ns3 source code reading (3) Application object

 In ns3, an object can be created according to the TypeId in the object. The cc file of each object has such a macro NS_OBJECT_ENSURE_REGISTERED to register the object. The content of this macro:

#define NS_OBJECT_ENSURE_REGISTERED(type)       \
  static struct X ## type ## RegistrationClass      \
  {                                             \
    X ## type ## RegistrationClass () {             \
      ns3::TypeId tid = type::GetTypeId ();     \
      tid.GetParent ();                         \
    }                                           \
  } x_ ## type ## RegistrationVariable

type::GetTypeId(); Executes the static function GetTypeId()  implemented in each class . Take the Application of PacketSink as an example, its GetTypeId function:

TypeId 
PacketSink::GetTypeId (void)
{
  static TypeId tid = TypeId ("ns3::PacketSink")
    .SetParent<Application> ()
    .AddConstructor<PacketSink> ()
    .AddAttribute ("Local", "The Address on which to Bind the rx socket.",
                   AddressValue (),
                   MakeAddressAccessor (&PacketSink::m_local),
                   MakeAddressChecker ())
    .AddAttribute ("Protocol", "The type id of the protocol to use for the rx socket.",
                   TypeIdValue (UdpSocketFactory::GetTypeId ()),
                   MakeTypeIdAccessor (&PacketSink::m_tid),
                   MakeTypeIdChecker ())
    .AddTraceSource ("Rx", "A packet has been received",
                     MakeTraceSourceAccessor (&PacketSink::m_rxTrace))
  ;
  return tid;
}

 AddConstructor<PacketSink>Register constructors for object types. This associates the TypeID of a class with the instantiation of the class.

TypeId::AddConstructor (void)
{
  struct Maker {
    static ObjectBase * Create () {
      ObjectBase * base = new T ();
      return base;
    }
  };
  Callback<ObjectBase *> cb = MakeCallback (&Maker::Create);
  DoAddConstructor (cb);
  return *this;
}

 So I see that PacketSinkHelper sets the TypeID of ns3::PacketSink to ObjectFactory.

PacketSinkHelper::PacketSinkHelper (std::string protocol, Address address)
{
  m_factory.SetTypeId ("ns3::PacketSink");
  m_factory.Set ("Protocol", StringValue (protocol));//PacketSink中的属性
  m_factory.Set ("Local", AddressValue (address));
}

On the surface, it is seen that Application is instantiated in PacketSinkHelper, but since the TypeID of ns3::PacketSink is registered before, the subclass of PacketSink is finally instantiated.

Ptr<Application> app = m_factory.Create<Application>