@c ========================================================================

@c ns-3 Object model

@c ========================================================================

@node Object model

@chapter Object model

@menu

* Object model::

* Object-oriented behavior::

* Object base classes::

* Memory management and class Ptr::

* Object factories::

* Downcasting::

@end menu

@command{ns-3} is fundamentally a C++ object system.  Objects can

be declared and instantiated as usual, per C++ rules.  ns-3 also

adds some features to traditional C++ objects, as described below,

to provide greater functionality and features.  This manual chapter

is intended to introduce the reader to the ns-3 object model.

This section describes the C++ class design for ns-3 objects.  In brief,

several design patterns in use include classic object-oriented design

(polymorphic interfaces and implementations), separation of interface

and implementation, the non-virtual public interface design pattern,

an object aggregation facility, and reference counting

for memory management.  Those familiar with component models such

as COM or Bonobo will recognize elements of the design in the 

ns-3 object aggregation model, although

the ns-3 design is not strictly in accordance with either.

@node Object-oriented behavior

@section Object-oriented behavior

C++ objects, in general, provide common object-oriented capabilities 

(abstraction, encapsulation, inheritance, and polymorphism) that are part 

of classic object-oriented design.  ns-3 objects make use of these 

properties; for instance:

@verbatim

class Address

{

public:

  Address ();

  Address (uint8_t type, const uint8_t *buffer, uint8_t len);

  Address (const Address & address);

  Address &operator = (const Address &address);

  ...

private:

  uint8_t m_type;

  uint8_t m_len;

  ...

};

@end verbatim

@node Object base classes

@section Object base classes

There are three special base classes used in ns-3.  Classes that inherit

from these base classes can instantiate objects with special properties.

These base classes are:

@itemize @bullet

@item @code{class Object}

@item @code{class ObjectBase}

@item @code{class SimpleRefCount}

@end itemize

It is not required that ns-3 objects inherit from these class, but 

those that do get special properties.  Classes deriving from 

@code{class Object} get the following properties.

@itemize @bullet

@item the ns-3 type and attribute system (see @ref{Attributes})

@item an object aggregation system

@item a smart-pointer reference counting system (class Ptr)

@end itemize

Classes that derive from @code{class ObjectBase} get the first two

properties above, but do not get smart pointers.  Classes that

derive from @code{class SimpleRefCount} get only the smart-pointer

reference counting system.

In practice, @code{class Object} is the variant of the three above that

the ns-3 developer will most commonly encounter.

@node Memory management and class Ptr

@section Memory management and class Ptr

Memory management in a C++ program is a complex process, and is

often done incorrectly or inconsistently.  We have settled on 

a reference counting design described as follows.

All objects using reference counting maintain an internal reference

count to determine when an object can safely delete itself.  

Each time that a pointer is obtained to an interface, the object's

reference count is incremented by calling @code{Ref()}.

It is the obligation of the

user of the pointer to explicitly @code{Unref()} the pointer when done.

When the reference count falls to zero, the object is deleted.

@itemize @bullet

@item When the client code obtains a pointer from the object itself

through object creation, or via GetObject, it does not have

to increment the reference count.   

@item When client code obtains a pointer from another source (e.g.,

copying a pointer) it must call @code{Ref()} to increment the

reference count.

@item All users of the object pointer must call @code{Unref()} to

release the reference.

@end itemize

The burden for calling @code{Unref()} is somewhat relieved by the

use of the reference counting smart pointer class described below. 

Users using a low-level API who wish to explicitly allocate

non-reference-counted objects on the heap, using operator new, 

are responsible for deleting such objects.

@subsection Reference counting smart pointer (Ptr)

Calling @code{Ref()} and @code{Unref()} all the time would be cumbersome,

so ns-3 provides a smart pointer @code{class Ptr} similar to 

@code{Boost::intrusive_ptr}.

This smart-pointer class assumes that the underlying type provides 

a pair of Ref and Unref methods that are expected to increment and 

decrement the internal refcount of the object instance.  

This implementation allows you to manipulate the smart pointer

as if it was a normal pointer: you can compare it with zero,

compare it against other pointers, assign zero to it, etc.

It is possible to extract the raw pointer from this

smart pointer with the GetPointer and PeekPointer methods.

If you want to store a newed object into a smart pointer,

we recommend you to use the CreateObject template functions

to create the object and store it in a smart pointer to avoid

memory leaks. These functions are really small convenience

bit of typing.

@subsection CreateObject and Create

Objects in C++ may be statically, dynamically, or automatically created.

This holds true for ns-3 also, but some objects in the system 

have some additional frameworks

available.  Specifically, reference counted objects are usually

allocated using a templated Create or CreateObject method, as follows.

For objects deriving from @code{class Object}:

@verbatim

 Ptr<WifiNetDevice> device = CreateObject<WifiNetDevice> ();

@end verbatim

Please do not create such objects using @code{operator new}; create them

using @code{CreateObject()} instead.

For objects deriving from @code{class SimpleRefCount}, or other

objects that support usage of the smart pointer class,

a templated helper function is available and recommended to be used:

@verbatim

  Ptr<B> b = Create<B> ();

@end verbatim  

This is simply a wrapper around operator new that correctly handles

the reference counting system.

In summary, use @code{Create<B>} if B is not an object but just uses

reference counting (e.g. @code{class Packet}), and use @code{CreateObject<B>}

if B derives from @code{ns3::Object}.

@subsection Aggregation

The ns-3 object aggregation system is motivated in strong part by 

a recognition that a common use case for ns-2 has been the use of 

inheritance and polymorphism to extend protocol models.  For instance, 

specialized versions of TCP such as RenoTcpAgent derive from (and override 

functions from) class TcpAgent.  

