When does reflect.IsValid return false?

Question

I am curious about IsValid function, because during my use of this function, it never returned false. So when does it return a negative result?

Answer 1

As the doc reflect.IsValid() says:

It returns false if v is the zero Value. [...] Most functions and methods never return an invalid value. If one does, its documentation states the conditions explicitly.

Value.IsValid() is supposed to report whether the reflect.Value itself is valid, not the value it wraps (if any).

All the examples below print false. You can try them on the Go Playground.

The simplest example is calling IsValid() on the zero value of reflect.Value (which is a struct):

fmt.Println(reflect.Value{}.IsValid())

The 2nd simplest example is when passing nil to reflect.ValueOf():

fmt.Println(reflect.ValueOf(nil).IsValid())

Another example: start with a pointer being nil, in this case there is no "pointed" value, a nil pointer points to nowhere. Attempting to get the reflect.Value of the pointed value using Value.Elem() results in a zero reflect.Value whose IsValid() method will return false:

var i *int
v := reflect.ValueOf(i)
v2 := v.Elem()
fmt.Println(v2.IsValid())

Or in one line:

fmt.Println(reflect.ValueOf((*int)(nil)).Elem().IsValid())

Same thing if you call Value.Indirect() on the above reflect.Value():

fmt.Println(reflect.Indirect(v).IsValid())

Or attempting to get a non-existing struct field by name using Value.FieldByName():

s := struct{}{}
fmt.Println(reflect.ValueOf(s).FieldByName("").IsValid())

Or attempting to get a non-existing method by name using Value.MethodByName():

fmt.Println(reflect.ValueOf(s).MethodByName("").IsValid())

Or attempting to get a value from a map by a non-existing key using Value.MapIndex():

m := map[int]int{}
fmt.Println(reflect.ValueOf(m).MapIndex(reflect.ValueOf(3)).IsValid())

The list goes on...