Method Pointer Receivers in Interfaces

Evan Hicks

—February 26, 2021

The Problem

The error: ./prog.go:23:7: cannot use b (type Bar) as type Foo in argument to Fizzy: Bar does not implement Foo (Fizz method has pointer receiver)

Click to Copy

package main

import (
    "fmt"
)

type Foo interface {
    Fizz()
}

type Bar struct {}

func (b *Bar) Fizz() {
    fmt.Println("fizz")
}

func Fizzy(foo Foo) {
    foo.Fizz()
}

func main() {
    b := Bar{}
    Fizzy(b)
}

The Solution

This error happens because the pointer to a type is its own unique type. In this example, *Bar is a different type to Bar. The method receiver Fizz is defined on the *Bar type, not the Bar type. So only *Bar satisfies the interface Foo.

The fix:

Click to Copy

package main

import (
    "fmt"
)

type Foo interface {
    Fizz()
}

type Bar struct {}

func (b *Bar) Fizz() {
    fmt.Println("fizz")
}

func Fizzy(foo Foo) {
    foo.Fizz()
}

func main() {
    b := &Bar{}
    Fizzy(b)
}

Change b to a pointer to a Bar. Since *Bar satisfies the interface Foo, b can now be used as a function argument to Fizzy.

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