Question

I am just learning Go and wrote the following struct (Image) to implement the image.Image interface.

package main

import (
    "image"
    "image/color"
    "code.google.com/p/go-tour/pic"
)

type Image struct{}

func (img Image) ColorModel() color.Model {
    return color.RGBAModel
}

func (img Image) Bounds() image.Rectangle {
    return image.Rect(0, 0, 100, 100)
}

func (img Image) At(x, y int) color.Color {
    return color.RGBA{100, 100, 255, 255}   
}

func main() {
    m := Image{}
    pic.ShowImage(m)
}

If I just import image/color and not import image, image.Rect is undefined. Why? Shouldn’t image/color already cover the methods and properties of image?

Also, if I change the function receivers from (img Image) to (img *Image), an error arises:

Image does not implement image.Image (At method requires pointer receiver)

Why is that? Doesn’t (img *Image) indicate a pointer receiver?

Answer 1

If you check out the source for the image package and its sub-packages, you will see that image/color does not depend on image at all, so it never imports it.

image does however import image/color

For the second part of your question, where you change all of the receivers to pointers, that means you should also be passing an Image pointer to ShowImage:

func main() {
    m := Image{}
    pic.ShowImage(&m)
}

Methods defined on a pointer receiver must be accessed on a pointer. But methods defined on just the struct can be accessed from a pointer or the value.

Here is some documentation explaining the difference between a pointer or a value receiver of a method:

1. Should I define methods on values or pointers?
2. Why do T and *T have different method sets?