The strange behavior of defer

[duan...@google.com]

For the following piece of code.

func main() {

start := time.Now()

defer fmt.Println(time.Since(start))

time.Sleep(1000000000)

fmt.Println("Hello, playground")

fmt.Println(time.Since(start))

}

The output is:

Hello, playground
1s
0s

I'm new at golang. Should the two time.Since both be called after the sleep? 

Thank you!

[Kurtis Rader]

No. From https://golang.org/ref/spec#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.

To keep that from happening you need to use a lambda (or function literal): https://play.golang.org/p/9Avbf_Mwx4g

--

Kurtis Rader

Caretaker of the exceptional canines Junior and Hank

[Dan Kortschak]

The parameter to fmt.Println is evaluated at the time of the defer
statement's execution. You should do something like this instead

func main() {
start := time.Now()

defer func() { fmt.Println(time.Since(start)) }()

time.Sleep(1000000000)
fmt.Println("Hello, playground")
fmt.Println(time.Since(start))
}

https://play.golang.org/p/30IQaP3jg5_w

[Kebo Duan]

I see, thank you very much!