Why is a custom initializer necessary in my Swift class?

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I'm working through the 100 Days of SwiftUI exercises and ran into an issue in a playground. I had the following code, where I added a custom initializer (lines 4–7):

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", speed: 50)
let interceptor = TIE(name: "cool", speed: 70)

I noticed that if I remove the custom initializer, the code breaks—even though all properties seem to be provided. Can someone explain why this initializer is required for the code to work correctly?

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Answers 3

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3 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

In Swift, classes don't automatically provide a memberwise initializer like structures do. When you have properties that don't have default values, Swift requires you to create a custom initializer to ensure that all properties are properly initialized before an instance is created.

Here’s what’s happening:

Custom Initializer:
The initializer in lines 4–7 takes parameters for each property and assigns them to the instance properties. This guarantees that every new instance of TIE has its name and speed set.
Why It Breaks Without It:
If you remove the custom initializer and don't provide default values for the properties, the Swift compiler doesn't know how to create an instance of your class because it cannot automatically generate an initializer for classes with non-default properties.
Solution Options:
Either keep the custom initializer or provide default values for the properties. For example:
```
SWIFT
class TIE {
    var name: String = ""
    var speed: Int = 0
}
let fighter = TIE()
```
However, in this case, you lose the ability to initialize with specific values at instantiation, or you'll need to set them later.

Thus, the custom initializer is essential because it ensures your instances are fully and correctly initialized with the necessary values.

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 MeteoricTraveler750 • 1 month ago

Adding to Matt's answer, it might help to explain why the language doesn't synthesize memberwise initializes for classes. It feels like an arbitrary limitation, but there's two good reasons for it:

Classes should be more than just dumb buckets of properties. They should have a meaningful identity, expose behaviour through methods, and hide the implementation details of their internal storage.
Classes have inheritance, and initializers are inherited by default. Each intiailizer in a subclass needs to call a required initializer on itself or the superclass. If there are multiple of them, it becomes really complicated and non-obvious to decide which one a memberwiser initializer should call. It's better to just make it explicit.

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Answer by PlanetarySatellite735 • 1 month ago

One of the differences between a class and a struct in Swift is that a struct provides, automatically, a memberwise initializer — meaning that if this were a struct, you would be able to say

SWIFT
let fighter = TIE(name: "wow", speed: 50)

without having declared an init(name:speed:) in your struct. The initializer, and its implementation, are implicit.

But a class does not provide an automatic memberwise initializer. Since this is a class, you have to write the initializer yourself, explicitly.

Why is a custom initializer necessary in my Swift class?

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