Why is `ResponseWriter` passed by value and `Request` passed by pointer in Go HTTP handlers?

Difference Between ResponseWriter and Request in Go HTTP Handlers

In Go HTTP handlers, the ResponseWriter is defined as a value, while the Request is passed as a pointer. To understand this distinction, let's examine the underlying types.

The ResponseWriter is an interface defined as:

type ResponseWriter interface {
    ...
}

This means that ResponseWriter is simply a contract that any concrete type can implement. In contrast, Request is defined as a struct:

type Request struct {
    ...
}

Since Request is a concrete type, it must be passed as a reference using a pointer to allow modifications to its internal state. This is in contrast to the ResponseWriter, which is passed by value as it is an interface.

The specific implementation of ResponseWriter used in GAE is actually a pointer to the unexported http.response type, giving the impression of passing a value. However, as seen above, the fundamental definition of ResponseWriter is an interface, which cannot be passed by value.

Ultimately, the decision to have ResponseWriter as a value and Request as a pointer allows a clean separation of concerns. The ResponseWriter interface ensures consistency and stability in response handling, while the pointer to the Request struct enables direct modifications to its internal state.

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