Shadowed variables

yourbasic.org/golang

Why doesn’t n change?

func main() {
	n := 0
	if true {
		n := 1
		n++
	}
	fmt.Println(n) // 0
}

Answer

The statement n := 1 declares a new variable which shadows the original n throughout the scope of the if statement.

To reuse n from the outer block, write n = 1 instead.

func main() {
	n := 0
	if true {
		n = 1
		n++
	}
	fmt.Println(n) // 2
}

Detecting shadowed variables

To help detect shadowed variables, you may use the experimental -shadow feature provided by the vet tool. It flags variables that may have been unintentionally shadowed. Passing the original version of the code to vet gives a warning message.

$ go vet -shadow main.go
main.go:4: declaration of "n" shadows declaration at main.go:2

Go 1.12 no longer supports this. Instead you may do

go install golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
go vet -vettool=$(which shadow)

Additionally, the Go compiler detects and disallows some cases of shadowing.

func Foo() (n int, err error) {
	if true {
		err := fmt.Errorf("Invalid")
		return
	}
	return
}

../main.go:4:3: err is shadowed during return

Share this page: