[英]Why use a pointer when using a custom http.Handler in Go?
When calling http.Handle()
in the code snippet below, I'm using my own templateHandler
type which implements the http.Handler
interface. 在下面的代码片段中调用http.Handle()
时,我使用的是自己的templateHandler
类型,该类型实现了http.Handler
接口。
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"}
. 现在由于某种原因,我必须使用&templateHandler{filename: "chat.html"}
传递指向http.Handle()
的指针。 Without the &
I get the following error: 如果没有&
,则会出现以下错误:
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? 为什么会这样呢? What difference does using a pointer make in this case? 在这种情况下,使用指针有什么区别?
http.Handle()
expects a value (any value) that implements http.Handler
, which means it must have a ServeHTTP()
method. http.Handle()
需要一个实现http.Handler
的值(任何值),这意味着它必须具有ServeHTTP()
方法。
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. 您为templateHandler.ServeHTTP()
方法使用了指针接收器,这意味着仅指向templateHandler
的指针值具有此方法,而没有非指针templateHandler
类型的指针。
A type may have a method set associated with it. 类型可能具有与之关联的方法集 。 The method set of an interface type is its interface. 接口类型的方法集是其接口。 The method set of any other type
T
consists of all methods declared with receiver typeT
. 其他任何类型T
的方法集都包含以接收者类型T
声明的所有方法 。 The method set of the corresponding pointer type*T
is the set of all methods declared with receiver*T
orT
(that is, it also contains the method set ofT
). 对应的指针类型*T
的方法集是用接收者*T
或T
声明的所有方法的集合(也就是说,它还包含T
的方法集)。
A non-pointer type only has methods with non-pointer receivers. 非指针类型仅具有带有非指针接收器的方法。 A pointer type has methods both with pointer and non-pointer receivers. 指针类型同时具有指针接收器和非指针接收器的方法。
Your ServeHTTP()
method modifies the receiver, so it must be a pointer. 您的ServeHTTP()
方法会修改接收方,因此它必须是一个指针。 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: 但是,如果不需要其他处理程序,则可以使用非指针接收器创建ServeHTTP()
方法,在这种情况下,您可以将非指针值用作http.Handler
,例如以下示例:
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{})
}
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