GoHow-ToBeginner · 3 min read

How to Use defer in Go: Syntax, Examples, and Tips

In Go, defer schedules a function call to run after the current function finishes, helping manage resources like files or locks. Use defer before a function call to ensure it executes last, even if the function returns early or panics.

📐

Syntax

The defer statement is followed by a function call. This call is postponed until the surrounding function completes. You can defer any function call, including anonymous functions.

defer: keyword to delay execution
function call: the function you want to run later

defer functionName(arguments)

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Example

This example shows how defer is used to close a file after reading it. The defer ensures the file closes even if the function returns early.

package main

import (
	"fmt"
	"os"
)

func readFile() {
	file, err := os.Open("example.txt")
	if err != nil {
		fmt.Println("Error opening file:", err)
		return
	}
	defer file.Close() // This will run when readFile returns

	buffer := make([]byte, 100)
	n, err := file.Read(buffer)
	if err != nil {
		fmt.Println("Error reading file:", err)
		return
	}
	fmt.Println("Read bytes:", n)
}

func main() {
	readFile()
}

Output

Read bytes: 100

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Common Pitfalls

One common mistake is deferring a function call with arguments that change later. The arguments are evaluated immediately when defer runs, not when the deferred function executes.

Also, multiple defer calls run in last-in, first-out order.

package main

import "fmt"

func main() {
	number := 5
	defer fmt.Println("Deferred number:", number) // number is evaluated now
	number = 10
	fmt.Println("Current number:", number)
}

Output

Current number: 10 Deferred number: 5

📊

Quick Reference

defer delays a function call until the surrounding function returns.
Arguments to deferred calls are evaluated immediately.
Deferred calls execute in reverse order.
Use defer to close files, unlock mutexes, or clean up resources.

✅

Key Takeaways

Use defer to run cleanup code after a function finishes, even if it returns early.

Arguments in deferred calls are evaluated immediately, so be careful with variables that change.

Multiple deferred calls run in last-in, first-out order.

Common uses include closing files, unlocking resources, and logging.

Defer helps write cleaner and safer code by managing resource cleanup automatically.