Golang反射无法识别来自映射成员的标签。

huangapple
go评论124阅读模式
英文:

golang reflect cannot recognize tag from map members

问题

我想使用反射(reflect)提取结构体的映射成员的标签，但是我发现如果从MapIndex中检索成员的值，它的类型将被识别为"*interface{}"，因此所有类型信息都丢失了，更不用说反射可以提取详细信息了。

在运行后，我得到了以下错误：

  1. Starting: C:\Users\Mento\go\bin\dlv.exe dap --check-go-version=false --listen=127.0.0.1:50308 from d:\Coding\Golang\demo
  2. DAP server listening at: 127.0.0.1:50308
  3. Key: Sen, Value: {Sen}, value pointer 0xc000044230
  4. Traversal tag with obj: type *main.Student, value &{Sen}
  5. Tag name Sname, value MyTag:"student-name"
  6. Key: Sen, Value: {Sen}, value pointer 0xc0000442c0
  7. Traversal tag with obj: type *interface {}, value 0xc0000442c0
  8. panic: reflect: NumField of non-struct type interface {}
  10. goroutine 1 [running]:
  11. reflect.(*rtype).NumField(0xec2d20)
  12.     C:/Program Files/Go/src/reflect/type.go:1015 +0xc8
  13. main.traversalTag({0xebd9e0, 0xc0000442c0})
  14.     d:/Coding/Golang/demo/demo.go:31 +0x1cb
  15. main.tryMapWithReflect({0xec3d40, 0xc00007a480})
  16.     d:/Coding/Golang/demo/demo.go:48 +0x2c9
  17. main.main()
  18.     d:/Coding/Golang/demo/demo.go:54 +0x38
  19. Process 8716 has exited with status 2
  20. dlv dap (11448) exited with code: 0

有人可以提示如何获取具有原始类型信息的映射成员的指针吗？

谢谢！
Mento

英文:

I want to extract struct's map members' tag with reflect, while I found if retrieve member's value from MapIndex, the type of it will be recognized as "*interface{}" and hence all type information are lost, no mention reflect can extract detail information.

  1. package main
  3. import (
  4. 	"fmt"
  5. 	"reflect"
  6. )
  8. type Student struct {
  9. 	Sname string `MyTag:"student-name"`
  10. }
  12. type Teacher struct {
  13. 	Name     string             `MyTag:"teacher-name"`
  14. 	Students map[string]Student `MyTag:"teacher-students"`
  15. }
  17. var sam = Teacher{
  18. 	Name: "Sam",
  19. 	Students: map[string]Student{
  20. 		"Sen": {
  21. 			Sname: "Sen",
  22. 		},
  23. 	},
  24. }
  26. func traversalTag(obj interface{}) {
  27. 	theType := reflect.TypeOf(obj)
  28. 	fmt.Printf("Traversal tag with obj: type %v, value %v\n", theType.String(), obj)
  30. 	elem := reflect.TypeOf(obj).Elem()
  31. 	for i := 0; i < elem.NumField(); i++ {
  32. 		fmt.Printf("Tag name %s, value %s\n", elem.Field(i).Name, elem.Field(i).Tag)
  33. 	}
  34. }
  36. func tryMapWithType(students map[string]Student) {
  37. 	for key, theValue := range students {
  38. 		fmt.Printf("Key: %v, Value: %v, value pointer %p\n", key, theValue, &theValue)
  39. 		traversalTag(&theValue)
  40. 	}
  41. }
  43. func tryMapWithReflect(obj interface{}) {
  44. 	reflectMap := reflect.ValueOf(obj)
  45. 	for _, key := range reflectMap.MapKeys() {
  46. 		theValue := reflectMap.MapIndex(key).Interface()
  47. 		fmt.Printf("Key: %v, Value: %v, value pointer %p\n", key, theValue, &theValue)
  48. 		traversalTag(&theValue) // Will have error
  49. 	}
  50. }
  52. func main() {
  53. 	tryMapWithType(sam.Students)
  54. 	tryMapWithReflect(sam.Students)
  55. }

After run I got following error:

