Why Do Go\'s Interface Setter Methods Require Pointers?

Polymorphism in Go: Interface Implementation

While Go does not support traditional polymorphism, interfaces provide a means to achieve a similar effect. However, limitations exist when implementing setter methods.

The Problem

Consider the following code:

type MyInterfacer interface {
    Get() int
    Set(i int)
}

type MyStruct struct {
    data int
}

func (this MyStruct) Get() int {
    return this.data
}

Attempting to implement the Set method results in an error:

func (this MyStruct) Set(i int) {
    this.data = i
}

Solution

To circumvent this limitation, the receiver in the Set method must be a pointer, as in:

func (this *MyStruct) Set(i int) {
    this.data = i
}

This allows the method to modify the underlying data, which is reflected in the interface variable.

Corrected Code

The following corrected code works as intended:

type MyInterfacer interface {
    Get() int
    Set(i int)
}

type MyStruct struct {
    data int
}

func (this *MyStruct) Get() int {
    return this.data
}

func (this *MyStruct) Set(i int) {
    this.data = i
}

func main() {
    s := &MyStruct{123}
    fmt.Println(s.Get())

    s.Set(456)
    fmt.Println(s.Get())

    var mi MyInterfacer = s
    mi.Set(789)
    fmt.Println(mi.Get())
}

While not true polymorphism, this technique allows for a similar level of flexibility through interface implementation in Go.

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