无法在非接口值上键入switch

I am playing with type assertion using the following dummy code, and I got the error:

cannot type switch on non-interface value

Does anyone know what does that mean?

package main

import "fmt"
import "strconv"

type Stringer interface {
    String() string
}

type Number struct {
    v int
}

func (number *Number) String() string {
    return strconv.Itoa(number.v)
}

func main() {
    n := &Number{1}
    switch v := n.(type) {
    case Stringer:
        fmt.Println("Stringer:", v)
    default:
        fmt.Println("Unknown")
    }
}

http://play.golang.org/p/Ti4FG0m1mc

Type switches require an interface to introspect. If you are passing a value of known type to it it bombs out. If you make a function that accepts an interface as a parameter, it will work:

func typeSwitch(tst interface{}) {
    switch v := tst.(type) {
        case Stringer:
           fmt.Println("Stringer:", v)
        default:
           fmt.Println("Unknown")
    }
}

See the full code here http://play.golang.org/p/QNyf0eG71_ and the golang documentation on interfaces http://golang.org/doc/effective_go.html#interfaces.

I figured out the answer, which is to cast n to interface{} before the type assertion:

switch v := interface{}(n).(type)

There are no typecasts in Go. You are doing a type conversion.

There are two kinds of type conversions

1. conversion between basic data types. For this we can use the direct casting

   i := 48

   str := string(i)

2. But type conversion using value.(type) is for conversion among the class hierarchy, (e.g. where we want to get the specific implementation from the interface). Else, we get the above error.