Golang: Defer

By Xah Lee. Date: .

Defer statement is a new flow control, not in other popular languages.

it's used like this:

defer function_name(args)

When this is run, the args are evaluated, but the function call is not. The function is called right before the outer function exits.

package main

import "fmt"

func ff(x int) int {
	fmt.Printf("ff called. arg is %v\n", x)
	return x
}

func main() {

	var x = 3

	fmt.Printf("main. x is %v\n", x)

	defer ff(x)

	x = 4

	fmt.Printf("main. x is %v\n", x)

}

// main. x is 3
// main. x is 4
// ff called. arg is 3

Defer is used similar to “finally” in other languages, to do some clean up.

Usually, it's used right after the file opening call, to close a file.

package main

import "fmt"

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)
}

// 2018-08-26 code from
// https://blog.golang.org/defer-panic-and-recover

func main() {}

Reference

The Go Programming Language Specification - The Go Programming Language#Defer_statements

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Golang

  1. Compile, Run
  2. Source Encoding
  3. Package, Import
  4. Comment
  5. Print
  6. String
  7. Print String
  8. String Backslash Escape
  9. Rune
  10. Variable
  11. Zero Value
  12. Constant
  13. If Then Else
  14. Switch/Case
  15. Loop
  16. Basic Types
  17. Array
  18. Slice
  19. Map
  20. Struct
  21. Function
  22. Closure
  23. Pointer
  24. String, Byte Slice, Rune Slice
  25. regexp
  26. Read File
  27. Write to File
  28. Walk Dir
  29. Check File Exist
  30. System Call
  31. Get Script Path
  32. Defer
  33. Random Number

Examples

  1. Match Any Regexp
  2. Find Replace
  3. Validate Links
  4. Generate Sitemap

Reference

  1. Go Spec