花时间在Go中执行一个函数

Question

延迟语句推迟执行函数,直到周围的函数返回.但是,如果我尝试打印执行以下功能所花费的时间,它总是打印0.

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

    //expecting to print non zero value but always gets 0
    defer fmt.Println(time.Now().Sub(start))

    sum := 0
    for i := 1; i < 101; i++ {
        sum += i
    }
    time.Sleep(1 * time.Second)
    fmt.Println(sum)
}

代码:https://play.golang.org/p/46dxtS5beET

Answer 1

延迟函数的参数在函数延迟时进行计算.更改以下代码以按预期评估已用时间:

defer func() { fmt.Println(time.Now().Sub(start)) }()

这里使用的时序逻辑可以放在一个可重用的函数中:

// timer returns a function that logs message and the elapsed time from
// the call to timer and the returned function. The returned function is
// intended to be used in a defer statement:
//   defer timer("sum")()
func timer(message string) func() {
    start := time.Now()
    return func() { fmt.Println(message, time.Since(start)) }
}

func sum() {
    defer timer("sum")()

    sum := 0
    for i := 1; i < 101; i++ {
        sum += i
    }
    time.Sleep(1 * time.Second)
    fmt.Println(sum)
}