Go by Example: 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 {
        panic(err)
    }
}

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

 
$ go run defer.go
creating
writing
closing

Next example: .

by Mark McGranaghan and Eli Bendersky | source | license