Question

Suppose I have a method of type http.HandleFunc

type HandlerFunc func(ResponseWriter, *Request)

And I want to wrap it around another method of the same type, somewhat like this:

func MyWrapper(res http.ResponseWriter, req *http.Request) {

    // do stuff

    AnotherMethod(res, req) // <- question refers to this line

    // do more stuff

}

func AnotherMethod(res http.ResponseWriter, req *http.Request) {

    // main logic

}

If I’m getting this right; when I call AnotherMethod(res, req) I’m passing a copy of (the value of) res to AnotherMethod, meaning that this object is now duplicated in memory.

Is there a way I could pass a pointer to res to AnotherMethod and then dereference there, in order to not copy the value of res? or am I not understanding something?

(Working with a pointer to (the value of) res inside AnotherMethod won’t work because the receivers of all methods in http.ResponseWriter are values and not pointers)

Answer 1

http.ResponseWriter is an interface type. Which means it can be both a reference or value type, depending on what the underlying type is and how it implements the interface.

In this case, res is an instance of the unexported type *http.response. As you can see, it is a pointer type, which means you can pass it around without creating a copy of the whole structure.

To see what real type is held inside an interface value you are receiving, you can do this: fmt.Printf("%T\n", res). It should print: *http.resonse.

For more information on how interface types work in Go, I recommend reading the Go specification on the subject.