go中的“ defer”是什么意思？ [重复]

Question

This question already has an answer here:

• Golang defer behavior 3 answers

golang's documentation says this:

A deferred function's arguments are evaluated when the defer statement is evaluated.

This confuses me.

Question: Does "evaluated" mean that the value is already known? I just don't understand why two examples print different. This confuse me.

I have two examples below:

 //It prints 0.
 func deferA() {
     i := 0
     defer fmt.Println(i)
     i++
     return
 }

 //It prints 1.
 func deferB() {
      i := 0
      defer func() {
          fmt.Println(i)
      }()
      i++
      return
 }

</div>

Answer 1

defer take a function, so fmt.Println(i) is a function and its argument i is evaluated as 0 when the defer statement is evaluated.

defer func() { fmt.Println(i) }, the func() { fmt.Println(i) } take no argument and its a closure, so i is not evaluated but close over by the closure.

You can more information about closure at https://gobyexample.com/closures