带有指针接收器的Golang方法

Question

I have this example code

package main

import (
    "fmt"
)

type IFace interface {
    SetSomeField(newValue string)
    GetSomeField() string
}

type Implementation struct {
    someField string
}

func (i Implementation) GetSomeField() string {
    return i.someField
}

func (i Implementation) SetSomeField(newValue string) {
    i.someField = newValue
}

func Create() IFace {
    obj := Implementation{someField: "Hello"}
    return obj // <= Offending line
}

func main() {
    a := Create()
    a.SetSomeField("World")
    fmt.Println(a.GetSomeField())
}

SetSomeField does not work as expected because its receiver is not of pointer type.

If I change the method to a pointer receiver, what I would expect to work, it looks like this:

func (i *Implementation) SetSomeField(newValue string) { ...

Compiling this leads to the following error:

prog.go:26: cannot use obj (type Implementation) as type IFace in return argument:
Implementation does not implement IFace (GetSomeField method has pointer receiver)

How can I have the struct implement the interface and the method SetSomeField change the value of the actual instance without creating a copy?

Here's a hackable snippet: https://play.golang.org/p/ghW0mk0IuU

I've already seen this question In go (golang), how can you cast an interface pointer into a struct pointer?, but I cannot see how it is related to this example.

Answer 1

Your pointer to the struct should implement the Interface. In that way you can modify its fields.

Look at how I modified your code, to make it working as you expect:

package main

import (
    "fmt"
)

type IFace interface {
    SetSomeField(newValue string)
    GetSomeField() string
}

type Implementation struct {
    someField string
}    

func (i *Implementation) GetSomeField() string {
    return i.someField
}

func (i *Implementation) SetSomeField(newValue string) {
    i.someField = newValue
}

func Create() *Implementation {
    return &Implementation{someField: "Hello"}
}

func main() {
    var a IFace
    a = Create()
    a.SetSomeField("World")
    fmt.Println(a.GetSomeField())
}