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

Examples

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

Reference

  1. Go Spec