Interfaces #

// _Interfaces_ are named collections of method
// signatures.

package main

import (
	"fmt"
	"math"
)

// 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 `rect`s.
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 `circle`s.
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) {
	fmt.Println(g)
	fmt.Println(g.area())
	fmt.Println(g.perim())
}

// Sometimes it's useful to know the runtime type of an
// interface value. One option is using a *type assertion*
// as shown here; another is a [type `switch`](switch).
func detectCircle(g geometry) {
	if c, ok := g.(circle); ok {
		fmt.Println("circle with radius", c.radius)
	}
}

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`.
	measure(r)
	measure(c)

	detectCircle(r)
	detectCircle(c)
}

$ go run interfaces.go
{3 4}
12
14
{5}
78.53981633974483
31.41592653589793
circle with radius 5

# To understand how Go's interfaces work under the hood,
# check out this [blog post](https://research.swtch.com/interfaces).