listing31.go

// Code listing 31: https://play.golang.org/p/313UebA3rD

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

package main

import "fmt"
import "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())
}

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)
}