Why defer is used in Go - See It in Action

Why defer is used

📖 Scenario: Imagine you are writing a Go program that opens a file, reads some data, and then closes the file. You want to make sure the file is always closed properly, even if something goes wrong while reading.

🎯 Goal: You will learn how to use defer in Go to ensure a file is closed automatically at the right time, making your code safer and cleaner.

📋 What You'll Learn

Create a variable to open a file

Use defer to close the file

Read from the file

Print the read content

💡 Why This Matters

🌍 Real World

In real programs, resources like files or network connections must be closed properly to avoid problems. Using <code>defer</code> helps make sure this happens even if errors occur.

💼 Career

Understanding <code>defer</code> is important for writing safe and clean Go code, which is a valuable skill for backend development and systems programming jobs.

1

Open a file

Write code to open a file named example.txt using os.Open and assign it to a variable called file. Also handle the error by assigning it to err.

Go

package main

import (
	"fmt"
	"os"
)

func main() {
	file, err := os.Open("example.txt")
	// Your code here
}

Hint

Use os.Open("example.txt") and assign the results to file and err.

2

Use defer to close the file

Add a defer statement to call file.Close() right after opening the file to ensure it will be closed automatically when the function ends.

Go

package main

import (
	"fmt"
	"os"
)

func main() {
	file, err := os.Open("example.txt")
	if err != nil {
		fmt.Println("Error opening file:", err)
		return
	}
	// Your code here
}

Hint

Write defer file.Close() right after checking the error.

3

Read from the file

Create a byte slice called data with length 100. Use file.Read(data) to read from the file and assign the number of bytes read to n and error to err. Handle the error if it is not nil.

Go

package main

import (
	"fmt"
	"os"
)

func main() {
	file, err := os.Open("example.txt")
	if err != nil {
		fmt.Println("Error opening file:", err)
		return
	}
	defer file.Close()
	// Your code here
}

Hint

Create data with make([]byte, 100) and read into it with file.Read(data).

4

Print the read content

Print the string version of the bytes read from the file using fmt.Println(string(data[:n])).

Go

package main

import (
	"fmt"
	"os"
)

func main() {
	file, err := os.Open("example.txt")
	if err != nil {
		fmt.Println("Error opening file:", err)
		return
	}
	defer file.Close()
	data := make([]byte, 100)
	n, err := file.Read(data)
	if err != nil {
		fmt.Println("Error reading file:", err)
		return
	}
	// Your code here
}

Hint

Use fmt.Println(string(data[:n])) to print the content read.