如何定义一个空变量来存储File结构的值？

Question

I am trying to figure out a way to declare an empty variable to store the return values of os.Create and os.Open. Code:

func main() {

  var path = "list.txt"
  // Some attempts:
  // var file File{}
  // var file *File

  // Check if file exists
  var _, err = os.Stat(path)

  // If new file, create it
  if os.IsNotExist(err) {
    var file, err = os.Create(path)

    // If file exists, open it
  } else {
    var file, err = os.Open(path)
  }

  // Check errors opening and creating file
  if err != nil {
    log.Fatal(err)
    return
  }

  // Defer closing file
  defer file.Close()

}

Both attempts result in the following error:

./main.go:13: undefined: File

I'm sure this is one of those things that I don't know I don't know. What I do know:

• Per os/file.go, type of the return value I'm looking for is *File
• That type is defined in os/file_unix.go as a struct

Can someone explain to me:

1. How do I create an empty variable that can then be used to store the first variable in the results of os.Create and os.Open.
2. Why were my two attempts wrong?
3. Anything else that I'm misunderstanding.

Answer 1

Defining variable is var <variable-name> <type>, learn more about variables.

var file *os.File
var err error

Your updated code:

func main() {
  path := "list.txt"

  var file *os.File
  var err error

  // Check if file exists
  if err = os.Stat(path); os.IsNotExist(err) {
    file, err = os.Create(path)
  } else { // If file exists, open it
    file, err = os.Open(path)
  }

  // Check errors opening and creating file
  if err != nil {
    log.Fatal(err)
    return
  }

  // Defer closing file
  defer file.Close()
}