golang defer语句是在return语句之前还是之后执行？

Question

I have a question about golang defer: Is golang defer statement execute before or after return statement?

I have read Defer_statements. But I do not got the answer.

I made a simple test:

func test1() (x int) {
    defer fmt.Printf("in defer: x = %d
", x)

    x = 7
    return 9
}

func test2() (x int) {
    defer func() {
        fmt.Printf("in defer: x = %d
", x)
    }()

    x = 7
    return 9
}

func test3() (x int) {
    x = 7
    defer fmt.Printf("in defer: x = %d
", x)
    return 9
}

func main() {
    fmt.Println("test1")
    fmt.Printf("in main: x = %d
", test1())
    fmt.Println("test2")
    fmt.Printf("in main: x = %d
", test2())
    fmt.Println("test3")
    fmt.Printf("in main: x = %d
", test3())
}

In test1(), using Printfto print x after defer. In test2(), using a anonymous function to print x after defer. In test3(), using Printfto print x after defer, but defer after x = 7.

But the result is:

test1
in defer: x = 0
in main: x = 9
test2
in defer: x = 9
in main: x = 9
test3
in defer: x = 7
in main: x = 9

So, is any one can explain: 1. why got this result? why test1 prints 0, test2 print9, test3 prints 7. 2. is defer statement excutes after return or before return?

Thanks a lot.

Answer 1

Thanks @dev.bmax @Tranvu Xuannhat @rb16. With your help, I found the key explanation from Defer_statements.

Each time a "defer" statement executes, the function value and parameters to the call are evaluated as usual and saved anew but the actual function is not invoked.

We can break up defer ... into three parts.

1. invoke defer, evaluating the value of function parameter.
2. execute defer, pushing a function in to stack.
3. execute functions in stack after return or panic.

I made a new test4 to explain.

func test4() (x int) {
    defer func(n int) {
        fmt.Printf("in defer x as parameter: x = %d
", n)
        fmt.Printf("in defer x after return: x = %d
", x)
    }(x)

    x = 7
    return 9
}

In test4,

1. invoke defer, evaluating the value of n, n = x = 0, so x as parameter is 0.
2. execute defer, pushing func(n int)(0) onto stack.
3. execute func(n int)(0) after return 9, n in fmt.Printf("in defer x as parameter: x = %d ", n) has been evaluated, x in fmt.Printf("in defer x after return: x = %d ", x) will be evaluated now, x is 9。

So, got the result:

test4
in defer x as parameter: x = 0
in defer x after return: x = 9
in main: x = 9