Swift Academy · Lesson

defer for Resource Cleanup

Close files and release resources reliably.

defer for Resource Cleanup is a free Swift Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

The Cleanup Problem

Resources like files, locks, or connections must be released. Forgetting to release on some path causes leaks.

func work() {
    print("acquire resource")
    print("use resource")
    print("release resource")
}
work()

defer Guarantees Release

Putting the release in a defer guarantees it runs no matter how the function exits.

func work() {
    print("acquire resource")
    defer { print("release resource") }
    print("use resource")
}
work()

Simulating a File Handle

Here a simple struct models open/close. The defer ensures close is always called.

struct File {
    let name: String
    func close() { print("closed", name) }
}
func readFile() {
    let f = File(name: "data.txt")
    defer { f.close() }
    print("reading", f.name)
}
readFile()

Lock Acquire/Release

Model a lock: acquire, then defer the release so the critical section is always unlocked.

func critical() {
    print("lock acquired")
    defer { print("lock released") }
    print("critical section work")
}
critical()

Cleanup Next to Setup

Keeping the cleanup right after setup makes the pairing obvious and hard to forget.

func session() {
    print("connect")
    defer { print("disconnect") }
    print("send request")
    print("receive response")
}
session()

Multiple Resources

With several resources, declare each cleanup right after its setup; LIFO order closes them correctly.

func transaction() {
    print("open connection")
    defer { print("close connection") }
    print("begin transaction")
    defer { print("commit transaction") }
    print("run query")
}
transaction()

Counter Reset

A defer can restore state, such as resetting a flag, when a scope ends.

func tracked() {
    var active = true
    defer { active = false; print("active reset to", active) }
    print("active is", active)
}
tracked()

Avoiding Duplication

Without defer you would repeat the cleanup on every exit path. defer writes it once.

func validate(_ ok: Bool) {
    print("start")
    defer { print("cleanup done once") }
    if !ok {
        print("invalid")
        return
    }
    print("valid")
}
validate(false)

Cleanup With Computed Value

A defer can log a final measurement, like how many items were processed.

func batch() {
    var processed = 0
    defer { print("processed", processed, "items") }
    for _ in 1...3 { processed += 1 }
}
batch()

Temporary State Restore

Use defer to temporarily change and then restore a setting reliably.

func withVerbose() {
    var verbose = false
    verbose = true
    defer { verbose = false; print("verbose restored:", verbose) }
    print("verbose during work:", verbose)
}
withVerbose()

Best Practice

Whenever you acquire something that must be released, register the release with defer on the very next line.

func handle() {
    print("acquire")
    defer { print("release") }
    print("process")
}
handle()

Quick Check

Test defer for cleanup.

Recap

Register resource release with defer right after acquisition. The cleanup is guaranteed to run on every exit path, keeps teardown next to setup, and avoids duplicating cleanup across return paths.

func work() {
    print("acquire")
    defer { print("release") }
    print("use")
}
work()

Frequently asked questions

Is the “defer for Resource Cleanup” lesson free?

Yes — the full text of “defer for Resource Cleanup” is free to read here on the web, and the Swift Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Swift Academy course, upgrade to CoddyKit PRO.

What will I learn in “defer for Resource Cleanup”?

Close files and release resources reliably. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “defer for Resource Cleanup” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Swift Academy lesson?

Yes. Every Swift Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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