This "create a default-constructed value just so you can return a reference" logic is pretty terrible tbh. For insert: first create a default-constructed value, then assign to it. For retrieval: in the not found case, (permanently) insert a default-constructed value into the map. Need to return a valid reference!

Qt containers do it better: upsert with insert() and retrieve with value(), which, in the not found case, will return a default-constructed value (or a caller-supplied value) but without inserting it into the map.