Why Can't I Assign to a Struct Field in a Map?

Error: Cannot Assign to Map's Struct Field

When working with maps that store structs, developers often encounter the error "cannot assign to struct field in a map." This issue arises when attempting to modify a subfield within a struct stored in the map.

The provided example demonstrates this scenario. A map snapshots is initialized with a key-value pair where the value is a struct Snapshot containing a slice of Users structs. However, when trying to append to the Users slice, the mentioned error is thrown.

To resolve this issue, it's essential to understand that the original struct stored in the map is not addressable. Instead, it's a copy. Therefore, any modifications will not be reflected in the original struct in the map.

The correct approach is to retrieve the struct from the map, make the necessary modifications, and then reassign it to the map. Here's a revised code that implements this approach:

func main() {
  snapshots := make(map[string]Snapshot, 1)

  snapshots["test"] = Snapshot{
    Key: "testVal",
    Users: make([]Users, 0),
  }

  user := Users{...} // Initialize a new Users struct

  // Retrieve the Snapshot value from the map
  snapshot := snapshots["test"]

  // Append to the Users slice
  snapshot.Users = append(snapshot.Users, user)

  // Reassign the modified Snapshot value to the map
  snapshots["test"] = snapshot
}

By following this approach, the original Snapshot struct in the map will be successfully updated with the appended Users slice.

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