Use of defer in Go

Question

What is the use of defer in Go? The language documentation says it is executed when the surrounding function returns. Why not just put the code at end of given

Answer 1

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())
}