Doubts about nested structures and instances check

I have a class with nested structures that receive data from two different devices.

class Command
{
  public:

    struct A
    {
      uint8_t A_command;
      std::vector<std::string> A_fields;
    };

    struct B
    {
      uint16_t B_command;
      uint16_t B_command_alt;
      std::vector<std::string> B_fields;
    };

    A a_cmd;
    B b_cmd;
};

And I would have a loop that receives commands from the devices and process them in this way

...
  Command command = {0};
  command.a_cmd = get_command_from_device_A();

But when trying to check if some of the structures is different from zero the compiler sends this message

  if (command.a_cmd != 0)
  {
    std::cout << "Command A was received" << std::endl;
  }

error C2451: conditional expression of type 'Command::A' is illegal Note: No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

Is it possible to do a comparison without having to add an additional boolean?

Thanks in advance