Why can't embedded types be passed as pointers

Question

Can someone explain why this doesn't work? It works if DoMove takes a struct instead of a pointer.

package main

import (
    "fmt"
)

type Vehicle interface {
    Move()
}

type Car interface {
    Vehicle
    Wheels() int
}

type car struct {}

func (f car) Move() { fmt.Println("Moving...") }
func (f car) Colour() int { return 4 }

func DoMove(v *Vehicle) {
    v.Move()
}

func main() {
    f := car{}
    DoMove(&f)

}

See https://stackoverflow.com/questions/44370277/type-is-pointer-to-interface-not-interface-confusion — maerics, Sep 06 '18 at 21:10

score 0 · Answer 1 · answered Sep 06 '18 at 21:14

Very simple. In your DoMove() function, the variable is of *Vehicle type (pointer to the Vehicle interface). The pointer has no method Move at all.

The usual practice is to use the interface as function argument, but pass in pointer to struct (and make sure the pointer implements the interface). Example,

package main

import (
    "fmt"
)

type Vehicle interface {
    Move()
}

type Car interface {
    Vehicle
    Wheels() int
}

type car struct {
        status string
}

func (f *car) Move() {
        fmt.Println("Moving...")
        f.status = "Moved"
}
func (f car) Status() string {
        return f.status
}

func DoMove(v Vehicle) {
    v.Move()
}

func main() {
    f := car{status: "Still"}
    DoMove(&f)
    fmt.Println(f.Status())
}

Output:

Moving...
Moved

The *car content is indeed changed.

Why can't embedded types be passed as pointers

1 Answers1