Go cmp – 如何为以结构体为键的映射定义自定义相等性？

huangapple
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英文:

Go cmp - how to define custom equality for a map with a struct as its keys?

问题

给定一个以结构体作为键的地图，其中结构体的值是指向另一个结构体的指针：

  1. type Dog struct {
  2. 	Name string
  3. }
  5. type Cat struct {
  6. 	Name string
  7. }
  9. type MapKey struct {
  10. 	dog *Dog
  11. 	cat *Cat
  12. }
  14. myMap := make(map[MapKey]int)

我该如何使用cmp包使下面的地图相等，其中它们被认为是相等的，因为MapKey具有相同的值（使用reflect.DeepEquals或cmp.Equals）？

  1. keyOne := MapKey{
  2. 	&Dog{Name: "bob"},
  3. 	&Cat{Name: "clive"},
  4. }
  5. keyTwo := MapKey{
  6. 	&Dog{Name: "bob"},
  7. 	&Cat{Name: "clive"},
  8. }
  10. got := map[MapKey]int{
  11. 	keyOne: 1,
  12. }
  13. want := map[MapKey]int{
  14. 	keyTwo: 1,
  15. }

在cmp文档中，它说我可以使用cmpopts.SortMaps（https://pkg.go.dev/github.com/google/go-cmp/cmp#Equal），但我不明白这与我的情况有何关系。

我尝试在MapKey结构上定义一个自定义的Equals函数，但它从未被调用。

可以使用Go Playground重现此问题：
https://go.dev/play/p/qMxaya3S26M

英文:

Given a map that uses a struct as its key, where the values of the struct are pointers to another struct:

  1. type Dog struct {
  2. 	Name string
  3. }
  5. type Cat struct {
  6. 	Name string
  7. }
  9. type MapKey struct {
  10. 	dog *Dog
  11. 	cat *Cat
  12. }
  14. myMap := make(map[MapKey]int)

How would I use the cmp package to make the below maps equal, where they are considered equal because the MapKey has the same values (reflect.DeepEquals or cmp.Equals)?

  1. keyOne := MapKey{
  2. 	&Dog{Name: "bob"},
  3. 	&Cat{Name: "clive"},
  4. }
  5. keyTwo := MapKey{
  6. 	&Dog{Name: "bob"},
  7. 	&Cat{Name: "clive"},
  8. }
  10. got := map[MapKey]int{
  11. 	keyOne: 1,
  12. }
  13. want := map[MapKey]int{
  14. 	keyTwo: 1,
  15. }

In the cmp documentation, it says I could use cmpopts.SortMaps (https://pkg.go.dev/github.com/google/go-cmp/cmp#Equal), however I don't see how this is relevant to my scenario.

I've tried defining a custom Equals function on the MapKey struct but it never gets called.

Go playground to reproduce this:
https://go.dev/play/p/qMxaya3S26M

答案1

得分: 1

cmp.Equal是使用map[MapKey]int类型的参数调用的，而不是MapKey类型。

因此，自定义的Equal函数必须在map[MapKey]int类型上进行定义。

但是，为了定义这个函数，我们需要从map[MapKey]int定义一个新类型。

这是一个完整工作示例的Playground链接：https://go.dev/play/p/deteHANWQ_3

  1. type MapKeyInt map[MapKey]int
  3. func (m MapKeyInt) Equal(other MapKeyInt) bool {
  4. 	if len(m) != len(other) {
  5. 		return false
  6. 	}
  8. 	keys, keysOther := make([]MapKey, 0), make([]MapKey, 0)
  9. 	values, valuesOther := make([]int, 0), make([]int, 0)
  11. 	for k, v := range m {
  12. 		keys = append(keys, k)
  13. 		values = append(values, v)
  14. 	}
  15. 	for k, v := range other {
  16. 		keysOther = append(keysOther, k)
  17. 		valuesOther = append(valuesOther, v)
  18. 	}
  20. 	for i := 0; i < len(m); i++ {
  21. 		if (keys[i].dog.Name != keysOther[i].dog.Name) || (keys[i].cat.Name != keysOther[i].cat.Name) {
  22. 			return false
  23. 		}
  24. 		if values[i] != valuesOther[i] {
  25. 			return false
  26. 		}
  27. 	}
  29. 	return true
  30. }
英文:

The cmp.Equal is called with parameters of type map[MapKey]int, not MapKey

So the custom Equal function has to be defined on the type map[MapKey]int.

But to define this function, we need to define a new type from map[MapKey]int.

Here's a playground to the full working example: https://go.dev/play/p/deteHANWQ_3

  1. type MapKeyInt map[MapKey]int
  3. func (m MapKeyInt) Equal(other MapKeyInt) bool {
  4. 	if len(m) != len(other) {
  5. 		return false
  6. 	}
  8. 	keys, keysOther := make([]MapKey, 0), make([]MapKey, 0)
  9. 	values, valuesOther := make([]int, 0), make([]int, 0)
  11. 	for k, v := range m {
  12. 		keys = append(keys, k)
  13. 		values = append(values, v)
  14. 	}
  15. 	for k, v := range other {
  16. 		keysOther = append(keysOther, k)
  17. 		valuesOther = append(valuesOther, v)
  18. 	}
  20. 	for i := 0; i &lt; len(m); i++ {
  21. 		if (keys[i].dog.Name != keysOther[i].dog.Name) || (keys[i].cat.Name != keysOther[i].cat.Name) {
  22. 			return false
  23. 		}
  24. 		if values[i] != valuesOther[i] {
  25. 			return false
  26. 		}
  27. 	}
  29. 	return true
  30. }

huangapple
  • 本文由 发表于 2022年11月7日 21:29:02
  • 转载请务必保留本文链接：https://go.coder-hub.com/74347375.html
  • go
  • go-cmp
  • unit-testing
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