英文:
JSON unmarshalling of array of objects crashes with nil map
问题
我有以下的最小复现代码，当这段代码运行时，在basicMap[key] = value处会抛出一个异常，异常信息是assignment to entry in nil map，然而，只有当类型是[]CustomMap时才会出现这个问题——当我使用CustomMap作为单个项时，它运行得很好。
在Go语言中，有没有一种方法可以解组自定义对象的数组？
package main
import "encoding/json"
type CustomMap map[string]interface{}
type Payload struct {
Items []CustomMap `json:"items"`
}
func mapToBasicMap(basicMap CustomMap, aMap map[string]interface{}) {
for key, value := range aMap {
basicMap[key] = value
}
}
func (this CustomMap) UnmarshalJSON(data []byte) error {
var aMap map[string]interface{} = make(map[string]interface{})
json.Unmarshal(data, &aMap)
mapToBasicMap(this, aMap)
return nil
}
func main() {
payload := Payload{}
json.Unmarshal([]byte(`{"items":[{"item1": 1}]}`), &payload)
}
英文:
I have the following minimal reproduction, when this code runs it throws an exception at basicMap[key] = value because assignment to entry in nil map, however it only does this when the type is []CustomMap - when I use a CustomMap for a single item, it works just fine.
Is there a way of unmarshalling arrays of custom objects in go?
package main
import "encoding/json"
type CustomMap map[string]any
type Payload struct {
Items []CustomMap `json:"items"`
}
func mapToBasicMap(basicMap CustomMap, aMap map[string]any) {
for key, value := range aMap {
basicMap[key] = value
}
}
func (this CustomMap) UnmarshalJSON(data []byte) error {
var aMap map[string]any = make(map[string]any)
json.Unmarshal(data, &aMap)
mapToBasicMap(this, aMap)
return nil
}
func main() {
payload := Payload{}
json.Unmarshal([]byte("{\"items\":[{\"item1\": 1}]}"), &payload)
}
</details>
# 答案1
**得分**: 2
Unmarshal应该具有指针接收器。在使用其值之前，先创建目标映射。
```go
func (this *CustomMap) UnmarshalJSON(data []byte) error {
var aMap map[string]any = make(map[string]any)
json.Unmarshal(data, &aMap)
*this = make(CustomMap)
mapToBasicMap(*this, aMap)
return nil
}
另一种修复方法是删除CustomMap.UnmarshalJSON方法。
英文:
Unmarshal should have a pointer receiver. Make the target map before using assigning to its values.
func (this *CustomMap) UnmarshalJSON(data []byte) error {
var aMap map[string]any = make(map[string]any)
json.Unmarshal(data, &aMap)
*this = make(CustomMap)
mapToBasicMap(*this, aMap)
return nil
}
A different fix is to delete method CustomMap.UnmarshalJSON
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