英文:
Why must I import both "image/color" and "image"?
问题
我只是学习Go，并编写了以下结构体（Image）来实现image.Image接口。
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)
}
如果我只导入image/color而不导入image，image.Rect会被定义为未定义。为什么？难道image/color不应该已经包含image的方法和属性吗？
另外，如果我将函数接收者从(img Image)更改为(img *Image)，会出现错误：
Image does not implement image.Image (At method requires pointer receiver)
为什么会这样？(img *Image)不是表示指针接收者吗？
英文:
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?
答案1
得分: 12
如果你查看image包的源代码及其子包，你会发现image/color不依赖于image，因此它从未导入过它。
对于你问题的第二部分，当你将所有的接收者都改为指针时，这意味着你应该将一个Image指针传递给ShowImage：
func main() {
m := Image{}
pic.ShowImage(&m)
}
在指针接收者上定义的方法必须通过指针访问。但是在结构体上定义的方法可以通过指针或值访问。
这里有一些解释方法的指针接收者和值接收者之间的区别的文档：
英文:
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:
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