What is the defer operator in Go language?

The defer operator is used for deferred execution of a function or method. Functions marked with defer are executed immediately before exiting the function that contains them.

Principles of defer operation:

• Executed upon exiting the current function, whether normally or due to a panic.
• Multiple defer statements in one function are executed in LIFO (Last-In, First-Out) order.
• Useful for cleaning up resources, such as closing files or network connections.

Example of usage:

package main

import (
	"fmt"
	"os"
)

func main() {
	fmt.Println("Opening file.")

	file, err := os.Open("example.txt")
	if err != nil {
		// Error handling
		return
	}

	// Deferred closing of the file
	defer file.Close()
	defer fmt.Println("File will be closed.") // This defer will execute first (LIFO)

	fmt.Println("Processing the file...")

	// Further work with the file
}

Program output:

Opening file.
Processing the file...
File will be closed.