Defer execution order in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the defer keyword do in Go?

The defer keyword schedules a function call to run after the surrounding function finishes, just before it returns.

beginner

In what order are multiple deferred functions executed in Go?

Deferred functions are executed in last-in, first-out (LIFO) order. The last deferred function runs first.

beginner

Consider this code snippet:

func example() {
  defer fmt.Println("first")
  defer fmt.Println("second")
  fmt.Println("middle")
}

What is the output?

The output will be:
middle
second
first

Because deferred calls run after the function ends, in reverse order.

intermediate

Why is defer useful for resource management in Go?

Defer helps ensure resources like files or locks are released properly, even if the function returns early or an error occurs.

intermediate

Can deferred functions modify named return values in Go?

Yes, deferred functions can modify named return values because they run just before the function returns.

What happens when multiple defer statements are used in a Go function?

A They execute in the order they appear.

B Only the first defer executes.

C They execute in reverse order (last deferred runs first).

D They execute simultaneously.

Deferred functions run in last-in, first-out order, so the last deferred function runs first.

When exactly do deferred functions run in Go?

A Immediately when deferred.

B After the surrounding function returns, before it actually returns to the caller.

C At the start of the function.

D Only if the function panics.

Deferred functions run after the surrounding function finishes but before it returns to the caller.

Which of these is a common use case for defer in Go?

A Closing files or releasing locks.

B Opening network connections.

C Starting goroutines.

D Declaring variables.

Defer is often used to close files or release locks to ensure cleanup happens.

If a deferred function modifies a named return variable, when does that modification take effect?

A Before the function starts.

B After the function returns to the caller.

C It never takes effect.

D Just before the function returns to the caller.

Deferred functions run just before returning, so they can change named return values.

What will this Go code print?

func f() {
  defer fmt.Println(1)
  defer fmt.Println(2)
  defer fmt.Println(3)
}

f()

A 3 2 1

B 1 2 3

C 1 3 2

D 2 3 1

Deferred calls run in reverse order, so it prints 3, then 2, then 1.

Explain how the defer keyword controls the execution order of deferred functions in Go.

Think about a stack of plates where the last plate placed is the first one taken.

Describe a practical example where using defer improves code safety or clarity.

Imagine you open a file and want to make sure it always closes.

Practice

1. What happens when multiple defer statements are used in a Go function?

easy

A. Deferred calls execute in last-in, first-out order when the function returns.

B. Deferred calls execute immediately when encountered.

C. Deferred calls execute in the order they appear in the code.

D. Deferred calls never execute automatically.

Solution

1. Step 1: Understand defer behavior

   In Go, deferred functions are postponed until the surrounding function returns.

2. Step 2: Execution order of deferred calls

   Deferred calls run in reverse order of their appearance, like a stack (last-in, first-out).

3. Final Answer:

   Deferred calls execute in last-in, first-out order when the function returns. -> Option A

4. Quick Check:

   Defer order = last-in, first-out [OK]

Hint: Remember: defer calls run backwards at function end [OK]

Common Mistakes:

• Thinking defer calls run immediately
• Assuming defer calls run in code order
• Believing defer calls never run

2. Which of the following is the correct syntax to defer a function call in Go?

easy

A. defer() fmt.Println("Hello")

B. defer := fmt.Println("Hello")

C. defer fmt.Println("Hello")

D. defer call fmt.Println("Hello")

Solution

1. Step 1: Recall defer syntax

   The keyword defer is followed directly by the function call without assignment or parentheses around defer.

2. Step 2: Identify correct syntax

   defer fmt.Println("Hello") correctly uses defer fmt.Println("Hello"). Other options misuse defer syntax.

3. Final Answer:

   defer fmt.Println("Hello") -> Option C

4. Quick Check:

   defer + function call = correct syntax [OK]

Hint: Use 'defer' directly before function call without extra symbols [OK]

Common Mistakes:

• Adding := after defer
• Using parentheses after defer keyword
• Writing extra words like 'call' after defer

3. What is the output of this Go program?

package main
import "fmt"
func main() {
    defer fmt.Println("First")
    defer fmt.Println("Second")
    fmt.Println("Third")
}

medium

A. Third
First
Second

B. Third
Second
First

C. First
Second
Third

D. Second
First
Third

Solution

1. Step 1: Identify immediate and deferred prints

   "Third" is printed immediately. The two deferred prints run after main ends.

2. Step 2: Determine deferred call order

   Deferred calls run last-in, first-out, so "Second" prints before "First".

3. Final Answer:

   Third Second First -> Option B

4. Quick Check:

   Immediate then deferred reverse order = Third, Second, First [OK]

Hint: Immediate prints first; deferred prints run backward after [OK]

Common Mistakes:

• Assuming deferred prints run in code order
• Thinking deferred prints run before immediate prints
• Confusing the order of deferred calls

4. Consider this Go code snippet:

func example() {
    defer fmt.Println("A")
    defer fmt.Println("B")
    defer fmt.Println("C")
    fmt.Println("D")
}

If the output is not as expected, what is the likely mistake?

medium

A. The function does not call fmt.Println("D") before defers.

B. The deferred calls are not in reverse order.

C. The deferred calls are missing parentheses.

D. The deferred calls execute immediately, not at function end.

Solution

1. Step 1: Understand expected output order

   "D" should print first immediately, then deferred calls print "C", "B", "A" in reverse order.

2. Step 2: Identify cause of unexpected output

   If "D" is missing or not printed before defers, output order changes unexpectedly.

3. Final Answer:

   The function does not call fmt.Println("D") before defers. -> Option A

4. Quick Check:

   Immediate prints must run before deferred calls [OK]

Hint: Ensure immediate prints run before deferred calls [OK]

Common Mistakes:

• Expecting deferred calls to run in code order
• Forgetting to call immediate print before defers
• Misunderstanding when deferred calls execute

5. You want to open three files in a Go function and ensure all are closed properly using defer. Which code snippet correctly closes the files in the right order?

hard

A. file1.Close()
file2.Close()
file3.Close()

B. defer file3.Close()
defer file2.Close()
defer file1.Close()

C. defer Close(file1)
defer Close(file2)
defer Close(file3)

D. defer file1.Close()
defer file2.Close()
defer file3.Close()

Solution

1. Step 1: Understand defer closing order

   Deferred calls run last-in, first-out, so the last deferred closes first.

2. Step 2: Apply to file closing

   Deferring file1.Close() first, then file2.Close(), then file3.Close() means file3 closes first, then file2, then file1, matching open order reversed.

3. Final Answer:

   defer file1.Close() defer file2.Close() defer file3.Close() -> Option D

4. Quick Check:

   Defer in open order closes files in reverse [OK]

Hint: Defer closes in reverse order of defer statements [OK]

Common Mistakes:

• Deferring closes in reverse order of opening
• Calling Close() immediately without defer
• Using incorrect function calls for closing