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Julian-Chu
Julian-Chu

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[Go] Ensuring struct has implemented interface

#go

Structural typing vs Class-based interface

In other OOP languages, the interface is a contract by implementing class. By Contrast, Golang checks the interface implementation for struct when you call struct with interface, not by implementing struct.

Let's see some code

C#

interface IMessageService{ void Send() } // compile time error to show the class doesn't implement the interface public class MyService: IMessageService{ }

Go

func main() { // compile time error happens when using it var service IMessageService = &MyService{} service.Send() } type IMessageService interface { Send() } type MyService struct { }

Can we check the interface implementation before calling it?

Of course, we can. There're 2 options to do this.

  • Constructor returns interface: 
func NewMyMessageService() IMessageService { return &MyService{} }
  • Using underscore to do check 
var _ IMessageService = &MyService{}

In case you have multiple interfaces to implement:

  • Embedding multiple interfaces into one, and return it 
type IMyService interface { IMessageService IEmailService } type IMessageService interface { Send() } type IEmailService interface { SendEmail() } type MyService struct { } func NewMyService() *IMyService { return &MyService{} }

Or

  • Using underscore to do multiple checks 
var _ IMessageService = &MyService{} var _ IEmailService = &MyService{}

Underscore is more useful

"_" is also useful to check whether the 3rd party struct has implement client side interface.

import( "thirdparty" ) var _ IMessageService = &thirdparty.NotMyService{}

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