Why Does Measuring Execution Time with Defer in Go Always Yield Zero?

Measuring Execution Time in Go: Resolving the Defer Mystery

Defer statements await the completion of a function before executing their associated functions. However, printing the execution time of the following function using defer always yields zero:

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

    // Expected 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)
}

Issue:
defer evaluates the arguments of the deferred function when the containing function defers them. Therefore, start is evaluated immediately, before the loop or sleep.

Solution:
To measure elapsed time accurately, evaluate time.Now() inside the deferred function:

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

By doing so, the time difference is calculated when the deferred function executes, ensuring accurate results.

Tip: Consider using a reusable function to encapsulate the timing logic for easier and more consistent measurement.

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