X does not implement Y (… method has a pointer receiver)

Issue

There are already several Q&As on this “X does not implement Y (… method has a pointer receiver)” thing, but to me, they seems to be talking about different things, and not applying to my specific case.

So, instead of making the question very specific, I’m making it broad and abstract — Seems like there are several different cases that can make this error happen, can someone summary it up please?

I.e., how to avoid the problem, and if it occurs, what are the possibilities? Thx.

Solution

This compile-time error arises when you try to assign or pass (or convert) a concrete type to an interface type; and the type itself does not implement the interface, only a pointer to the type.

Short summary: An assignment to a variable of interface type is valid if the value being assigned implements the interface it is assigned to. It implements it if its method set is a superset of the interface. The method set of pointer types includes methods with both pointer and non-pointer receiver. The method set of non-pointer types only includes methods with non-pointer receiver.

Let’s see an example:

type Stringer interface {
    String() string
}

type MyType struct {
    value string
}

func (m *MyType) String() string { return m.value }

The Stringer interface type has one method only: String(). Any value that is stored in an interface value Stringer must have this method. We also created a MyType, and we created a method MyType.String() with pointer receiver. This means the String() method is in the method set of the *MyType type, but not in that of MyType.

When we attempt to assign a value of MyType to a variable of type Stringer, we get the error in question:

m := MyType{value: "something"}

var s Stringer
s = m // cannot use m (type MyType) as type Stringer in assignment:
      //   MyType does not implement Stringer (String method has pointer receiver)

But everything is ok if we try to assign a value of type *MyType to Stringer:

s = &m
fmt.Println(s)

And we get the expected outcome (try it on the Go Playground):

something

So the requirements to get this compile-time error:

  • A value of non-pointer concrete type being assigned (or passed or converted)
  • An interface type being assigned to (or passed to, or converted to)
  • The concrete type has the required method of the interface, but with a pointer receiver

Possibilities to resolve the issue:

  • A pointer to the value must be used, whose method set will include the method with the pointer receiver
  • Or the receiver type must be changed to non-pointer, so the method set of the non-pointer concrete type will also contain the method (and thus satisfy the interface). This may or may not be viable, as if the method has to modify the value, a non-pointer receiver is not an option.

Structs and embedding

When using structs and embedding, often it’s not "you" that implement an interface (provide a method implementation), but a type you embed in your struct. Like in this example:

type MyType2 struct {
    MyType
}

m := MyType{value: "something"}
m2 := MyType2{MyType: m}

var s Stringer
s = m2 // Compile-time error again

Again, compile-time error, because the method set of MyType2 does not contain the String() method of the embedded MyType, only the method set of *MyType2, so the following works (try it on the Go Playground):

var s Stringer
s = &m2

We can also make it work, if we embed *MyType and using only a non-pointer MyType2 (try it on the Go Playground):

type MyType2 struct {
    *MyType
}

m := MyType{value: "something"}
m2 := MyType2{MyType: &m}

var s Stringer
s = m2

Also, whatever we embed (either MyType or *MyType), if we use a pointer *MyType2, it will always work (try it on the Go Playground):

type MyType2 struct {
    *MyType
}

m := MyType{value: "something"}
m2 := MyType2{MyType: &m}

var s Stringer
s = &m2

Relevant section from the spec (from section Struct types):

Given a struct type S and a type named T, promoted methods are included in the method set of the struct as follows:

  • If S contains an anonymous field T, the method sets of S and *S both include promoted methods with receiver T. The method set of *S also includes promoted methods with receiver *T.
  • If S contains an anonymous field *T, the method sets of S and *S both include promoted methods with receiver T or *T.

So in other words: if we embed a non-pointer type, the method set of the non-pointer embedder only gets the methods with non-pointer receivers (from the embedded type).

If we embed a pointer type, the method set of the non-pointer embedder gets methods with both pointer and non-pointer receivers (from the embedded type).

If we use a pointer value to the embedder, regardless of whether the embedded type is pointer or not, the method set of the pointer to the embedder always gets methods with both the pointer and non-pointer receivers (from the embedded type).

Note:

There is a very similar case, namely when you have an interface value which wraps a value of MyType, and you try to type assert another interface value from it, Stringer. In this case the assertion will not hold for the reasons described above, but we get a slightly different runtime-error:

m := MyType{value: "something"}

var i interface{} = m
fmt.Println(i.(Stringer))

Runtime panic (try it on the Go Playground):

panic: interface conversion: main.MyType is not main.Stringer:
    missing method String

Attempting to convert instead of type assert, we get the compile-time error we’re talking about:

m := MyType{value: "something"}

fmt.Println(Stringer(m))

Answered By – icza

Answer Checked By – Mary Flores (GoLangFix Volunteer)

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