Meaning of a struct with embedded anonymous interface?

Question

sort package:

type Interface interface {
    Len() int
    Less(i, j int) bool
    Swap(i, j int)
}

...

type reverse struct {
    Interface
}
         


        

Answer 1

I find this feature very helpful when writing mocks in tests.

Here is such an example:

package main_test

import (
    "fmt"
    "testing"
)

// Item represents the entity retrieved from the store
// It's not relevant in this example
type Item struct {
    First, Last string
}

// Store abstracts the DB store
type Store interface {
    Create(string, string) (*Item, error)
    GetByID(string) (*Item, error)
    Update(*Item) error
    HealthCheck() error
    Close() error
}

// this is a mock implementing Store interface
type storeMock struct {
    Store
    // healthy is false by default
    healthy bool
}

// HealthCheck is mocked function
func (s *storeMock) HealthCheck() error {
    if !s.healthy {
        return fmt.Errorf("mock error")
    }
    return nil
}

// IsHealthy is the tested function
func IsHealthy(s Store) bool {
    return s.HealthCheck() == nil
}

func TestIsHealthy(t *testing.T) {
    mock := &storeMock{}
    if IsHealthy(mock) {
        t.Errorf("IsHealthy should return false")
    }

    mock = &storeMock{healthy: true}
    if !IsHealthy(mock) {
        t.Errorf("IsHealthy should return true")
    }
}

By using:

type storeMock struct {
    Store
    ...
}

One doesn't need to mock all Store methods. Only HealthCheck can be mocked, since only this method is used in the TestIsHealthy test.

Below the result of the test command:

$ go test -run '^TestIsHealthy$' ./main_test.go           
ok      command-line-arguments  0.003s

A real world example of this use case one can find when testing the AWS SDK.

---

To make it even more obvious, here is the ugly alternative - the minimum one needs to implement to satisfy the Store interface:

type storeMock struct {
    healthy bool
}

func (s *storeMock) Create(a, b string) (i *Item, err error) {
    return
}
func (s *storeMock) GetByID(a string) (i *Item, err error) {
    return
}
func (s *storeMock) Update(i *Item) (err error) {
    return
}

// HealthCheck is mocked function
func (s *storeMock) HealthCheck() error {
    if !s.healthy {
        return fmt.Errorf("mock error")
    }
    return nil
}

func (s *storeMock) Close() (err error) {
    return
}