Defer statement for cleanup in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print "Cleanup done" after the function finishes.

Swift

func example() {
    [1] {
        print("Cleanup done")
    }
    print("Function running")
}

Drag options to blanks, or click blank then click option'

Aguard

Bwhile

Cif

Ddefer

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'if' or 'while' instead of 'defer' does not delay execution until function end.

The defer statement schedules code to run just before the function returns, perfect for cleanup.

2fill in blank

medium

Complete the code to ensure the file is closed after reading.

Swift

func readFile() {
    let file = openFile()
    [1] {
        file.close()
    }
    print("Reading file")
}

Drag options to blanks, or click blank then click option'

Aguard

Brepeat

Cdefer

Dswitch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'guard' or 'switch' does not guarantee cleanup after function ends.

The defer statement ensures file.close() runs when the function ends, even if errors occur.

3fill in blank

hard

Fix the error by completing the code to defer the cleanup print statement.

Swift

func process() {
    [1] {
        print("Cleanup after processing")
    }
    print("Processing data")
}

Drag options to blanks, or click blank then click option'

Aif

Bdefer

Cguard

Drepeat

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'if' or 'guard' does not delay execution to the end of the function.

Only defer correctly delays the cleanup print until the function ends.

4fill in blank

hard

Fill both blanks to defer two cleanup print statements in order.

Swift

func cleanup() {
    [1] {
        print("First cleanup")
    }
    [2] {
        print("Second cleanup")
    }
    print("Function body")
}

Drag options to blanks, or click blank then click option'

Adefer

Bguard

Cif

Drepeat

Attempts:

3 left

💡 Hint

Common Mistakes

Using different keywords for the two blocks breaks the defer behavior.

Using defer twice schedules both cleanup prints to run in reverse order after the function ends.

5fill in blank

hard

Fill all three blanks to defer cleanup and print variable values correctly.

Swift

func example() {
    let x = 10
    [1] {
        print("Cleanup x: \(x)")
    }
    let y = 20
    [2] {
        print("Cleanup y: \(y)")
    }
    print("Values: \(x), \(y)")
    [3] {
        print("Final cleanup")
    }
}

Drag options to blanks, or click blank then click option'

Adefer

Bguard

Cif

Drepeat

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'guard' or 'if' instead of 'defer' breaks the cleanup scheduling.

All three defer statements schedule cleanup prints to run in reverse order after the function ends.

Practice

1. What does the defer statement do in Swift?

easy

A. Skips the rest of the function

B. Runs code just before the function returns

C. Runs code immediately when the function starts

D. Runs code only if an error occurs

Solution

1. Step 1: Understand the purpose of defer

   The defer statement schedules code to run at the end of the current function, just before it returns.

2. Step 2: Compare options with this behavior

   Only Runs code just before the function returns matches this behavior exactly; others describe different timings or conditions.

3. Final Answer:

   Runs code just before the function returns -> Option B

4. Quick Check:

   defer runs code before function ends = C [OK]

Hint: Remember: defer runs code at function exit, not start [OK]

Common Mistakes:

• Thinking defer runs immediately
• Confusing defer with error handling
• Assuming defer skips code

2. Which of the following is the correct syntax to use a defer statement in Swift?

easy

A. defer -> { print("Cleanup") }

B. defer: print("Cleanup")

C. defer() print("Cleanup")

D. defer { print("Cleanup") }

Solution

1. Step 1: Recall correct defer syntax

   The defer statement uses the keyword followed by a block in curly braces: defer { ... }.

2. Step 2: Check each option

   Only defer { print("Cleanup") } matches the correct syntax. Others use invalid punctuation or structure.

3. Final Answer:

   defer { print("Cleanup") } -> Option D

4. Quick Check:

   defer uses braces {} for code block = D [OK]

Hint: defer always uses braces { } for its code block [OK]

Common Mistakes:

• Using parentheses instead of braces
• Adding colons after defer
• Using arrow symbols incorrectly

3. What will be the output of this Swift code?

func test() {
    defer { print("First") }
    defer { print("Second") }
    print("Inside")
}
test()

medium

A. Inside
First
Second

B. First
Second
Inside

C. Inside
Second
First

D. Second
First
Inside

Solution

1. Step 1: Understand defer execution order

   Multiple defer statements run in reverse order of appearance when the function ends.

2. Step 2: Trace the output

   "Inside" prints first. Then the last defer prints "Second", then the first defer prints "First".

3. Final Answer:

   Inside Second First -> Option C

4. Quick Check:

   defer runs last-in-first-out = A [OK]

Hint: Multiple defers run in reverse order at function end [OK]

Common Mistakes:

• Assuming defers run in order added
• Thinking defer runs immediately
• Mixing print order

4. Identify the error in this Swift function using defer:

func cleanup() {
    defer print("Done")
    print("Working")
}

medium

A. Missing braces {} after defer

B. Cannot use print inside defer

C. defer must be last statement

D. No error, code is correct

Solution

1. Step 1: Check defer syntax

   The defer statement requires a code block enclosed in braces {}.

2. Step 2: Analyze the given code

   The code uses defer print("Done") without braces, which is invalid syntax.

3. Final Answer:

   Missing braces {} after defer -> Option A

4. Quick Check:

   defer needs braces {} for code block = B [OK]

Hint: Always use braces {} after defer keyword [OK]

Common Mistakes:

• Omitting braces for defer block
• Thinking defer can take single statement without braces
• Assuming defer must be last line

5. You want to open a file, write data, and ensure the file is always closed even if an error occurs. Which is the best way to use defer for cleanup in Swift?

hard

A. Open file, write data, then use defer { close file } immediately after opening

B. Write data, then open file, then defer { close file }

C. Use defer { close file } only if no error occurs

D. Close file manually without defer after writing

Solution

1. Step 1: Understand defer for cleanup

   defer ensures cleanup code runs at function end, even if errors happen.

2. Step 2: Apply defer after resource acquisition

   Place defer { close file } immediately after opening the file to guarantee closure.

3. Final Answer:

   Open file, write data, then use defer { close file } immediately after opening -> Option A

4. Quick Check:

   defer after open ensures cleanup always runs = A [OK]

Hint: Place defer right after resource acquisition for safe cleanup [OK]

Common Mistakes:

• Placing defer after writing data
• Skipping defer on errors
• Closing file manually risking leaks