Defer statement for cleanup in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the defer statement in Swift?

The defer statement schedules code to run just before the current scope exits, ensuring cleanup code runs no matter how the scope is exited.

beginner

When exactly does the code inside a defer block execute?

The code inside a defer block executes right before the function or scope returns or exits, even if an error is thrown or a return happens earlier.

intermediate

Can multiple defer statements be used in the same scope? If yes, in what order do they execute?

Yes, multiple defer statements can be used. They execute in reverse order of appearance (last-in, first-out).

beginner

Why is defer useful for resource management?

defer ensures resources like files or network connections are properly closed or released, even if the function exits early or throws an error.

beginner

Example: What will this Swift code print?

func test() {
  defer { print("Cleanup") }
  print("Start")
  return
  print("End")
}
test()

It will print:<br>Start<br>Cleanup<br>The defer block runs after print("Start") and before the function returns. The print("End") is never reached.

What happens if a function with a defer block throws an error?

A The function ignores the error.

B The <code>defer</code> block is skipped.

C The <code>defer</code> block still runs before the error propagates.

D The program crashes immediately.

The defer block always runs before the function exits, even if an error is thrown.

If you have two defer blocks in a function, which one runs first?

A Only the last one runs.

B The first one written in the code.

C They run simultaneously.

D The second one written in the code.

Defer blocks run in reverse order, so the last one written runs first.

Which of these is a good use case for defer?

A Declaring a variable.

B Opening a file and ensuring it closes after use.

C Starting a loop.

D Printing a message at the start of a function.

defer is great for cleanup tasks like closing files.

What will this code print?

func example() {
  defer { print("First") }
  defer { print("Second") }
  print("Hello")
}
example()

A Hello\nSecond\nFirst

B First\nSecond\nHello

C Second\nHello\nFirst

D Hello\nFirst\nSecond

Defer blocks run in reverse order, so "Second" prints before "First" after "Hello".

Can defer be used outside of functions, like in global scope?

A No, only inside functions or methods.

B Yes, anywhere in code.

C Only inside loops.

D Only inside classes.

defer works only inside functions or methods to manage cleanup at scope exit.

Explain how the defer statement helps with cleanup in Swift functions.

Think about what happens when a function ends early or throws an error.

Describe the order of execution when multiple defer statements are used in the same function.

Imagine stacking plates and removing them one by one.