What is it

Schedules function calls to execute BEFORE the surrounding function returns.

• commonly used for cleanup tasks like closing files or releasing resources.

Without returns, defer executes after a non-defer execution.

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

/*
hello
world
*/

In below example, the defer function increments the return value i BEFORE the surrounding function returns.

• A return statement that specifies results sets the result parameters before any deferred functions are executed.
• deferred functions are invoked immediately before the surrounding function returns

Meaning, the return parameter i has already been set to return value 1 immediate as the function starts executing.

• Then, the defer functions begin execution.
• Then, the return parameter i is set again to 2 since i++ is executed via the defer func().
• Since there is only one i, i is no longer 1, it is a 2.

func main() {
  c()
}

func c() (i int) {
  defer func() {
    i++
    print(i)
  }()

  defer func() { print("third") }()
  defer func() { print("second") }()
  defer func() { print("first") }()

  print(i)

  return 1
}

/*
// i is initialized to a default int value of 0
0 // print(i) prints initial value
// i is then set to return value 1, but function does not return yet
first // Since defer is LIFO, last defer function is first to execute
second
third
2 // i++ print(i) // last element in stack
// since return value is i, not what follows after "return", printing c() would result in 2
 */