Why does serving a struct via http create a copy?-CodePudding

I noticed some strange behaviour when I create a struct in go that registers an http-Handler in its New function.

Consider the following code:

package main

import (
    "fmt"
    "net/http"
)

type Counter struct {
    name  string
    value int
}

func New(name string) Counter {
    c := Counter{
        name:  name,
        value: 0,
    }

    http.HandleFunc("/", c.serve)
    return c
}

func (c *Counter) inc()             { c.value   }
func (c *Counter) reset()           { c.value = 0 }
func (c *Counter) nameApp(n string) { c.name  = n }
func (c *Counter) print()           { fmt.Printf("[%s]: %d (%p)\n", c.name, c.value, &c) }

func (c *Counter) Reinit(name string, value int) {
    c.name = name
    c.value = value
}

func (c *Counter) serve(w http.ResponseWriter, req *http.Request) {
    c.inc()
    c.nameApp("-foo")
    fmt.Println("Counter served:")
    c.print()

    w.WriteHeader(http.StatusOK)
    w.Write([]byte{})
}

func main() {
    c := New("My New Counter")
    fmt.Println("New Counter:")
    c.print()

    c.Reinit("My reinit Counter", 10)
    fmt.Println("Counter after Reinit() call:")
    c.print()

    http.ListenAndServe("localhost:9000", nil)
}

When running it it creates the following output:

New Counter:
[My New Counter]: 0 (0xc00012a2a0)
Counter after Reinit() call:
[My reinit Counter]: 10 (0xc00012a2a0)

After sending two requests to the server the output is:

Counter served:
[My New Counter-foo]: 1 (0xc00012a2c0) // expected "[My reinit Counter]: 11 (0xc00012a2a0)"
Counter served:
[My New Counter-foo-foo]: 2 (0xc00012a2c0) // expected "[My reinit Counter]: 12 (0xc00012a2a0)"

Why does the struct not behave as I had expected, even though I am properly using pointer receivers?

How can I modify my struct from a main routine or really any other routine to and have these changes being reflected in the associated http request handlers?

CodePudding user response：

Found the solution while writing up the question:

The return value of the new function needs to be a pointer, otherwise it will return a copy. The following change solves the problem:

func New(name string) *Counter { // add the *
    // ... as before
    return &c // add the &
}

(What I don't really get, though, is why the address in initial output stays the same!?)

CodePudding user response：

func (c *Counter) print()

You are printing

fmt.Printf("[%s]: %d (%p)\n", c.name, c.value, &c)

Most notably, you are printing &c via %p. c is the pointer receiver argument to the method. &c is a pointer to the pointer receiver. In other words, &c is a pointer to a local variable, the receiver variable. If you want to print just the address of the Counter which was called on, use the plain c. For example:

fmt.Printf("[%s]: %d (%p)\n", c.name, c.value, c)