deinit

Deinit, short for deinitializer, is a programming construct that refers to a special method or hook invoked when an instance is about to be destroyed. It provides a place to perform cleanup tasks such as releasing resources, closing file handles, or saving state before memory is reclaimed.

In Swift, deinit is a distinguished instance method used with classes. The syntax is simply deinit { /* cleanup code */ }. It has no parameters and no return value, and it cannot be called directly by code. The Swift runtime invokes deinit automatically when the last strong reference to a class instance is released and the instance is about to be deallocated. Deinitializers are not inherited and cannot be overridden in the traditional sense; each class can define its own deinit implementation. They are not available for structures or enums, which are value types and do not require deallocation in the same way as reference types.

Within a deinitializer, you can access the instance’s properties to perform cleanup, but you should avoid creating new strong references to self or triggering new side effects that could extend the object's lifetime. Deinitializers run only at the moment of deallocation and are not guaranteed to execute if a program terminates unexpectedly.

Beyond Swift, other languages offer analogous mechanisms. C++ uses destructors that are invoked when an object goes out of scope or is explicitly deleted, providing a deterministic cleanup point. Objective-C uses dealloc for similar cleanup, whereas languages with garbage collection may provide finalizers or destructors with different guarantees and performance implications.

Overall, deinit represents the programmatic point at which a class instance ends its lifecycle, enabling orderly resource management as memory is released.