如何在golang的接口中添加新方法

Question

I have a golang interface I

type I interface {
    A()
    B()
}

This interface is an element of type S struct. Now I want to add a function C() to this interface which will be called an object of type S. But this interface is implemented by many other types(for ex: T). And on compiling I get an error as T does not implement C().

One workaround for this is add a dummy implementation of C() in T which just returns a value of the T's return type.

Is there any better way to do this?

Answer 1

You can implement multiple interfaces with a single struct, as below. You would then have methods accept the different interfaces as arguments.

If you need a single function which utilises methods from both interfaces you could just pass the pointer to your struct as separate arguments (one for each interface) but there is nothing stopping one interface satisfying multiple interfaces with a smaller scope so you could create a third interface which encapsulates the functionality of both to handle these situations (See IJ interface example).

package main

import (
    "fmt"
)

type I interface {
    A()
    B()
}

// interface 'J' could be defined in an external package, it doesn't matter
type J interface {
    C()
}

// encapsulate I & J interfaces as IJ
type IJ interface {
    J
    I
}

// S will satisfy interfaces I, J & IJ
type S struct {}

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

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

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


func main() {
    s := &S{}
    doWithI(s)
    doWithJ(s)
    fmt.Println("===================================")
    doWithIAndJ(s)
}

func doWithI(in I){
    in.A()
    in.B()
}

func doWithJ(in J){
    in.C()
}

func doWithIAndJ(in IJ){
    in.A()
    in.B()
    in.C()
}

https://play.golang.org/p/DwH7Sr3zf_Y