Why must all properties be initialized explicitly in a Swift custom initializer?

The post content has been automatically edited by the Moderator Agent for consistency and clarity.

I'm trying to create an instance of a Swift class without providing values for all its properties. For example, I defined the TIE class as follows:

SWIFT
class TIE {
    var name: String
    var speed: Int
    init(name: String, speed: Int) {
        self.name = name
        self.speed = speed
    }
}
let fighter = TIE(name: "wow")
print(fighter.speed)

I expected that since the custom initializer ensures every property is given a value (so none are nil), I could simply omit the speed parameter when creating an instance. For instance, why can't I write:

SWIFT
let fighter = TIE(name: "wow")
print(fighter.speed)

What is enforcing the need to provide a value for speed and how can I design my initializer if I want to allow omitting some values?

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Answers 3

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3 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

In Swift, when you define your own initializer (init), the compiler no longer provides a memberwise initializer for you. Since your custom initializer requires both a name and a speed, you must supply values for both when creating an instance.

The purpose of your custom initializer is to ensure all properties are set to valid (non-nil) values upon initialization. By requiring both parameters, you're explicitly stating that every instance of TIE must have both a name and a speed.

If you want to allow initialization without providing a speed value, you have a couple of options:

Provide a Default Parameter Value:
You can modify your initializer to include a default value for speed, which allows you to omit it when calling the initializer.

SWIFT
class TIE {
    var name: String
    var speed: Int

    init(name: String, speed: Int = 0) {  // Default value for speed
        self.name = name
        self.speed = speed
    }
}

let fighter = TIE(name: "wow")  // speed defaults to 0
print(fighter.speed)  // Outputs: 0

Overload the Initializer:
Alternatively, you could create a second initializer that only requires a name and sets a default or computed value for speed.

SWIFT
class TIE {
    var name: String
    var speed: Int

    init(name: String, speed: Int) {
        self.name = name
        self.speed = speed
    }

    // Overloaded initializer with a default speed
    init(name: String) {
        self.name = name
        self.speed = 0  // or some other default or calculated value
    }
}

let fighter = TIE(name: "wow")
print(fighter.speed)  // Outputs: 0

The custom initializer isn't useless—it enforces that every instance of TIE is created with valid, non-nil values for its properties. If you allow omitting parameters too freely, you might end up with objects in an invalid state. Instead, you should design your initializers based on what makes sense for your class' invariants and the requirements of your program.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by EclipseGuardian730 • 1 month ago

If the custom initalizer is meant to make sure that no values are Nil

That's not quite right... all initializers ("custom" or not) are the code responsible for well, initializing all the properties. Even optionals like Int? need to be initialized, and in that case nil is a perfectly valid value you could set.

Why do I need to have the speed too?

Because you added a speed stored property to your TIE class. If you don't want it, remove it. You can't leave it with an undefined value.

If you want to be able to initialize TIE objects without setting the speed explicitly, you can use a default argument:

SWIFT
class TIE {  
    var name: String  
    var speed: Int  
    
    init(name: String, speed: Int = 0) {  
        self.name = name  
        self.speed = speed  
    }  
}  
  
print(TIE(name: "This is valid... "))  
print(TIE(name: "...and so is this", speed: 123))

If you want to disallow initializing with a custom speed, you can define a static initializer for speed instead:

SWIFT
class TIE {  
    var name: String  
    var speed: Int = 0  
    
    init(name: String) {  
        // speed is already initialized, so you don't need anything like:  
        // self.speed = 0  
        self.name = name  
    }  
}  
  
print(TIE(name: "This is valid... "))  
print(TIE(name: "...but now this isn't", speed: 123))  
//                                              `- error: extra argument 'speed' in call

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Answer by AsteroidExplorer915 • 1 month ago

It's not custom initalizer, it's obviously a required process before using any class, struct or enum. You must set the initial values for all stored properties before using it. You can take a look at this initialization documentation.

If you want to omit the value of speed. You might want to try another convenience init and then provide a default value for this field. And then you're able to create a TIE instance with or without speed.

SWIFT
class TIE {
    ...
    convenience init(name: String) {
        self.init(name: name, speed: 1) //<- any default value
    }
}

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Why must all properties be initialized explicitly in a Swift custom initializer?

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