A Close Look at Golang's defer

I’ve never come across anything like Go’s defer in any of the languages I’ve worked on before. As far as I know there isn’t an equivalent of it in PHP and the closest we have in Java is try-with-resources.

If you’ve been coding in Go you’ve probably used the defer statement plenty of times.

With defer, I can put the Close statement immediately after opening. I love it because it helps me never to forget closing that file reader.

f, err := os.Open("myfile")
defer f.Close()
if err != nil {
    panic(err)
}
...

What does it do?

A defer statement defers the execution of a function until the surrounding function returns.

• https://tour.golang.org/flowcontrol/12

A straight and simple definition from A Tour of Go. Any function or method call can be deferred by prepending the keyword. It is usually used for clean-ups like closing a file or a connection.

Any number of functions can be deferred which will then be executed in a last in first out order when the function that contains the defer statement returns.

In action

func greet() {
    defer fmt.Println("Good")
    defer fmt.Println("day")
    defer fmt.Println("beautiful")
    defer fmt.Println("person")
    fmt.Println("How are you today?") // not deferred
}

Output

How are you today?
person
beautiful
day
Good

It is also good to keep in mind that when deferring a function, it’s arguments are evaluated immediately.

func doStuff() {
    i := 0
    defer fmt.Println(i)
    i++
}

Output:

0

How awesome is defer, right? There’s so much more you can do with it. You can even recover from a panic.

For me, It is just one of the things that makes Go very enjoyable to write in.