Nil in the Go language

This is a creation in Article, where the information may have evolved or changed.

This article is Francesc Campoy on the GoConf Understanding Nil Lecture notes.

(1) nil no type .

(2) in a Go language, variables that do not appear to be initialized have their type zero value . 6the common types of variables zero value are:,,, nil pointer slice map channel , function and interface . The specific meanings are as follows:

type	Nil value meaning
Pointer	Point to Nothing
Slice	3 member values in the slice variable: buf is nil (no backing array), Len and Cap are all 0
Map,channel,function	A nil pointer, pointing to Nothing
Interface	Interface contains "type, value", and a nil interface must both be nil: "Nil, nil"

For that interface reason, the value is just because <type, value> two values in this tuple are needed nil interface nil . So pay attention to the following two points:

A) do not declare concrete error VARs:

func do() error {    var err *doError // nil of type *doError    return err // error (*doError, nil)}func main() {    err := do() // error (*doError, nil)    fmt.Println(err == nil) // false}

b) do not return concrete error types:

func do() *doError {    return nil // nil of type *doError}func main() {    err := do() // nil of type *doError    fmt.Println(err == nil) // true}func do() *doError {    return nil // nil of type *doError}func wrapDo() error { // error (*doError, nil)    return do() // nil of type *doError}func main() {    err := wrapDo() // error (*doError, nil)    fmt.Println(err == nil) // false}

(3) nil Some useful usage scenarios:

type	Nil Value usage scenario
Pointer	Methods can called on nil receivers
Slice	Perfectly valid zero values
Map	Perfect as read-only values (adding members to nil map will cause panic)
Channel	Essential for some concurrency patterns
function	Needed for completeness
Interface	The most used signal in Go (err! = nil)