  const Value &Get(const std::string &key) const {
    static Value null_value;
    assert(IsObject());
    Object::const_iterator it = object_value_.find(key);
    return (it != object_value_.end()) ? it->second : null_value;
  }
  const Value &Get(const std::string &key) const {
    static Value null_value;
    assert(IsObject());
    Object::const_iterator it = object_value_.find(key);
    return (it != object_value_.end()) ? it->second : null_value;
  }