How to avoid interface method implementations that needs a pointer receiver?

Question

Consider the following example: http://play.golang.org/p/eAot_sVwND

package main

import "fmt"

type Incrementor interface {
    Increment()
}

type Counter struct {
    i int

    Incrementor
}

func (c *Counter) Increment(){
    c.i++
} 

func main() {
    var c Incrementor
    c = &Counter{}
    c.Increment()
    fmt.Println(c)  
}

Unfortunatelly I need to c = &Counter{} because Counter.Increment() implementation has a pointer receiver otherwise c.Increment() calls won't be able to modify c.x property:

func (c Counter) Increment(){
    c.i++ // no errors, but now we increment c.x having a copy of c as context
}

How to make original implementation works without & on c = &Counter{}? In other words, how to avoid the need for the pointer receiver on C.Increment implementation at all?

This is just a nit, but I think that maybe a pointer is not necessary to do that in Go.

VonC · Accepted Answer · 2014-12-07 09:42:29Z

4

This is just a nit, but I think that maybe a pointer is not necessary to do that in Go.

Considering that Go uses to pass everything by value, a pointer receiver is the natural way to achieve what you want.
This is supported by the Go FAQ:

First, and most important, does the method need to modify the receiver? If it does, the receiver must be a pointer.

You would find a similar conclusion in "Things I Wish Someone Had Told Me About Golang: pointers"

answered Dec 7, 2014 at 9:42

VonC

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3 Comments

marcio Over a year ago

So there is no way to get rid of the & &Counter{} initialization?

marcio Over a year ago

Motive for this concern is that I find it quite unpleasant to initialize structs by reference when using tirhd party packages, so I try to avoid it as much as possible. You end never knowing how to initialize things until you see a compile error.

VonC Over a year ago

@marcioAlmada I understand, but I don't know of a natural way to avoid pointer intialization in Go, when you have a method which modifies its content.

Chris Cherry · Accepted Answer · 2014-12-07 11:21:21Z

2

You could consider defining an NewCounter function that encapsulates the initialization of your type (or some 3rd party) and returns a *Counter. Usage could look something like this:

func main() {
    c := NewCounter()
    c.Increment()
    fmt.Println(c)  
}

answered Dec 7, 2014 at 11:21

Chris Cherry

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1 Comment

marcio Over a year ago

Nice, we see this pattern all over the place within go std libs.

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