[英]Go: type assertion for maps
I'm reading data structures from JSON. 我正在从JSON读取数据结构。 There's a little bit of conversions going on and at the end I have a
struct
where one of the fields is of type interface{}
. 有一些转换正在进行,最后我有一个
struct
,其中一个字段是interface{}
类型。 It's actually a map, so JSON puts it inside a map[string]inteface{}
. 它实际上是一个地图,所以JSON将它放在一个
map[string]inteface{}
。
I actually know that the underlying structure is map[string]float64
and I would like to use it like that, so I try to do an assertion. 我实际上知道底层结构是
map[string]float64
,我想这样使用它,所以我尝试做一个断言。 The following code reproduces the behaviour: 以下代码重现了该行为:
type T interface{}
func jsonMap() T {
result := map[string]interface{}{
"test": 1.2,
}
return T(result)
}
func main() {
res := jsonMap()
myMap := res.(map[string]float64)
fmt.Println(myMap)
}
I get the error: 我收到错误:
panic: interface conversion: main.T is map[string]interface {}, not map[string]float64
I can do the following: 我可以做以下事情:
func main() {
// A first assertion
res := jsonMap().(map[string]interface{})
myMap := map[string]float64{
"test": res["test"].(float64), // A second assertion
}
fmt.Println(myMap)
}
This works fine, but I find it very ugly since I need to reconstruct the whole map and use two assertions. 这工作正常,但我发现它非常难看,因为我需要重建整个地图并使用两个断言。 Is there a correct way to force the first assertion to drop the
interface{}
and use float64
? 是否有正确的方法强制第一个断言删除
interface{}
并使用float64
? In other words, what is the correct way to do the original assertion .(map[string]float64)
? 换句话说,执行原始断言的正确方法是什么
.(map[string]float64)
?
Edit: 编辑:
The actual data I'm parsing looks like this: 我正在解析的实际数据如下所示:
[
{"Type":"pos",
"Content":{"x":0.5 , y: 0.3}} ,
{"Type":"vel",
"Content":{"vx": 0.1, "vy": -0.2}}
]
In Go I use a struct
and encoding/json
in the following way. 在Go中,我使用
struct
和encoding/json
以下列方式。
type data struct {
Type string
Content interface{}
}
// I read the JSON from a WebSocket connection
_, event, _ := c.ws.ReadMessage()
j := make([]data,0)
json.Unmarshal(event, &j)
You cannot type assert map[string]interface{}
to map[string]float64
. 您不能键入assert
map[string]interface{}
来map[string]float64
。 You need to manually create new map. 您需要手动创建新地图。
package main
import (
"encoding/json"
"fmt"
)
var exampleResponseData = `{
"Data":[
{
"Type":"pos",
"Content":{
"x":0.5,
"y":0.3
}
},
{
"Type":"vel",
"Content":{
"vx":0.1,
"vy":-0.2
}
}
]
}`
type response struct {
Data []struct {
Type string
Content interface{}
}
}
func main() {
var response response
err := json.Unmarshal([]byte(exampleResponseData), &response)
if err != nil {
fmt.Println("Cannot process not valid json")
}
for i := 0; i < len(response.Data); i++ {
response.Data[i].Content = convertMap(response.Data[i].Content)
}
}
func convertMap(originalMap interface{}) map[string]float64 {
convertedMap := map[string]float64{}
for key, value := range originalMap.(map[string]interface{}) {
convertedMap[key] = value.(float64)
}
return convertedMap
}
Are you sure you cannot define Content
as map[string]float64
? 您确定无法将
Content
定义为map[string]float64
吗? See example below. 见下面的例子。 If not, how can you know that you can cast it in the first place?
如果没有,你怎么知道你可以把它放在首位?
type response struct {
Data []struct {
Type string
Content map[string]float64
}
}
var response response
err := json.Unmarshal([]byte(exampleResponseData), &response)
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