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 &quot;encoding/json&quot;

type CustomMap map[string]any

type Payload struct {
	Items []CustomMap `json:&quot;items&quot;`
}

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, &amp;aMap)
	mapToBasicMap(this, aMap)
	return nil
}
func main() {
	payload := Payload{}
	json.Unmarshal([]byte(&quot;{\&quot;items\&quot;:[{\&quot;item1\&quot;: 1}]}&quot;), &amp;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, &amp;aMap)
	*this = make(CustomMap)
	mapToBasicMap(*this, aMap)
	return nil
}

A different fix is to delete method CustomMap.UnmarshalJSON

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