runtimeType key

runtime_type_key.dart

class Storage<T> {
  final _internal = <Type, T>{};

  void add(T value, [Type? type]) {
    _internal[type ?? T] = value;
  }

  T get<T>() {
    return _internal[T] as T;
  }
}

class Boxed<T> {}

class Container {
  final storage = Storage<Boxed>();

  add(Boxed value) {
    storage.add(value, value.runtimeType);
    //                 ^^^^^^^^^^^^^^^^^^
    //                 can't use generic
  }

  Boxed<T> get<T>() {
    return storage.get<Boxed<T>>();
  }
}

final container = Container();

void main() {
  final intValue = Boxed<int>();
  final stringValue = Boxed<String>();
  final doubleValue = Boxed<double>();

  final items = [intValue, stringValue, doubleValue];

  items.forEach((element) {
    container.add(element);
  });

  assert(container.get<int>() == intValue);
  assert(container.get<String>() == stringValue);
  assert(container.get<double>() == doubleValue);
}