Use of defer in Go
We usually use defer
to close or deallocate resources.
A surrounding function executes all deferred function calls before it returns, even if it panics. If you just place a function call at the end of a surrounding function, it is skipped when panic happens.
Moreover a deferred function call can handle panic by calling the recover
built-in function. This cannot be done by an ordinary function call at the end of a function.
Each deferred call is put on stack, and executed in reverse order when the surrounding function ends. The reversed order helps deallocate resources correctly.
The defer
statement must be reached for a function to be called.
You can think of it as another way to implement try-catch-finally
blocks.
Closing like try-finally
:
func main() {
f, err := os.Create("file")
if err != nil {
panic("cannot create file")
}
defer f.Close()
// no matter what happens here file will be closed
// for sake of simplicity I skip checking close result
fmt.Fprintf(f,"hello")
}
Closing and panic handling like try-catch-finally
func main() {
defer func() {
msg := recover()
fmt.Println(msg)
}()
f, err := os.Create(".") // . is a current directory
if err != nil {
panic("cannot create file")
}
defer f.Close()
// no matter what happens here file will be closed
// for sake of simplicity I skip checking close result
fmt.Fprintf(f,"hello")
}
The benefit over try-catch-finally is that there is no nesting of blocks and variable scopes. This simplifies the structure of the surrounding function.
Just like finally blocks, deferred function calls can also modify the return value if they can reach the returned data.
func yes() (text string) {
defer func() {
text = "no"
}()
return "yes"
}
func main() {
fmt.Println(yes())
}
Key benefit of using defer
- it will be called any way no matter how function will return. If an extraordinary situation would occur deferred function will be called.
So it gives nice things:
Recover after
panic
. This allows yes realizetry ... catch
behavior.Not to forget clean up (close files, free memory, etc) before normal exit. You may open some resource and you have to close it before exit. But function can have several exit points - so you have to add freeing in every return point. That’s very tedious in maintenance. Or you can put only one deferred statement - and resources will be released automatically.
Well, it's not always guaranteed that your code may reach the end of the function (e.g. an error or some other condition may force you to return well ahead of the end of a function). The defer statement makes sure that whatever function is assigned to it gets executed for sure even if the function panics or the code returns well before the end of the function.
The defer statement also helps keep the code clean esp. in cases when there are multiple return statements in a function esp. when one needs to free resources before return (e.g. imagine you have an open call for accessing a resource at the beginning of the function - for which a corresponding close must be called before the function returns for avoiding a resource leak. And say your function has multiple return statements, maybe for different conditions including error checking. In such a case, without defer, you normally would call close for that resource before each return statement). The defer statement makes sure the function you pass to it is always called irrespective of where the function returns, and thus saves you from extraenous housekeeping work.
Also defer can be called multiple times in the same function. E.g.: In case you have different resources being allocated through your function which need to be eventually freed before returning, then you can call defer for each of them after allocation and these functions are executed in the reverse order of the sequence in which they were called when the function exits.
There are already good answers here. I would like to mention one more use case.
func BillCustomer(c *Customer) error {
c.mutex.Lock()
defer c.mutex.Unlock()
if err := c.Bill(); err != nil {
return err
}
if err := c.Notify(); err != nil {
return err
}
// ... do more stuff ...
return nil
}
The defer
in this example ensures that no matter how BillCustomer
returns, the mutex
will be unlocked
immediately prior to BillCustomer
returning. This is extremely useful because without defer
you would have to remember to unlock
the mutex
in every place that the function could possibly return
.
ref.