Golang method with pointer receiver [duplicate]

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()) } 

Answer 2

The simple answer is that you won't be able to have the struct implement your interface while having SetSomeField work the way you want.

However, a pointer to the struct will implement the interface, so changing your Create method to do return &obj should get things working.

The underlying problem is that your modified SetSomeField method is no longer in the method set of Implementation. While the type *Implementation will inherit the non-pointer receiver methods, the reverse is not true.

The reason for this is related to the way interface variables are specified: the only way to access the dynamic value stored in an interface variable is to copy it. As an example, imagine the following:

var impl Implementation var iface IFace = &impl 

In this case, a call to iface.SetSomeField works because it can copy the pointer to use as the receiver in the method call. If we directly stored a struct in the interface variable, we'd need to create a pointer to that struct to complete the method call. Once such a pointer is made, it is possible to access (and potentially modify) the interface variable's dynamic value without copying it.