core,internet,lte: Prevent calling ConstructSelf in constructors of Object-derived classes

doc/manual/source/attributes.rst

@@ -788,12 +788,20 @@ consistently to allow correct operation. To this end we do allow for consistency
 checking *when the attribute is used* (*cf*. ``NS_ASSERT_MSG``
 or ``NS_ABORT_MSG``).
 
-In general, the attribute code to assign values to the underlying class member
-variables is executed after an object is constructed. But what if you need the
-values assigned before the constructor body executes, because you need them in
-the logic of the constructor? There is a way to do this, used for example in the
-class :cpp:class:`ConfigStore`: call :cpp:func:`ObjectBase::ConstructSelf()` as
-follows::
+For classes deriving from the :cpp:class:`Object` class, the attribute code to assign initial
+values to the underlying class member variables is executed after an object is constructed.
+Therefore, you cannot access the values assigned to the attributes of an object from within
+its constructor. Instead, you can override :cpp:func:`ObjectBase::NotifyConstructionCompleted()`
+and access the values assigned to the attributes of the object from within that function.
+A practical example of how this is used can be found in the ns-3 class :cpp:class:`RttEstimator`
+in the `internet` module.
+
+For classes deriving directly from :cpp:class:`ObjectBase`, it is instead needed to explicitly
+call :cpp:func:`ObjectBase::ConstructSelf()` to assign initial values to the class attributes.
+Such a call can be made in the class constructor, in which case the values assigned to the
+attributes are accessible from within its constructor. There are not many examples of this usage
+in the ns-3 codebase because most classes with attributes typically derive from :cpp:class:`Object`.
+One example is the :cpp:class:`ConfigStore` class, whose constructor is shown as follows::
 
     ConfigStore::ConfigStore()
     {

src/core/model/object-base.cc

@@ -83,6 +83,19 @@ ObjectBase::ConstructSelf(const AttributeConstructionList& attributes)
     // loop over the inheritance tree back to the Object base class.
     NS_LOG_FUNCTION(this << &attributes);
     TypeId tid = GetInstanceTypeId();
+    TypeId objectTid;
+    // the TypeId of a class derived from Object is initialized to "ns3::Object"; for a class
+    // deriving from Object, check that this function is called after that the correct TypeId is set
+    // to ensure that the attributes of the class are initialized
+    NS_ABORT_MSG_IF(TypeId::LookupByNameFailSafe("ns3::Object", &objectTid) && tid == objectTid,
+                    "ObjectBase::ConstructSelf() has been called on an object of a class derived "
+                    "from the Object class, but the TypeId is still set to ns3::Object.\n"
+                    "This is known to happen in two cases:\n"
+                    "- ObjectBase::ConstructSelf() is called in the class constructor; in this "
+                    "case, override ObjectBase::NotifyConstructionCompleted() to access the "
+                    "initial values of the object attributes as soon as object construction is "
+                    "completed (see issue #1249)\n"
+                    "- the class deriving from Object does not define a static GetTypeId() method");
     do // Do this tid and all parents
     {
         // loop over all attributes in object type

src/internet/model/rtt-estimator.cc

@@ -63,12 +63,13 @@ RttEstimator::RttEstimator()
     : m_nSamples(0)
 {
     NS_LOG_FUNCTION(this);
+}
 
-    // We need attributes initialized here, not later, so use the
-    // ConstructSelf() technique documented in the manual
-    ObjectBase::ConstructSelf(AttributeConstructionList());
+void
+RttEstimator::NotifyConstructionCompleted()
+{
+    NS_LOG_FUNCTION(this);
     m_estimatedRtt = m_initialEstimatedRtt;
-    m_estimatedVariation = Time(0);
     NS_LOG_DEBUG("Initialize m_estimatedRtt to " << m_estimatedRtt.GetSeconds() << " sec.");
 }
 

src/internet/model/rtt-estimator.h

@@ -89,6 +89,8 @@ class RttEstimator : public Object
     Time m_initialEstimatedRtt; //!< Initial RTT estimation
 
   protected:
+    void NotifyConstructionCompleted() override;
+
     Time m_estimatedRtt;       //!< Current estimate
     Time m_estimatedVariation; //!< Current estimate variation
     uint32_t m_nSamples;       //!< Number of samples

src/lte/helper/emu-epc-helper.cc

@@ -31,8 +31,14 @@ EmuEpcHelper::EmuEpcHelper()
     : NoBackhaulEpcHelper()
 {
     NS_LOG_FUNCTION(this);
-    // To access the attribute value within the constructor
-    ObjectBase::ConstructSelf(AttributeConstructionList());
+}
+
+void
+EmuEpcHelper::NotifyConstructionCompleted()
+{
+    NoBackhaulEpcHelper::NotifyConstructionCompleted();
+
+    NS_LOG_FUNCTION(this);
 
     // Create EmuFdNetDevice for SGW
     EmuFdNetDeviceHelper emu;

src/lte/helper/emu-epc-helper.h

@@ -55,6 +55,9 @@ class EmuEpcHelper : public NoBackhaulEpcHelper
                 std::vector<uint16_t> cellIds) override;
     void AddX2Interface(Ptr<Node> enbNode1, Ptr<Node> enbNode2) override;
 
+  protected:
+    void NotifyConstructionCompleted() override;
+
   private:
     /**
      * helper to assign addresses to S1-U NetDevices

src/lte/helper/no-backhaul-epc-helper.cc

@@ -45,8 +45,12 @@ NoBackhaulEpcHelper::NoBackhaulEpcHelper()
       m_s5LinkMtu(3000)
 {
     NS_LOG_FUNCTION(this);
-    // To access the attribute value within the constructor
-    ObjectBase::ConstructSelf(AttributeConstructionList());
+}
+
+void
+NoBackhaulEpcHelper::NotifyConstructionCompleted()
+{
+    EpcHelper::NotifyConstructionCompleted();
 
     int retval;
 

src/lte/helper/no-backhaul-epc-helper.h

@@ -76,6 +76,8 @@ class NoBackhaulEpcHelper : public EpcHelper
     int64_t AssignStreams(int64_t stream) override;
 
   protected:
+    void NotifyConstructionCompleted() override;
+
     /**
      * @brief DoAddX2Interface: Call AddX2Interface on top of the Enb device pointers
      *

src/lte/helper/point-to-point-epc-helper.cc

@@ -28,8 +28,14 @@ PointToPointEpcHelper::PointToPointEpcHelper()
     : NoBackhaulEpcHelper()
 {
     NS_LOG_FUNCTION(this);
-    // To access the attribute value within the constructor
-    ObjectBase::ConstructSelf(AttributeConstructionList());
+}
+
+void
+PointToPointEpcHelper::NotifyConstructionCompleted()
+{
+    NoBackhaulEpcHelper::NotifyConstructionCompleted();
+
+    NS_LOG_FUNCTION(this);
 
     // since we use point-to-point links for the backhaul links,
     // we use a /30 subnet which can hold exactly two addresses

src/lte/helper/point-to-point-epc-helper.h

@@ -51,6 +51,9 @@ class PointToPointEpcHelper : public NoBackhaulEpcHelper
                 Ptr<NetDevice> lteEnbNetDevice,
                 std::vector<uint16_t> cellIds) override;
 
+  protected:
+    void NotifyConstructionCompleted() override;
+
   private:
     /**
      * S1-U interfaces