在Go中使用自定义http.Handler时为什么要使用指针？

Question

When calling http.Handle() in the code snippet below, I'm using my own templateHandler type which implements the http.Handler interface.

package main

import (
    "html/template"
    "log"
    "net/http"
    "path/filepath"
    "sync"
)

type templateHandler struct {
    once     sync.Once
    filename string
    templ    *template.Template
}

func (t *templateHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    t.once.Do(func() {
        t.templ = template.Must(template.ParseFiles(filepath.Join("templates", t.filename)))
    })
    t.templ.Execute(w, nil)
}

func main() {
    http.Handle("/", &templateHandler{filename: "chat.html"})
    if err := http.ListenAndServe(":8080", nil); err != nil {
        log.Fatal("ListenAndServe: ", err)
    }
}

Now for some reason I have to pass a pointer to http.Handle() using &templateHandler{filename: "chat.html"}. Without the & I get the following error:

cannot use (templateHandler literal) (value of type templateHandler) 
as http.Handler value in argument to http.Handle: 
missing method ServeHTTP

Why exactly is this happening? What difference does using a pointer make in this case?

Answer 1

http.Handle() expects a value (any value) that implements http.Handler, which means it must have a ServeHTTP() method.

You used pointer receiver for the templateHandler.ServeHTTP() method, which means only a pointer value to templateHandler has this method, but not that of a non-pointer templateHandler type.

Spec: Method sets:

A type may have a method set associated with it. The method set of an interface type is its interface. The method set of any other type T consists of all methods declared with receiver type T. The method set of the corresponding pointer type *T is the set of all methods declared with receiver *T or T (that is, it also contains the method set of T).

A non-pointer type only has methods with non-pointer receivers. A pointer type has methods both with pointer and non-pointer receivers.

Your ServeHTTP() method modifies the receiver, so it must be a pointer. But if some other handler does not need to, the ServeHTTP() method may be created using a non-pointer receiver, and in that case you can use a non-pointer value as the http.Handler, like in this example:

type myhandler struct{}

func (m myhandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {}

func main() {
    // non-pointer struct value implements http.Handler:
    http.Handle("/", myhandler{})
}