defer

what is it?

pushes a function call onto a list. the list of saved calls is executed after the surrounding function returns

example

func CopyFile(dstName, srcName string) (written int64, err error) {
    src, err := os.Open(srcName)
    if err != nil {
        return
    }
    defer src.Close()
 
    dst, err := os.Create(dstName)
    if err != nil {
        return
    }
    defer dst.Close()
 
    return io.Copy(dst, src)
}

allows us to think about closing each file right after opening it

rules

1. a deferred function’s arguments are evaluated when the defer statement is evaluated

   func a() {
       i := 0
       defer fmt.Println(i)
       i++
       return
   }
   // the deferred call will print 0 after the function returns

2. deferred function calls are executed in last in first out order

   func a() {
       i := 0
       defer fmt.Println(i)
       i++
       return
   }
   // prints 3210

3. deferred functions may read and assign to the returning function’s named return values

uses of defer

// releasing a mutex
mu.Lock()
defer mu.Unlock()
 
// printing a footer
printHeader()
defer printFooter()