The defer Pattern

Using defer ensures that all resources are properly closed or given back to the pool when the resources are not needed anymore. Secondly, it is paramount in Go to recover from panicking.

Closing a file stream

// open a file f
defer f.Close()

Unlocking a locked resource (a mutex)

mu.Lock()
defer mu.Unlock()

Closing a channel (if necessary)

ch := make(chan float64)
defer close(ch)

or with 2 channels:

answerα, answerβ := make(chan int), make(chan int)
defer func() { close(answerα); close(answerβ) }()

Recovering from a panic

defer func() {
  if err := recover(); err != nil {
    log.Printf("run time panic: %v", err)
}()

Stopping a ticker

tick1 := time.NewTicker(updateInterval)
defer tick1.Stop()

Release of a process

p, err := os.StartProcess(..., ..., ...)

defer p.Release()

Stopping CPUprofiling and flushing the info

pprof.StartCPUProfile(f)
defer pprof.StopCPUProfile()

A goroutine signaling a WaitGroup

func HeavyFunction1(wg *sync.WaitGroup) {
defer wg.Done()
// Do a lot of stuff
}

It can also be used when not forgetting to print a footer in a report.

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That’s it about the defer pattern, and its uses. The next lesson brings you the highlights of playing around operators without violating the visibility rule.