为什么Go反射NumField（）仅在类型上可用？

Question

I have the following Go code (play.golang.org):

package main

import (
    "reflect"
    "fmt"
)

type User struct {
    name string
    email string
}

func main() {   
    uS := User{}
    uSt := reflect.TypeOf(uS)   

    fmt.Println( uSt )
    fmt.Println( uSt.NumField() )
    // fmt.Println( uS.NumField() ) // this doesn't work, why?

}

I'm just curious here. Why we need to get the type of the struct first before calling NumField()?

Why can't we just call it on the struct itself i.e. uS.NumField()?

From docs:

type Value struct {
        // contains filtered or unexported fields
}

func (v Value) NumField() int

I'm not quite getting what is Value really means here. I do really appreciate concise explanation and examples.

Answer 1

NumField() is a function of reflected structs and can be applied on any struct once you have reflected it.

It is NOT a function of User. You did not create any functions for User, so I did to try to clarify

https://play.golang.org/p/c8oGa_56YfE