Cannot use struct that implements interface

Question

code:

package main

import "fmt"

type implementation struct {
    d []int
}

func (impl *implementation) getData() interface{} {
    return impl.d
}

type phase struct{}

type data interface {
    getData() interface{}
}

func MakeIntDataPhase() *phase {
    return &phase{}
}

func (p *phase) run(population []data) []data {
    return nil
}

func main() {
    var population []implementation
    MyPhase := MakeIntDataPhase()
    fmt.Println(MyPhase.run(population))

}

When running following code in playground I got following error: prog.go:30:25: cannot use population (type []implementation) as type []data in argument to MyPhase.run

I am new to golang and I don't understand why is this happening?

Struct implementation implements method getData() from data interface. Isn't it enough to use a slice of implementation in run method?

Where my reasoning is wrong?

That's not how slices work. See https://golang.org/doc/faq#convert_slice_of_interface — Volker, Oct 30 '18 at 12:31

ssemilla · Accepted Answer · 2018-10-30T12:51:33.560

This seems counter-intuitive but []data is of a different type to []implementation because of how slice types are represented in Go.

This is actually discussed in the the Go Wiki

Edit: Consider this

var impl []*implementation
var data []data = impl

The compiler will complain with

cannot use impl (type []*implementation) as type []data in assignment

It's more code but you actually have to create a slice of your interface as what the comments in this thread recommends, like so:

var impl []*implementation
var data []data

// assuming impl already has values
for _, v := range impl {
    data = append(data, v)
}

Cannot use struct that implements interface

1 Answers1