如何使用从接口实现的方法更改结构中变量的指针

Question

Edit: For everyone suggesting using a pointer receiver in the function: By changing the method to have a pointer receiver, the structure no longer implements the interface. I have a image at the bottom of my question showing that.

I am trying to make a setter that will mutate the pointer of a variable in a struct with a method implemented from an interface.

package main

import "fmt"

func main() {
    i := 1
    b := BlahImpl {id:&i}

    fmt.Println(b.ID())
    j := 2
    b.SetID(&j)

    fmt.Println(b.ID())
}

type BlahInterface interface {
    SetID(*int)
    ID() int
}

type BlahImpl struct {
    id *int
}

func (b BlahImpl) SetID(i *int) {
    b.id = i
}

func (b BlahImpl) ID() int {
    return *b.id
}

The current output is:

1
1

But I would like:

1
2

When I use pointer receiver I get this error because the struct is no longer implementing the interface.

Answer 1

Well, to be honest I do not quite get why it works this way, but this works as you want it to:

package main

import "fmt"

func main() {
    i := 1
    b := BlahImpl{id: &i}

    fmt.Println(b.ID())
    j := 2
    b.SetID(&j)

    fmt.Println(b.ID())
}

type BlahInterface interface {
    SetID(*int)
    ID() int
}

type BlahImpl struct {
    id *int
}

func (b *BlahImpl) SetID(i *int) {
    b.id = i
}

func (b *BlahImpl) ID() int {
    return *b.id
}

The difference is how the structure's methods are defined. I've added * before the name so reference on the structure is passed into the method. It looks like without * the method gets copy of the struct so the modification does not work.