Setter method not setting struct property Golang

Question

I need help with understanding why this error is being thrown:

I am using a pointer because I want it to update the field.

prog.go:56: cannot use MammalImpl literal (type MammalImpl) as type Mammal in array element: MammalImpl does not implement Mammal (SetName method has pointer receiver) prog.go:57: cannot use MammalImpl literal (type MammalImpl) as type Mammal in array element: MammalImpl does not implement Mammal (SetName method has pointer receiver)

I am not sure why this is unable to set/override the name property as follows.

 package main

import (
    "fmt"
)

type Mammal interface {
    GetID() int
    GetName() string
    SetName(s string)
}

type Human interface {
    Mammal

    GetHairColor() string
}

type MammalImpl struct {
    ID   int
    Name string
}

func (m MammalImpl) GetID() int {
    return m.ID
}

func (m MammalImpl) GetName() string {
    return m.Name
}

func (m *MammalImpl) SetName(s string) {
    m.Name = s
}

type HumanImpl struct {
    MammalImpl
    HairColor string
}

func (h HumanImpl) GetHairColor() string {
    return h.HairColor
}

func Names(ms []Mammal) *[]string {
    names := make([]string, len(ms))
    for i, m := range ms {
        m.SetName("Herbivorous") // This modification is not having any effect and throws and error
        names[i] = m.GetName()
    }
    return &names
}

func main() {
    mammals := []Mammal{
        MammalImpl{1, "Carnivorious"},
        MammalImpl{2, "Ominivorious"},
    }

    numberOfMammalNames := Names(mammals)
    fmt.Println(numberOfMammalNames)
}

Go Playground code is here http://play.golang.org/p/EyJBY3rH23

icza · Accepted Answer · 2015-02-08 18:24:41Z

3

The problem is that you have a method SetName() which has a pointer receiver:

func (m *MammalImpl) SetName(s string)

So if you have a value of type MammalImpl, the method set of that value does not contain the SetName() method therefore it does not implement the Mammal interface.

But the method set of a pointer to MammalImpl (*MammalImpl) will contain the SetName() method therefore it will implement the Mammal interface.

So when you populate the mammals slice, you have to populate it with *MammalImpl values, because that is the one that implements the element type of the slice (which is Mammal). You can easily obtain a pointer to a MammalImpl if you already have a MammalImpl value: use the address & operator to generate a pointer to the value:

mammals := []Mammal{
    &MammalImpl{1, "Carnivorious"},
    &MammalImpl{2, "Ominivorious"},
}

Try your modified program on the Go Playground.

edited Feb 8, 2015 at 18:24

answered Feb 8, 2015 at 18:19

icza

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