Why do classes have deinitializers and structs don’t?

Paul Hudson    @twostraws    May 28th 2020

Updated for Xcode 15

One small but important feature of classes is that they can have a deinitializer function – a counterpart to init() that gets run when the class instance gets destroyed. Structs don’t have deinitializers, which means we can’t tell when they are destroyed.

The job of deinitializers is to tell us when a class instance was destroyed. For structs this is fairly simple: the struct is destroyed when whatever owns it no longer exists. So, if we create a struct inside a method and the methods ends, the struct is destroyed.

However, classes have complex copying behavior that means several copies of the class can exist in various parts of your program. All the copies point to the same underlying data, and so it’s now much harder to tell when the actual class instance is destroyed – when the final variable pointing to it has gone away.

Behind the scenes Swift performs something called automatic reference counting, or ARC. ARC tracks how many copies of each class instance exists: every time you take a copy of a class instance Swift adds 1 to its reference count, and every time a copy is destroyed Swift subtracts 1 from its reference count. When the count reaches 0 it means no one refers to the class any more, and Swift will call its deinitializer and destroy the object.

So, the simple reason for why structs don’t have deinitializers is because they don’t need them: each struct has its own copy of its data, so nothing special needs to happen when it is destroyed.

You can put your deinitializer anywhere you want in your code, but I love this quote from Anne Cahalan: “Code should read like sentences, which makes me think my classes should read like chapters. So the deinitializer goes at the end, it's the ~fin~ of the class!”

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