Cast an interface/struct in golang

Oct 25, 2017 · Agus Sarwono

type casting interface in golang can be useful when you build a modular app using golang

says

package main

import (
	"fmt"
)

type Bird interface {
	Fly()
}

type Parrot struct {
	Bird
	Name string
}

func (p *Parrot) Fly() {
	fmt.Println("a Parrot name " + p.Name + " flying")
}

type Dove struct {
	Bird
	Name string
}

func (p *Dove) Fly() {
	fmt.Println("a Dove name " + p.Name + " flying")
}

func NewBird(birdType string, name string) Bird {
	var bird Bird
	switch birdType {
	case "dove":
		bird = &Dove{
			Name: name,
		}
	case "parrot":
		bird = &Parrot{
			Name: name,
		}
	default:
	}

	return bird
}

func main() {
	bird1 := NewBird("dove", "Doval")
	bird2 := NewBird("parrot", "Patriot")
	// to access Name in bird1, cast dove to type Dove
	dove, valid := bird1.(*Dove)
	if !valid {
		panic("invalid type")
	}
	fmt.Println(dove.Name)
	dove.Fly()
	// same as dove, you need to cast bird2
	parrot, valid := bird2.(*Parrot)
	if !valid {
		panic("invalid type")
	}
	fmt.Println(parrot.Name)
	parrot.Fly()
}

above example would return

machine@root$
Doval
a Dove name Doval flying
Patriot
a Parrot name Patriot flying

as you can see, you need to cast dove to Dove type to access Name property, that because NewBird only return interface Bird which doesn’t have Name property, you can only define method inside interface, therefore you need to cast them