Golang interface method – pointer receiver or value receiver

paradite

9 years ago

Recently I have been working with Golang interface.

There are lots of confusions on how to use interface in Golang, particularly how to implement the methods in interface. Should I implement pointer receiver, or value receiver? How to mutate a struct that implements an interface? Can I implement both?

Here is a sample code to clear the confusion (http://play.golang.org/p/6u6g2HM33X):

package main

import "fmt"

type User interface {
    SetIndex(int)
}

type UserMutable struct {
    Index int
}

// UserMutable DOES NOT implement User, *UserMutable implements User
func (u *UserMutable) SetIndex(index int) {
    u.Index = index
}

type UserImmutable struct {
    Index int
}

// UserImmutable implements User
func (u UserImmutable) SetIndex(index int) {
    u.Index = index
}

func main() {
    //userMutable is type *UserMutable
    userMutable := &UserMutable{}
    var user User
    user = userMutable
    user.SetIndex(9)

    //userImmutable is type UserImmutable
    userImmutable := UserImmutable{}
    // or userImmutable := &UserImmutable{}
    var user2 User
    user2 = userImmutable
    user2.SetIndex(9)

    fmt.Println("userMutable", userMutable.Index)
    // "userMutable 9"
    fmt.Println("userImmutable", userImmutable.Index)
    // "userImmutable 0"
}

package main

import "fmt"

type User interface {

SetIndex(int)

}

type UserMutable struct {

Index int

}

// UserMutable DOES NOT implement User, *UserMutable implements User

func (u *UserMutable) SetIndex(index int) {

u.Index = index

}

type UserImmutable struct {

Index int

}

// UserImmutable implements User

func (u UserImmutable) SetIndex(index int) {

u.Index = index

}

func main() {

//userMutable is type *UserMutable

userMutable := &UserMutable{}

var user User

user = userMutable

user.SetIndex(9)

//userImmutable is type UserImmutable

userImmutable := UserImmutable{}

// or userImmutable := &UserImmutable{}

var user2 User

user2 = userImmutable

user2.SetIndex(9)

fmt.Println("userMutable", userMutable.Index)

// "userMutable 9"

fmt.Println("userImmutable", userImmutable.Index)

// "userImmutable 0"

}

As you can see, a method in an interface can be implemented by either pointer receiver and value receiver.

In the case of pointer receiver, the pointer to the struct (*UserMutable in the example above) implements the interface.

In the case of value receiver, the the struct itself (UserImmutable in the example above) implements the interface.

Hope this clears confusions for people new to Golang.

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