Go by Example: Interfaces

Interfaces are named collections of method signatures.

package main
import (

Here’s a basic interface for geometric shapes.

type geometry interface {
    area() float64
    perim() float64

For our example we’ll implement this interface on rect and circle types.

type rect struct {
    width, height float64
type circle struct {
    radius float64

To implement an interface in Go, we just need to implement all the methods in the interface. Here we implement geometry on rects.

func (r rect) area() float64 {
    return r.width * r.height
func (r rect) perim() float64 {
    return 2*r.width + 2*r.height

The implementation for circles.

func (c circle) area() float64 {
    return math.Pi * c.radius * c.radius
func (c circle) perim() float64 {
    return 2 * math.Pi * c.radius

If a variable has an interface type, then we can call methods that are in the named interface. Here’s a generic measure function taking advantage of this to work on any geometry.

func measure(g geometry) {
func main() {
    r := rect{width: 3, height: 4}
    c := circle{radius: 5}

The circle and rect struct types both implement the geometry interface so we can use instances of these structs as arguments to measure.

$ go run interfaces.go
{3 4}

To learn more about Go’s interfaces, check out this great blog post.

Next example: Enums.