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