the-way-to-go_ZH_CN/eBook/examples/chapter_11/interfaces.go

package main

import "fmt"

type Shaper interface {
	Area() float32
	// Perimeter() float32
}

type Square struct {
	side float32
}

func (sq *Square) Area() float32 {
	return sq.side * sq.side
}

func main() {
	sq1 := new(Square)
	sq1.side = 5

	var areaIntf Shaper
	areaIntf = sq1
	// shorter, without separate declaration:
	// areaIntf := Shaper(sq1)
	// or even:
	// areaIntf := sq1
	fmt.Printf("The square has area: %f\n", areaIntf.Area())
}

// The square has area: 25.000000