Can not convert a generic struct type to a generic interface type in golang

Given there's a generic interface:

type Packet interface {
    Content() int
}

type Recycler[T any] interface {
    Get() *T
}

and their implementation:

type packet struct {
    content int
}

type BaseRecycler[T any] struct {
    t T
}

its impossible to convert the implementation to the interface:

    r := &BaseRecycler[packet]{}
    r1 := r.(Recycler[Packet]) 
        fmt.Println(r3.Get().Content()) // compile error: Unresolved reference 'Content'

here is a live code example: https://go.dev/play/p/qbnYfyDd22A

The error is because the Packet interface align with the *packet instead of packet. Could anyone suggest how to handle this?

Solution

You are assuming covariance in type parameters, which Go does not have - types must match exactly. You would need variance annotations like Kotlins in and out or Javas wildcards for that.

As the FAQ states:

In Go method types must match exactly, [...]. Programmers who want covariant result types are often trying to express a type hierarchy through interfaces. In Go it’s more natural to have a clean separation between interface and implementation.

To be a little more concrete, you could implement your packet like (Go Playground):

type packet struct {
    content int
}

func (p packet) Content() int {
    return p.content
}

and then an interface abstracting that:

type Packet interface {
    Content() int
}

Also the BaseRecycler:

type BaseRecycler[T any] struct {
    t T
}

func (b BaseRecycler[T]) Get() T {
    return b.t
}

and an abstraction thereof:

type Recycler[T any] interface {
    Get() T
}

Note some simplifications: The methods don't have pointer receivers (they serve no purpose in this example) and BaseRecycler does return a type, not an address.

Now you can simply use that:

    var r Recycler[Packet] = BaseRecycler[Packet]{t: packet{}}
    fmt.Println(r.Get().Content())

Also, you could define a PacketRecycler which has a concrete packet type and is not generic:

type PacketRecycler struct {
    t packet
}

func (p PacketRecycler) Get() Packet {
    return p.t
}

and use it in an identical fashion:

    var r1 Recycler[Packet] = PacketRecycler{}
    fmt.Println(r1.Get().Content())