Don't understand composition in Go

Question

In the example below I've embedded http.ResponseWriter into my own struct called Response. I've also added an extra field called Status. Why can't I access that field from inside my root handler function?

When I print out the type of w in my root handler function it says it's of type main.Response which seems correct and when I print out the values of the struct I can see that Status is there. Why can't I access by going w.Status?

This is the contents of stdout:

main.Response
{ResponseWriter:0xc2080440a0 Status:0}

Code:

package main

import (
    "fmt"
    "reflect"

    "net/http"
)

type Response struct {
    http.ResponseWriter
    Status int
}

func (r Response) WriteHeader(n int) {
    r.Status = n
    r.ResponseWriter.WriteHeader(n)
}

func middleware(h http.Handler) http.Handler {

    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {

        resp := Response{ResponseWriter: w}

        h.ServeHTTP(resp, r)
    })
}

func root(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("root"))
    fmt.Println(reflect.TypeOf(w))
    fmt.Printf("%+v\n", w)
    fmt.Println(w.Status) // <--- This causes an error.
}

func main() {
    http.Handle("/", middleware(http.HandlerFunc(root)))
    http.ListenAndServe(":8000", nil)
}

Answer 1

w is a variable of type http.ResponseWriter. ResponseWriter does not have a field or method Status, only your Response type.

http.ResponseWriter is an interface type, and since your Response type implements it (because it embeds ResponseWriter), the w variable may hold a value of dynamic type Response (and in your case it does).

But to access the Response.Status field, you have to convert it to a value of type Response. For that use Type assertion:

if resp, ok := w.(Response); ok {
    // resp is of type Response, you can access its Status field
    fmt.Println(resp.Status) // <--- properly prints status
}