map [string] interface {}和interface {}之间的区别

Question

I want to parse a JSON file to a map[string]interface{}:

var migrations map[string]interface{}
json.Unmarshal(raw, &migrations)

fmt.Println(migrations["create_user"])

But I modified my code to point data to interface{}:

var migrations interface{}
json.Unmarshal(raw, &migrations)

// compile wrong here
fmt.Println(migrations["create_user"])

I don't understand much about difference between map[string]interface{} and interface{} in above case.

Answer 1

It is because by default you need to type assert interface{} to get the underlying value of map[string]interface{}.

As per GoLang Specification

For an expression x of interface type and a type T, the primary expression

x.(T)

asserts that x is not nil and that the value stored in x is of type T. The notation x.(T) is called a type assertion.

Also Unmarshal function requires pointer to migration of type interface{} or map[string]interface{}

var migrations interface{}
json.Unmarshal(raw, &migrations)

fmt.Println(migrations.(interface{}).(map[string]interface{})["create_user"])

Since migrations is not a map. So you cannot use its key to get the value. Interface{} don't have keys