ns3中可以根据对象中的TypeId进行一个对象的创建。每一个对象的cc文件都有这样一个宏NS_OBJECT_ENSURE_REGISTERED，对对象进行注册。这个宏的内容：

#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 (); 执行每个类中实现的静态函数GetTypeId ()。以PacketSink这个Application为例，其GetTypeId函数：

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>为对象类型注册构造器。这样就将一个类的TypeID与类的实例化关联到一起。

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

 所以我看到PacketSinkHelper向ObjectFactory设置的是ns3::PacketSink的TypeID。

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));
}

表面上看到PacketSinkHelper中实例化的是Application，但是由于之前注册了ns3::PacketSink的TypeID，因此最终实例化的是PacketSink这个子类。

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