The defer Statement

defer is a promise to do something right before leaving the current block of code, no matter how you leave it.

In this page:

A defer block is written where cleanup logic conceptually belongs, but Swift delays actually running it until the function is about to exit.

Example: Basic defer Usage

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func processFile() {
    print("Opening file")
    defer {
        print("Closing file")
    }
    print("Processing file")
}
processFile()

When a scope has several defer blocks, they execute in the reverse order they were written, like a stack.

Example: Multiple defer Blocks Run in Reverse Order

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func demo() {
    defer { print("First defer (runs last)") }
    defer { print("Second defer (runs first)") }
    print("Function body")
}
demo()

Common Mistakes

Assuming defer runs immediately where it's written; it actually always runs at the very end of the enclosing scope.
Writing multiple defer blocks and expecting them to run in the order written; they actually run in reverse order, last-in-first-out.
Using defer for logic that must run conditionally only on success; defer always runs regardless of whether the scope exits normally or via a thrown error.

Chapter Summary

defer { } schedules code to run just before the current scope exits, regardless of how it exits.
Multiple defer blocks run in reverse (last-in-first-out) order.
defer is commonly used for cleanup, like closing a resource, that must always happen.
defer runs whether the scope exits normally, via return, or via a thrown error.

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