dynamodbattribute.UnmarshalMap将我的变量类型更改为map [string] interface {}

Question

Background

I am trying to Unmarshal an item returned by dynamodb.GetItem into an object, which type I do not know at that place. For this I have a function emptyItemConstructor which returns a new object t of the desired type.

Question

I have a function like this:

func GetItem(emptyItemConstructor func() interface{}) interface{} {
  myItem := emptyItemConstructor()
  fmt.Printf("Type is: %T
", myItem)
  _ = dynamodbattribute.UnmarshalMap(item, &myItem)
  fmt.Printf("Type now is: %T
", myItem)
}

and I am passing this function for emptyItemConstructor:

func constructor() MyDynamoDBItemType {
    return MyDynamoDBItemType{}
}

The output of the function is:

Type is: MyDynamoDBItemType
Type now is: map[string]interface

Why is UnmarshalMap changing the type of myItem?

Answer 1

The type of variable myItem is interface{}, and the type of UnmarshalMap input param is interface{} too. There is no comparison of underlying types when assigning values

A example is as follows:

package main

import "fmt"

func Item() interface{} {
    return struct {
        Name string
    }{Name: "poloxue"}
}

func ItemMap(item *interface{}) {
    *item = map[string]interface{}{
        "Name": "poloxue",
    }
}

func main() {
    m := Item()
    fmt.Printf("%T
", m)
    ItemMap(&m)
    fmt.Printf("%T
", m)
}

If you want to unmarshal map to struct , try mapstructure package?