Help to understand code from flag package

(Said Aspen) #1

I am learning go. First time I use one of the standard packages I try to read through it’s code to learn more about the package. I also Want to get a feel for how go code is written. This particular example is from the flag package.

Would someone be able to explain it to me? I just can’t wrap my head around it:

// isZeroValue determines whether the string represents the zero
// value for a flag.
func isZeroValue(flag *Flag, value string) bool {
	// Build a zero value of the flag's Value type, and see if the
	// result of calling its String method equals the value passed in.
	// This works unless the Value type is itself an interface type.
	typ := reflect.TypeOf(flag.Value)
	var z reflect.Value
	if typ.Kind() == reflect.Ptr {
		z = reflect.New(typ.Elem())
	} else {
		z = reflect.Zero(typ)
	return value == z.Interface().(Value).String()

I know reflection from Java and I do understand the purpose of this function but can’t understand how it accomplishes that.
The last line of the method is totally nonsensical to me:)

Thanks in advance

(Johan Dahl) #2


Hard for me too. But it compares the value to

z.Interface() gets the zero-type as an interface{} it is then converted into an flag.Value type and then the string representation of that. But why not just do z.(Value).String() ???