defer

In the Golang programming language you can use the defer statement. A defer statement defers the execution of a function until the surrounding function returns.

So in practice, that means first finish the current function before executing the deferred function.

Examples

The program below is a go program that imports the "fmt" module for output and then calls the Println function twice. It uses the defer statement;

package main

import "fmt"

func main() {
    defer fmt.Println("world")
    fmt.Println("hello")
}

The deferred call's arguments are evaluated immediately, but the function call is not executed until the surrounding function returns.

Instead of the output "hello world", it outputs:

hello
world

Program exited.

The Println function "world" is only called once the function main is finished.

Defer

If you would extend the function with several calls, it will output world last. The program remains in normal execution order.

package main

import "fmt"

func main() {
    defer fmt.Println("world")
    fmt.Println("hello")
    fmt.Println("there")
    fmt.Println("the")
}

You can defer more than one function call, then all the deferred functions are executed after the main cycle has completed:

defer fmt.Println("world")
defer fmt.Println("hello")
defer fmt.Println("there")
fmt.Println("the")

Related links:

• Defer statement
• Learn Go