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In this lesson, we’ll explore the defer statement in Go, a powerful feature that postpones a function’s execution until the surrounding function returns. While the arguments for a deferred function call are evaluated immediately, the function itself executes later. This behavior ensures that certain operations, such as resource cleanup, occur right before a function exits.
The image explains a "defer statement," which delays function execution until the surrounding function returns, with arguments evaluated immediately.

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How the Defer Statement Works

Consider the example below with three functions: printName, printRollNo, and printAddress.
  • The printName function outputs a provided string.
  • The printRollNo function prints an integer.
  • The printAddress function prints a string representing an address.
Within the main function, the program flow is as follows:
  1. The printName function is immediately called to print the name “Joe”.
  2. The defer keyword schedules the printRollNo function to execute after main finishes its other statements.
  3. The printAddress function is called to print the address “street-32”.
  4. Once main is ready to return, the deferred printRollNo(23) call is executed.
Even though the argument for printRollNo(23) is evaluated immediately, the deferred function call itself is executed only after the main function completes.

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Example Code

Below is the complete Go code that demonstrates the usage of the defer statement: 
package main

import "fmt"

// printName prints the provided name.
func printName(str string) {
    fmt.Println(str)
}

// printRollNo prints the student's roll number.
func printRollNo(rno int) {
    fmt.Println(rno)
}

// printAddress prints the provided address.
func printAddress(addr string) {
    fmt.Println(addr)
}

func main() {
    printName("Joe")
    defer printRollNo(23)
    printAddress("street-32")
}

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Running the Program

To execute the program, use the following command in your terminal: 
go run main.go
The output will be: 
Joe
street-32
23

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Output Breakdown

  1. printName("Joe") is executed immediately, displaying Joe.
  2. The defer printRollNo(23) statement schedules printRollNo to run after completion of the main function.
  3. printAddress("street-32") executes next, printing street-32.
  4. When main is about to return, the deferred call to printRollNo(23) is finally executed, printing 23.

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Additional Resources

For further details on Go’s control flow and function execution, consider visiting these resources: That concludes our discussion of the defer statement. Happy coding and enjoy exploring Go’s powerful features with your own hands-on exercises!

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