Defer Keyword in Golang

In Go language, defer statements delay the execution of the function or method or an anonymous method until the nearby functions returns. In other words, defer function or method call arguments evaluate instantly, but they don't execute until the nearby functions returns. You can create a deferred method, or function, or anonymous function by using the defer keyword.

Syntax:

// Function
defer func func_name(parameter_list Type)return_type{
// Code
}

// Method
defer func (receiver Type) method_name(parameter_list){
// Code
}

defer func (parameter_list)(return_type){
// code
}()

Important Points:

• In Go language, multiple defer statements are allowed in the same program and they are executed in LIFO(Last-In, First-Out) order as shown in Example 2.
• In the defer statements, the arguments are evaluated when the defer statement is executed, not when it is called.
• Defer statements are generally used to ensure that the files are closed when their need is over, or to close the channel, or to catch the panics in the program.

Let us discuss this concept with the help of an example:

Example 1: 

// Go program to illustrate the
// concept of the defer statement
package main

import "fmt"

// Functions
func mul(a1, a2 int) int {

    res := a1 * a2
    fmt.Println("Result: ", res)
    return 0
}

func show() {
    fmt.Println("Hello!, GeeksforGeeks")
}

// Main function
func main() {

    // Calling mul() function
    // Here mul function behaves
    // like a normal function
    mul(23, 45)

    // Calling mul()function
    // Using defer keyword
    // Here the mul() function
    // is defer function
    defer mul(23, 56)

    // Calling show() function
    show()
}

Output: 

Result:  1035
Hello!, GeeksforGeeks
Result:  1288

Explanation: In the above example we have two functions named mul() and show(). Where show() function is called normally in the main() function, mul() function is called in two different ways:

• First, we call mul function normally(without the defer keyword), i.e, mul(23, 45) and it executes when the function is called(Output: Result : 1035 ).
• Second, we call mul() function as a defer function using defer keyword, i.e, defer mul(23, 56) and it executes(Output: Result: 1288 ) when all the surrounding methods return.

Example 2:

// Go program to illustrate
// multiple defer statements, to illustrate LIFO policy
package main

import "fmt"

// Functions
func add(a1, a2 int) int {
    res := a1 + a2
    fmt.Println("Result: ", res)
    return 0
}

// Main function
func main() {

    fmt.Println("Start")

    // Multiple defer statements
    // Executes in LIFO order
    defer fmt.Println("End")
    defer add(34, 56)
    defer add(10, 10)
}

Output:

Start
Result:  20
Result:  90
End