可以使用typedef来进行类型断言吗？

huangapple
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英文:

Can I make a type assertion work with a typedef?

问题

我有一个自定义类型，可以更方便地使用 API：

  1. type Object map[string]interface{}

但是这个东西不起作用：

  1. var mockResponse = map[string]interface{}{"success": true}
  2. resp, ok := mockResponse.(Object)
  3. // ok = false

我能做些什么让 mockResponse.(Object) 的 ok 变为 true 吗？它们基本上是相同的类型...

英文:

I have a custom type that allows me to work with an API more conveniently:

  1. type Object map[string]interface{}

And this thing doesn't work:

  1. var mockResponse = map[string]interface{}{"success": true}
  2. resp, ok := mockResponse.(Object)
  3. // ok = false

Can I do anything so mockResponse.(Object)'s ok is true? It's basically the same type...

答案1

得分: 7

首先，你的“伪代码”是一个编译时错误。你不能在具体类型的值上使用类型断言（没有意义，具体类型就是具体类型，没有其他），你只能在接口值上使用它。

示例应该是：

  1. var mockResponse interface{} = map[string]interface{}{"success": true}
  2. resp, ok := mockResponse.(Object)

请注意，如果mockResponse的类型是map[string]interface{}，那么就不需要类型断言，你可以简单地将其转换为Object，如下所示：

  1. var mockResponse = map[string]interface{}{"success": true}
  2. obj := Object(mockResponse)

类型定义会创建一个新的、独立的类型。只有当你使用相同的具体类型时，才能进行到具体类型的类型断言。

你可以编写一个辅助函数，从接口值中提取Object，它可以处理map[string]interface{}，以及具体类型为Object的情况：

  1. func getObj(x interface{}) (o Object, ok bool) {
  2.     switch v := x.(type) {
  3.     case Object:
  4.         return v, true
  5.     case map[string]interface{}:
  6.         return Object(v), true
  7.     }
  8.     return nil, false
  9. }

测试一下：

  1. resp, ok := getObj(map[string]interface{}{"success": true})
  2. fmt.Println(resp, ok)
  4. resp, ok = getObj(Object{"success": true})
  5. fmt.Println(resp, ok)
  7. resp, ok = getObj("invalid")
  8. fmt.Println(resp, ok)

输出结果（在Go Playground上尝试）：

  1. map[success:true] true
  2. map[success:true] true
  3. map[] false

注意：上述的getObj()并没有处理所有可能的情况，其中一个Object可能从接口值中“获取”的情况。

例如，如果我们有这样的类型定义：

  1. type Object2 Object

将一个Object2的值传递给getObj()，将无法从中提取出Object，即使Object2的值可以转换为Object：

  1. resp, ok = getObj(Object2{"success": true})
  2. fmt.Println(resp, ok) // map[] false

如果你想处理所有可能的情况，其中具体值可以转换为Object，你可以使用反射：

  1. var objType = reflect.TypeOf(Object{})
  3. func getObj(x interface{}) (o Object, ok bool) {
  4.     if v := reflect.ValueOf(x); v.Type().ConvertibleTo(objType) {
  5.         o, ok = v.Convert(objType).Interface().(Object)
  6.         return
  7.     }
  8.     return nil, false
  9. }

测试一下（在Go Playground上尝试）：

  1. resp, ok = getObj(Object2{"success": true})
  2. fmt.Println(resp, ok) // map[success:true] true

使用反射比简单的类型断言或类型切换要慢，所以你可以混合使用这两种解决方案：

  1. func getObj(x interface{}) (o Object, ok bool) {
  2.     // 首先尝试简单的情况：
  3.     switch v := x.(type) {
  4.     case Object:
  5.         return v, true
  6.     case map[string]interface{}:
  7.         return Object(v), true
  8.     }
  10.     // 然后退回到反射：
  11.     if v := reflect.ValueOf(x); v.Type().ConvertibleTo(objType) {
  12.         o, ok = v.Convert(objType).Interface().(Object)
  13.         return
  14.     }
  15.     return nil, false
  16. }
英文:

First of all, your "pseudo code" is a compile time error. You can't use type assertion on a value with concrete type (there's no point, a concrete type is a concrete type and nothing else), you may only use it on an interface value.

The example should be:

  1. var mockResponse interface{} = map[string]interface{}{"success": true}
  2. resp, ok := mockResponse.(Object)

Note that if mockResponse would be of type map[string]interface{}, then no type assertion would be needed, you could simply convert it to Object like this:

  1. var mockResponse = map[string]interface{}{"success": true}
  2. obj := Object(mockResponse)

The type definition creates a new, distinct type. And the type assertion to a concrete type only holds if you use the same concrete type.

What you may do is write a helper function which extracts Object from an interface value, which may handle both map[string]interface{} and also if the concrete type is Object:

  1. func getObj(x interface{}) (o Object, ok bool) {
  2. 	switch v := x.(type) {
  3. 	case Object:
  4. 		return v, true
  5. 	case map[string]interface{}:
  6. 		return Object(v), true
  7. 	}
  8. 	return nil, false
  9. }

Testing it:

  1. resp, ok := getObj(map[string]interface{}{"success": true})
  2. fmt.Println(resp, ok)
  4. resp, ok = getObj(Object{"success": true})
  5. fmt.Println(resp, ok)
  7. resp, ok = getObj("invalid")
  8. fmt.Println(resp, ok)

Output (try it on the Go Playground):

  1. map[success:true] true
  2. map[success:true] true
  3. map[] false

Note: the above getObj() does not handle all possible cases where an Object could be "acquired" from the interface value.

For example if we'd have a type definition like this:

  1. type Object2 Object

Passing a value of Object2 to getObj(), an Object would not be extracted from it, even though a value of type Object2 could be converted to Object:

  1. resp, ok = getObj(Object2{"success": true})
  2. fmt.Println(resp, ok) // map[] false

If you want to handle all possible cases where the concrete value is convertible to Object, you may use reflection:

  1. var objType = reflect.TypeOf(Object{})
  3. func getObj(x interface{}) (o Object, ok bool) {
  4. 	if v := reflect.ValueOf(x); v.Type().ConvertibleTo(objType) {
  5. 		o, ok = v.Convert(objType).Interface().(Object)
  6. 		return
  7. 	}
  8. 	return nil, false
  9. }

Testing it (try this one on the Go Playground):

  1. resp, ok = getObj(Object2{"success": true})
  2. fmt.Println(resp, ok) // map[success:true] true

Using reflection is slower than a simple type assertion or type switch, so you can mix the 2 solutions:

  1. func getObj(x interface{}) (o Object, ok bool) {
  2. 	// First try the trivial cases:
  3. 	switch v := x.(type) {
  4. 	case Object:
  5. 		return v, true
  6. 	case map[string]interface{}:
  7. 		return Object(v), true
  8. 	}
  10. 	// Then revert to reflection:
  11. 	if v := reflect.ValueOf(x); v.Type().ConvertibleTo(objType) {
  12. 		o, ok = v.Convert(objType).Interface().(Object)
  13. 		return
  14. 	}
  15. 	return nil, false
  16. }

huangapple
  • 本文由 发表于 2021年7月27日 19:09:24
  • 转载请务必保留本文链接：https://go.coder-hub.com/68543788.html
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