03-interface-and-struct

Interface and Struct

1. Struct

1. Define a struct and its method

   type Parent struct {
       Id int
       Name string
   }
   
   func (p Parent) GetName() string {
       return p.Name
   }
   
   func (p *Parent) SetName(name string) {
       p.Name = name
   }

   1. p for GetName is a receiver.
   2. p for SetName is a pointer receiver, which is used to modify value of the receiver.

   Exercise: Pointer Receiver

2. Use the new struct.

   func main() {
       p := Parent{Id: 1, Name: "John"}
       fmt.Println(p)
       p.SetName("Bob")
       fmt.Println(p.GetName())
   }

3. Run.

   go run main.go
   {1 John}
   Bob
   

2. Embedding

1. Embed Parent struct in Child struct.

   type Child struct {
       Parent // Embed Parent struct
       OtherField string
   }

2. Child can use the Parent's methods.

   func main() {
       p := Parent{Id: 1, Name: "John"}
       ch := Child{Parent: p}
       fmt.Printf("id: %d, name: %s\n", ch.Id, ch.Name)
   
       ch = Child{Parent{Id: 1, Name: "child"}, "Other"}
       fmt.Println(ch.GetName())
   }

3. Run.

   go run main.go
   id: 1, name: John
   child
   

Example

Embedding is often used in Kubernetes operator too!

Example: memcached_controller.go

type MemcachedReconciler struct {
	client.Client // Embeded
	Scheme   *runtime.Scheme
	Recorder record.EventRecorder
}

In this example, MemcachedReconciler has all the methods of client.Client such as Get, Create, Update, etc.

Interface

1. Define an interface Product

   type Object interface {
       GetName() string
   }

2. Use it.

   func main() {
       p := Parent{Id: 1, Name: "John"}
       printObjectName(&p)
       ch := Child{Parent: p}
       printObjectName(&ch)
   
       ch = Child{Parent{Id: 1, Name: "child"}, "Other"}
       printObjectName(&ch)
   }
   
   func printObjectName(obj Object) {
       fmt.Println(obj.GetName())
   }

3. Run.

   go run main.go
   John
   John
   child
   

References

1. Structs and Interfaces
2. Method Pointer Receivers in Interfaces

Practices

1. Structs
2. Struct Fields
3. Pointer Receiver
4. Interfaces
5. Interfaces are implemented implicitly
6. Interface values
7. Interface values with nil underlying values
8. Nil interface values
9. The empty interface
10. Type assertions
11. Type switches
12. Stringers
13. Exercise: Stringers