Defer a function call (with return value)

yourbasic.org/golang

Defer statement basics

A defer statement postpones the execution of a function until the surrounding function returns, either normally or through a panic.

func main() {
    defer fmt.Println("World")
    fmt.Println("Hello")
}
Hello
World

Deferred calls are executed even when the function panics:

func main() {
    defer fmt.Println("World")
    panic("Stop")
    fmt.Println("Hello")
}
World
panic: Stop

goroutine 1 [running]:
main.main()
    ../main.go:3 +0xa0

Order of execution

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

If there are several deferred function calls, they are executed in last-in-first-out order.

func main() {
    fmt.Println("Hello")
    for i := 1; i <= 3; i++ {
        defer fmt.Println(i)
    }
    fmt.Println("World")
}
Hello
World
3
2
1

Use func to return a value

Deferred anonymous functions may access and modify the surrounding function’s named return parameters.

In this example, the foo function returns “Change World”.

func foo() (result string) {
    defer func() {
        result = "Change World" // change value at the very last moment
    }()
    return "Hello World"
}

Common applications

Defer is often used to perform clean-up actions, such as closing a file or unlocking a mutex. Such actions should be performed both when the function returns normally and when it panics.

Close a file

In this example, defer statements are used to ensure that all files are closed before leaving the CopyFile function, whichever way that happens.

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)
}

Error handling: catch a panic

The Recover from a panic code example shows how to use a defer statement to recover from a panic and update the return value.

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