defer statement in go

defer is one of the unique keywords in Go.

What does defer do? As the name suggests, it defers something. It defers function calls. A deferred function gets invoked right after the surrounding function returns.

func outer() {
    defer inner()
    log.Println("outer got called")
}

func inner() {
    log.Println("inner got called")
}

outer got called prints first then inner got called.

What the main use of defer statement? It helps mainly with actions that need to happen but not immediately. For eg. closing a file that’s opened or closing a database connection.

What’s the advantage of using defer? - Even when the surrounding function panics, the deferred function call gets invoked - Makes the code less bug free

Things to remember:

1. The function calls get stacked in Last In First Out (LIFO)

package main

import (
	"fmt"
)

func main() {
	defer func() {
		fmt.Println("1st call")
	}()
	defer func() {
		fmt.Println("2nd call")
	}()
	defer func() {
		fmt.Println("3rd call")
	}()
}

The above function outputs:

viggy28@MacBook-Pro defer % go run defer.go
3rd call
2nd call
1st call

1. The (deferred) function’s arguments are evaluated right away, not once the surrounding function returns.

   a. If you would like the deferred function’s arguments evaluated at the time of the surrounding function returns then make the argument as a pointer. However that involves changing the signature of the function being deferred.

   b. Another option is to convert the deferred function as a closure (not pass the argument to be evaluated)

func f() {
    i := 0
    j := 0
    defer func(i int) {
        fmt.Println(i, j)
    }(i)
    i++
    j++ 
}

The output of this function is 0,1. since i is an argument passed to the closure, its value is being evaluated at the time of defer statement whereas j is evaluated when the surrounding function returns.

The same logic applies to deferred methods too. (value receivers evaluated immediately and pointer receivers evaluated with whatever the final value is).

• Go