Update to gobook@c6d7a22edd03e7738c38d5baa2174ff325d8d863

ch6/coloredpoint/main.go

@@ -0,0 +1,89 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 161.
+
+// Coloredpoint demonstrates struct embedding.
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+//!+decl
+import "image/color"
+
+type Point struct{ X, Y float64 }
+
+type ColoredPoint struct {
+	Point
+	Color color.RGBA
+}
+
+//!-decl
+
+func (p Point) Distance(q Point) float64 {
+	dX := q.X - p.X
+	dY := q.Y - p.Y
+	return math.Sqrt(dX*dX + dY*dY)
+}
+
+func (p *Point) ScaleBy(factor float64) {
+	p.X *= factor
+	p.Y *= factor
+}
+
+func main() {
+	//!+main
+	red := color.RGBA{255, 0, 0, 255}
+	blue := color.RGBA{0, 0, 255, 255}
+	var p = ColoredPoint{Point{1, 1}, red}
+	var q = ColoredPoint{Point{5, 4}, blue}
+	fmt.Println(p.Distance(q.Point)) // "5"
+	p.ScaleBy(2)
+	q.ScaleBy(2)
+	fmt.Println(p.Distance(q.Point)) // "10"
+	//!-main
+}
+
+/*
+//!+error
+	p.Distance(q) // compile error: cannot use q (ColoredPoint) as Point
+//!-error
+*/
+
+func init() {
+	//!+methodexpr
+	p := Point{1, 2}
+	q := Point{4, 6}
+
+	distance := Point.Distance   // method expression
+	fmt.Println(distance(p, q))  // "5"
+	fmt.Printf("%T\n", distance) // "func(Point, Point) float64"
+
+	scale := (*Point).ScaleBy
+	scale(&p, 2)
+	fmt.Println(p)            // "{2 4}"
+	fmt.Printf("%T\n", scale) // "func(*Point, float64)"
+	//!-methodexpr
+}
+
+func init() {
+	red := color.RGBA{255, 0, 0, 255}
+	blue := color.RGBA{0, 0, 255, 255}
+
+	//!+indirect
+	type ColoredPoint struct {
+		*Point
+		Color color.RGBA
+	}
+
+	p := ColoredPoint{&Point{1, 1}, red}
+	q := ColoredPoint{&Point{5, 4}, blue}
+	fmt.Println(p.Distance(*q.Point)) // "5"
+	q.Point = p.Point                 // p and q now share the same Point
+	p.ScaleBy(2)
+	fmt.Println(*p.Point, *q.Point) // "{2 2} {2 2}"
+	//!-indirect
+}