However, two problems that have arisen in the ns-2 model are downcasts

and ``weak base class.''  Downcasting refers to the procedure of using

a base class pointer to an object and querying it at run time to

find out type information, used to explicitly cast the pointer to 

a subclass pointer so that the subclass API can be used.

Weak base class refers to the problems that arise when a class

cannot be effectively reused (derived from) because it lacks necessary

functionality, leading the developer to have to modify the

base class and causing proliferation of base class API calls, some

of which may not be semantically correct for all subclasses.

ns-3 is using a version of the query interface design pattern

to avoid these problems.  This design is based on elements of the 

@uref{http://en.wikipedia.org/wiki/Component_Object_Model,,Component Object Model} 

and @uref{http://en.wikipedia.org/wiki/Bonobo_(component_model),,GNOME Bonobo}

although full binary-level compatibility of replaceable components

is not supported and we have tried to simplify the syntax and impact

on model developers.  

@subsubsection Aggregation example

@code{class Node} is a good example of the use of aggregation in ns-3.

Note that there are not derived classes of Nodes in ns-3 such as

class InternetNode.  Instead, components (protocols) are aggregated to 

a node.  Let's look at how some Ipv4 protocols are added to a node.

@verbatim

static void

AddIpv4Stack(Ptr<Node> node)

{

  Ptr<Ipv4L3Protocol> ipv4 = CreateObject<Ipv4L3Protocol> ();

  ipv4->SetNode (node);

  node->AggregateObject (ipv4);

  Ptr<Ipv4Impl> ipv4Impl = CreateObject<Ipv4Impl> ();

  ipv4Impl->SetIpv4 (ipv4);

  node->AggregateObject (ipv4Impl);

}

@end verbatim

Note that the Ipv4 protocols are created using @code{CreateObject()}.  

Then, they are aggregated to the node.  In this manner, the Node base

class does not need to be edited to allow users with a base class Node

pointer to access the Ipv4 interface; users may ask the node for a pointer

to its Ipv4 interface at runtime.  How the user asks the node is described

in the next subsection.

Note that it is a programming error to aggregate more than one object of

the same type to an ns3::Object.  So, for instance, aggregation is not

an option for storing all of the active sockets of a node.

@subsubsection GetObject example

GetObject is a type-safe way to achieve a safe downcasting 

and to allow interfaces to be found on an object.  

Consider a node pointer @code{m_node} that points to a Node object 

that has an implementation of IPv4 previously aggregated to it.  The 

client code wishes to configure

a default route.  To do so, it must access an object within

the node that has an interface to the IP forwarding configuration.

It performs the following:

@verbatim

  Ptr<Ipv4> ipv4 = m_node->GetObject<Ipv4> ();

@end verbatim

If the node in fact does not have an Ipv4 object aggregated to it, then

the method will return null.  Therefore, it is good practice to check

the return value from such a function call.  If successful, the user can

now use the Ptr to the Ipv4 object that was previously aggregated to

the node.

Another example of how one might use aggregation is to add optional

models to objects.  

For instance, an existing Node object may have an ``Energy Model''

object aggregated to it at run time (without modifying

and recompiling the node class).  An existing model (such as a wireless

net device) can then later "GetObject" for the energy model and act 

appropriately if the interface has been either built in to the underlying

Node object or aggregated to it at run time.  However, other nodes need

not know anything about energy models.

We hope that this mode of programming will require much less 

need for developers to modify the base classes.

@node Object factories

@section Object factories

A common use case is to create lots of similarly configured objects.

One can repeatedly call @code{CreateObject} but there is also a 

factory design pattern in use in the ns-3 system.  It is heavily

used in the "helper" API. 

Class @code{ObjectFactory} can be used to instantiate objects and

to configure the attributes on those objects 

@verbatim

  void SetTypeId (TypeId tid);

  void Set (std::string name, const AttributeValue &value);

  Ptr<T> Create (void) const;

@end verbatim

The first method allows one to use the ns-3 TypeId system to specify

the type of objects created.  The second allows one to set 

attributes on the objects to be created, and the third allows one

to create the objects themselves. 

For example:

@verbatim

  ObjectFactory factory;

  // Make this factory create objects of type FriisPropagationLossModel

  factory.SetTypeId ("ns3::FriisPropagationLossModel")

  // Make this factory object change a default value of an attribute, for

  // subsequently created objects

  factory.Set ("SystemLoss", DoubleValue (2.0));

  // Create one such object

  Ptr<Object> object = m_factory.Create (); 

  factory.Set ("SystemLoss", DoubleValue (3.0));

  // Create another object

  Ptr<Object> object = m_factory.Create (); 

@node Downcasting

@section Downcasting

A question that has arisen several times is, "If I have a base class

pointer (Ptr) to an object and I want the derived class pointer, should

I downcast (via C++ dynamic cast) to get the derived pointer, or should

I use the object aggregation system to @code{GetObject<> ()} to find

a Ptr to the interface to the subclass API?"

The answer to this is that in many situations, both techniques will work.

ns-3 provides a templated function for making the syntax of Object

dynamic casting much more user friendly:

@verbatim

template <typename T1, typename T2>

Ptr<T1>

DynamicCast (Ptr<T2> const&p)

{

  return Ptr<T1> (dynamic_cast<T1 *> (PeekPointer (p)));

}

@end verbatim

DynamicCast works when the programmer has a base type pointer and is

testing against a subclass pointer.  GetObject works when looking for

different objects aggregated, but also works with subclasses, in the same

way as DynamicCast.  If unsure, the programmer should use GetObject, as it

works in all cases.   If the programmer knows the class hierarchy of the

object under consideration, it is more direct to just use DynamicCast.