Defer

Defer is used to ensure that a function call is performed later in a program’s execution, usually for purposes of cleanup. defer is often used where e.g. ensure and finally would be used in other languages.

package main

import (
    "fmt"
    "os"
)

// Suppose we wanted to create a file, write to it
// and then close when we’re done. Here’s how we could do that with defer.
func main() {

    // Immediately after getting a file object with createFile
    // we defer the closing of that file with closeFile.
    // This will be executed at the end of the enclosing function (main)
    // after writeFile has finished.
    f := createFile("/tmp/defer.txt")
    defer closeFile(f)
    writeFile(f)
}

func createFile(p string) *os.File {
    fmt.Println("creating")
    f, err := os.Create(p)
    if err != nil {
        panic(err)
    }
    return f
}

func writeFile(f *os.File) {
    fmt.Println("writing")
    fmt.Fprintln(f, "data")

}

// It’s important to check for errors when closing a file, even in a deferred function.
func closeFile(f *os.File) {
    fmt.Println("closing")
    err := f.Close()

    if err != nil {
        fmt.Fprintf(os.Stderr, "error: %v\n", err)
        os.Exit(1)
    }
}

Running the program confirms that the file is closed after being written.

$ go run defer.go
creating
writing
closing