When is go reflect CanInterface false?

Issue

As per this playground example (https://play.golang.org/p/Jr64yE4zSpQ), and the implementation of CanInterface in reflect/value.go, it looks like CanInterface is false only for private fields?

What are other scenarios when CanInterface is false?

Playground example:

num := 6
meta := reflect.ValueOf(num)
fmt.Println("canInterface:", meta.CanInterface() == true)

meta = reflect.ValueOf(&num)
fmt.Println("canInterface:", meta.CanInterface() == true)

foo := Foo{}
meta = reflect.ValueOf(&foo)
fmt.Println("canInterface:", meta.CanInterface() == true)
meta = meta.Elem()
fmt.Println("canInterface:", meta.CanInterface() == true)
publicField := meta.FieldByName("Number")
privateField := meta.FieldByName("privateNumber")
fmt.Println(
    "canInterface:", 
    publicField.CanInterface() == true,
    // Woah, as per the implementation (reflect/value.go) 
    // this is the only time it can be false
    privateField.CanInterface() != true)

var fooPtr *Foo
var ptr anInterface = fooPtr
meta = reflect.ValueOf(ptr)
fmt.Println("canInterface:", meta.CanInterface() == true)

meta = reflect.ValueOf(&foo)
meta = meta.Elem() // ptr to actual value
publicField = meta.FieldByName("Number")
ptrToField := publicField.Addr()
fmt.Println("canInterface:", ptrToField.CanInterface() == true)

reflect/value.go

func (v Value) CanInterface() bool {
if v.flag == 0 {
    panic(&ValueError{"reflect.Value.CanInterface", Invalid})
}
// I think "flagRO" means read-only?
return v.flag&flagRO == 0
}

Solution

If you dive into the source code for CanInterface, you can see this line:

return v.flag&flagRO == 0

And a bit below it, this block of code from the function valueInterface:

if safe && v.flag&flagRO != 0 {
    // Do not allow access to unexported values via Interface,
    // because they might be pointers that should not be
    // writable or methods or function that should not be callable.
    panic("reflect.Value.Interface: cannot return value obtained from unexported field or method")
}

Since v.flag&flagRO != 0 is equivalent to !CanInterface, we can conclude from the comment below it that CanInterface is false when the reflect.Value is an unexported struct field or method.

Answered By – Zippo

Answer Checked By – Mary Flores (GoLangFix Volunteer)

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