JSON unmarshalling of array of objects crashes with nil map-CodePudding

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)
}

CodePudding user response：

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