Sometimes we need to clean up resources, like when we open a file: we need to close the handle before we exit. We could do something like:

1f, err := os.Open("testdata/somefile.txt")
2... // do something with f
3f.Close()

However if something panics before we can close f, or if the function exits before that, then f will hang around in memory forever (it will leak)

To make sure the f.Close() line is executed no matter what, we can use the defer keyword.

A defer statements takes some function call and defers it, that is, it doesn’t call the function now, but it remembers to call it right before the function exits:

1f, err := os.Open("testdata/somefile.txt")
2if err != nil {
3  return err
4}
5defer f.Close()
6... // do stuff with f

We can call multiple defers in the same function for different function calls. All the defer functions will be executed when the function exits in inversed order. This is referred to as stacking defers.

A defer statement defers the execution of a function until the surrounding function returns.

The deferred call’s arguments are evaluated immediately, but the function call is not executed until the surrounding function returns.

References

• John Arundel, For the Love of Go
• https://go.dev/tour/flowcontrol/12
• https://go.dev/tour/flowcontrol/13

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