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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Reference