How to run code when an object is destroyed

Swift version: 5.1

Paul Hudson @twostraws May 28th 2019

All structs and classes can have initializers, which are special methods that run when those types are created. However, classes can also have deinitializers – code that gets run when an instance of the class is destroyed. This isn’t possible with structs because they only ever have one owner.

Deinitializers never take any parameters, so they are written just as deinit. For example, we could create a simple Person class with an initializer and a deinitializer:

class Person {
    init() {
        print("I'm alive!")
    }

    deinit {
        print("I'm dying!")
    }
}

If you want to try that in a playground, run this code:

do {
    let person = Person()
}

Putting the Person instance inside a do block ensures it will be destroyed before the playground finishes, so you should see “I’m alive!” and “I’m dying!”

Deinitializers are extremely important when handling memory that isn’t managed by Swift. For example, if you’re using an external C library and it has allocated RAM, you should free that RAM inside your deinitializer.

Available from iOS 8.0 – learn more in my book Pro Swift