calls the primary initializer to set the instance's name property.

Failable Initializer

Sometimes it may not be possible to create an instance of a struct or class due to invalid input parameters. A failable initializer can be used to handle these situations. A failable initializer returns an optional value and is created by placing a question mark after the keyword init.

1struct Person {
2   var name: String
3   var age: Int
4   
5   init?(name: String, age: Int) {
6      if name.isEmpty || age < 0 {
7         return nil
8      }
9      self.name = name
10      self.age = age
11   }
12}
13
14if let person = Person(name: "William", age: 25) {
15   print("Person's name is \(person.name) and age is \(person.age).")
16} else {
17   print("Invalid input parameter.")
18}

Output:

1Person's name is William and age is 25.

Memberwise Initializer for Structs

A memberwise initializer is an automatic initializer provided by Swift for struct types. It initializes each stored property of the struct with a value that is passed as an argument to the initializer.

1struct Person {
2   var name: String
3   var age: Int
4}
5
6let person = Person(name: "William", age: 25)
7print("Person's name is \(person.name) and age is \(person.age).")

Output:

1Person's name is William and age is 25.

In the above example, the memberwise initializer is used to create an instance of the Person struct. The struct has two properties: name and age. The memberwise initializer automatically initializes these properties with the values passed as arguments.