Struct with an pointer to an Interface mock error

Question

package main

import (
    "github.com/golang/mock/gomock"
    "testing"
)

type Talker interface {
    talk() string
}

type Person struct {
    moth *Talker
}

func (p *Person) speak() string {
    return (*p.moth).talk()
}

func TestPerson(t *testing.T) {
    ctrl := gomock.NewController(t)
    mockTalker := NewMockTalker(ctl)

    person := Person{moth: mockTalker}
}

Assuming that I have already created a mock for Talker interface using mockgen.

I am getting error when I am creating Person{moth: mockTalker}. I am not able to pass mockTalker.

jiang · Answer 1 · 2021-12-23T06:07:51.483

3

Don't user pointer interface. Essentially interface is pointer

type Person struct {
    moth Talker
}

Normally, if function want return interface, it's will return new struct by pointer.

import "fmt"

type I interface {
    M()
}

type S struct {
}

func (s *S) M() {
    fmt.Println("M")
}

func NewI() I {
    return &S{}
}

func main() {
    i := NewI()
    i.M()
}

edited Dec 23 '21 at 06:07

answered Dec 23 '21 at 03:54

jiang

31
4

score 1 · Answer 2 · answered Dec 23 '21 at 03:35

In your Person struct the moth field is *Talker type. It is a pointer type of Talker interface. NewMockTalker(ctl) returns Talker type mock implementation.

You can do two things to fix this.

Change the Person's moth field's type to Talker.

type Person struct {
    moth Talker
}

or

pass pointer reference of mockTalker to the person initialization`

person := Person{moth: &mockTalker}

Struct with an pointer to an Interface mock error

2 Answers2