Golang：隐式结构匹配

huangapple
go评论128阅读模式
英文:

Golang: implicit struct matching

问题

考虑以下代码：

  1. type Rectangle struct {
  2.     Width, Height, Area int
  3. }
  5. type Square struct {
  6.     Side, Area int
  7. }
  9. type Geometry struct {
  10.     Area int
  11. }
  13. func SumGeometries(geometries ...Geometry) (sum int) {
  14.     for _, g := range geometries {
  15.         sum += g.Area
  16.     }
  17.     return
  18. }
  20. func TestSumGeometries(t *testing.T) {
  21.     rect := Rectangle{5, 4, 20}
  22.     square := Square{5, 25}
  24.     got := SumGeometries(rect, square) // 无法将rect（Rectangle类型的变量）作为Geometry类型的参数传递给MyFunc，编译器报错
  25.     want := 45
  27.     if got != want {
  28.         t.Error("fail!")
  29.     }
  30. }

我希望MyFunc可以接受包含Apple的任何结构体，而不仅仅是特定的BStruct。在Go语言中是否可以实现这一点？

目前我能找到的唯一方法是：

  1. type Rectangle struct {
  2.     Width, Height, Area int
  3. }
  5. func (r *Rectangle) GetArea() int {
  6.     return r.Area
  7. }
  9. type Square struct {
  10.     Side, Area int
  11. }
  13. func (s *Square) GetArea() int {
  14.     return s.Area
  15. }
  17. type Areaer interface {
  18.     GetArea() int
  19. }
  21. func SumGeometries(geometries ...Areaer) (sum int) {
  22.     for _, s := range geometries {
  23.         sum += s.GetArea()
  24.     }
  25.     return
  26. }
  28. func TestArgs(t *testing.T) {
  29.     rect := Rectangle{5, 4, 20}
  30.     square := Square{5, 25}
  32.     got := SumGeometries(&rect, &square) // 无法将rect（Rectangle类型的变量）作为Geometry类型的参数传递给MyFunc，编译器报错
  33.     want := 45
  35.     if got != want {
  36.         t.Error("fail!")
  37.     }
  38. }

但这种方式可能不太符合惯用写法：当我已经满足于消费者直接访问数据时，是否需要向结构体中添加一个不必要的方法呢？

英文:

Consider this code:

  1. type Rectangle struct {
  2. 	Width, Height, Area int
  3. }
  5. type Square struct {
  6. 	Side, Area int
  7. }
  9. type Geometry struct {
  10. 	Area int
  11. }
  13. func SumGeometries(geometries ...Geometry) (sum int) {
  14. 	for _, g := range geometries {
  15. 		sum += g.Area
  16. 	}
  17. 	return
  18. }
  20. func TestSumGeometries(t *testing.T) {
  21.     rect := Rectangle{5, 4, 20}
  22. 	square := Square{5, 25}
  24.     got := SumGeometries(rect, square)		// cannot use rect (variable of type Rectangle) as Geometry value in argument to MyFunc compilerIncompatibleAssign
  25. 	want := 45
  27. 	if got != want {
  28. 		t.Error("fail!")
  29. 	}
  30. }

I want MyFunc to take whatever struct that contains Apple, not just BStruct in specific.
Is this achievable in Go?

The only way I can find ATM is the following:

  1. type Rectangle struct {
  2. 	Width, Height, Area int
  3. }
  5. func (r *Rectangle) GetArea() int {
  6. 	return r.Area
  7. }
  9. type Square struct {
  10. 	Side, Area int
  11. }
  13. func (s *Square) GetArea() int {
  14. 	return s.Area
  15. }
  17. type Areaer interface {
  18. 	GetArea() int
  19. }
  21. func SumGeometries(geometries ...Areaer) (sum int) {
  22. 	for _, s := range geometries {
  23. 		sum += s.GetArea()
  24. 	}
  25. 	return
  26. }
  28. func TestArgs(t *testing.T) {
  29.     rect := Rectangle{5, 4, 20}
  30. 	square := Square{5, 25}
  32.     got := SumGeometries(&rect, &square)		// cannot use rect (variable of type Rectangle) as Geometry value in argument to MyFunc compilerIncompatibleAssign
  33. 	want := 45
  35. 	if got != want {
  36. 		t.Error("fail!")
  37. 	}
  38. }

It feels perhaps not idiomatic: would I want to pollute my struct with an unnecessary method when I'd be already happy with consumers accessing the data directly?

答案1

得分: 2

将方法添加到类型中并不会“污染”它。

但是有一种方法可以实现你想要的功能而不重复定义。定义一个Area类型，其中包含具有GetArea()方法的公共字段（在这里是Area）：

  1. type Area struct {
  2.     Value int
  3. }
  5. func (a Area) GetArea() int {
  6.     return a.Value
  7. }

然后将其嵌入到其他类型中：

  1. type Rectangle struct {
  2.     Width, Height int
  3.     Area
  4. }
  6. type Square struct {
  7.     Side int
  8.     Area
  9. }

这样GetArea()方法就会被提升，并且Rectangle和Square将自动实现Areaer接口。测试一下：

  1. rect := Rectangle{5, 4, Area{20}}
  2. square := Square{5, Area{25}}
  4. got := SumGeometries(rect, square)
  6. want := 45
  8. if got != want {
  9.     fmt.Println("fail!")
  10. }

没有输出（没有错误）。在Go Playground上试一试。

请注意，如果Area只包含一个字段，甚至可以省略包装结构体，直接使用int作为底层类型：

  1. type Area int
  3. func (a Area) GetArea() int {
  4.     return int(a)
  5. }

然后使用起来更简单：

  1. rect := Rectangle{5, 4, 20}
  2. square := Square{5, 25}

在Go Playground上试一试。

英文:

Adding methods to types it not "polluting".

But there's a way to achieve what you want without repetition. Define an Area type holding the common (here Area) field with GetArea() method:

  1. type Area struct {
  2. 	Value int
  3. }
  5. func (a Area) GetArea() int {
  6. 	return a.Value
  7. }

And embed this in other types:

  1. type Rectangle struct {
  2. 	Width, Height int
  3. 	Area
  4. }
  6. type Square struct {
  7. 	Side int
  8. 	Area
  9. }

This way the GetArea() method gets promoted, and Rectangle and Square will automatically implement Areaer. Testing it:

  1. rect := Rectangle{5, 4, Area{20}}
  2. square := Square{5, Area{25}}
  4. got := SumGeometries(rect, square)
  6. want := 45
  8. if got != want {
  9. 	fmt.Println("fail!")
  10. }

Outputs nothing (no error). Try it on the Go Playground.

Note that if Area contains a single field only, you can even omit the wrapper struct and use int as the underlying type directly:

  1. type Area int
  3. func (a Area) GetArea() int {
  4. 	return int(a)
  5. }

Then using it is simpler:

  1. rect := Rectangle{5, 4, 20}
  2. square := Square{5, 25}

Try this one on the Go Playground.

huangapple
  • 本文由 发表于 2022年9月2日 19:04:47
  • 转载请务必保留本文链接：https://go.coder-hub.com/73581607.html
  • go
  • interface
  • methods
  • struct
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