  1. Starting: C:\Users\Mento\go\bin\dlv.exe dap --check-go-version=false --listen=127.0.0.1:50308 from d:\Coding\Golang\demo
  2. DAP server listening at: 127.0.0.1:50308
  3. Key: Sen, Value: {Sen}, value pointer 0xc000044230
  4. Traversal tag with obj: type *main.Student, value &{Sen}
  5. Tag name Sname, value MyTag:"student-name"
  6. Key: Sen, Value: {Sen}, value pointer 0xc0000442c0
  7. Traversal tag with obj: type *interface {}, value 0xc0000442c0
  8. panic: reflect: NumField of non-struct type interface {}
  10. goroutine 1 [running]:
  11. reflect.(*rtype).NumField(0xec2d20)
  12. 	C:/Program Files/Go/src/reflect/type.go:1015 +0xc8
  13. main.traversalTag({0xebd9e0, 0xc0000442c0})
  14. 	d:/Coding/Golang/demo/demo.go:31 +0x1cb
  15. main.tryMapWithReflect({0xec3d40, 0xc00007a480})
  16. 	d:/Coding/Golang/demo/demo.go:48 +0x2c9
  17. main.main()
  18. 	d:/Coding/Golang/demo/demo.go:54 +0x38
  19. Process 8716 has exited with status 2
  20. dlv dap (11448) exited with code: 0

Can anyone hint how to get pointer of map members with original type information?

Thanks you,
Mento

答案1

得分: 1

如你所知，使用&theValue会解析为类型*interface{}。类型*interface{}与类型*Student是不同的，而你从tryMapWithType传递给traversalTag的是*Student类型。

如果你想从tryMapWithReflect将*Student传递给traversalTag，你需要使用反射创建该指针值。普通的原生Go地址操作符&是不够的。

当你有一个可寻址的reflect.Value时，你只需要调用.Addr()方法就可以获得指向可寻址值的指针，然而映射元素是不可寻址的，因此reflectMap.MapIndex(key)是不可寻址的。所以，对你来说，不可能通过reflectMap.MapIndex(key).Addr().Interface()来获取*Student。

因此，你唯一的选择是使用反射来创建*Student类型的新值，将指向的值设置为映射中的值，然后返回该值的.Interface()。

另外，你可以从traversalTag中省略.Elem()，这样你就不必传递指针给它。

英文:

As you know using &theValue resolves to the type *interface{}. The type *interface{} is distinct from the type *Student which is what you are passing in to traversalTag from tryMapWithType.

If you want to pass *Student to traversalTag from tryMapWithReflect you need to create that pointer value using reflection. Plain native Go address operator & just isn't enough.

When you have a reflect.Value that is addressable all you need to do is to call the .Addr() method to get a pointer to the addressable value, however map elements are not addressable and therefore reflectMap.MapIndex(key) is not addressable. So, unfortunately for you, it's not possible to do reflectMap.MapIndex(key).Addr().Interface() to get *Student.

So your only option is to use reflection to create a new value of the *Student type, set the pointed-to value to the value in the map, and then return the .Interface() of that.

  1. func tryMapWithReflect(obj interface{}) {
  2. 	reflectMap := reflect.ValueOf(obj)
  3. 	for _, key := range reflectMap.MapKeys() {
  4. 		theValue := reflectMap.MapIndex(key).Interface()
  6. 		// allocate a new value of type *Student
  7. 		newValue := reflect.New(reflectMap.MapIndex(key).Type())
  9. 		// use Elem do dereference *Stunded
  10. 		// and then use Set to set the Student to the content of theValue
  11. 		newValue.Elem().Set(reflect.ValueOf(theValue))
  13. 		fmt.Printf("Key: %v, Value: %v, value pointer %p\n", key, newValue.Elem().Interface(), newValue.Interface())
  15. 		// return the newValue *Student
  16. 		traversalTag(newValue.Interface())
  17. 	}
  18. }

https://go.dev/play/p/pNL2wjsOW5y

Alternatively, just drop the .Elem() from the traversalTag and then you don't have to pass pointers to it.

  1. func traversalTag(obj interface{}) {
  2. 	theType := reflect.TypeOf(obj)
  3. 	fmt.Printf("Traversal tag with obj: type %v, value %v\n", theType.String(), obj)
  5. 	elem := reflect.TypeOf(obj)
  6. 	for i := 0; i < elem.NumField(); i++ {
  7. 		fmt.Printf("Tag name %s, value %s\n", elem.Field(i).Name, elem.Field(i).Tag)
  8. 	}
  9. }

https://go.dev/play/p/EwJ5e0uc2pd

huangapple
  • 本文由 发表于 2022年3月16日 21:22:46
  • 转载请务必保留本文链接：https://go.coder-hub.com/71497849.html
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