Deferred Function Execution in Go

Introduction

The defer statement is used to delay the execution of a function until the surrounding function returns. It is often used to ensure that some cleanup is performed after a function completes, regardless of the path taken to get there.

Defer

In this lab, you need to use defer to create a file, write to it, and then close it when you're done.

• The createFile function should create a file with the given path and return a pointer to the file.
• The writeFile function should write the string "data" to the file.
• The closeFile function should close the file and check for errors.

## Running the program confirms that the file is closed
## after being written.
$ go run defer.go
creating
writing
closing

There is the full code below:

// _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")

}

func closeFile(f *os.File) {
    fmt.Println("closing")
    err := f.Close()
    // It's important to check for errors when closing a
    // file, even in a deferred function.
    if err != nil {
        fmt.Fprintf(os.Stderr, "error: %v\n", err)
        os.Exit(1)
    }
}

Summary

In this lab, you learned how to use defer to ensure that a function call is performed later in a program's execution, usually for purposes of cleanup. You also learned how to create a file, write to it, and then close it using defer.