NS2 Shadow Object Construction

NS2: Shadow ObjectNS2: Shadow Object
ConstructionConstruction
by Teerawat Issariyakul
http://www.ns2ultimate.com
October 2010
http://www.ns2ultimate.com 1

OutlineOutline
Introduction
Creating an OTcl object
Shadow Object Construction Process
Summary

IntroductionIntroduction
This is a series on how NS2 binds C++ and OTcl together.This is the
second topic of the series:
1.WhyTwo Languages?
2. Binding C++ and OTcl classes
3. Variable binding
4. OTcl command: Invoking C++ statements from the OTcl domain
5. Eval and result: Invoking OTcl statements from the C++ domain
6. Object binding and object construction process.

MotivationMotivation
 NS2 consists of 2 languages:
◦ OTcl: A user interface language used to create objects
◦ C++: A language defining the created objects’ attributes and
behaviors
 Objects in NS2
◦ A user creates an object from the OTcl domain
◦ NS2 automatically create a “shadow” object in the C++ domain
 From the user perspective, these two objects are the same.

Suppose we have two bound classes
We are going to learn how EXACTLY
NS2 auto. create a shadow object.
What We Would Like to Do?What We Would Like to Do?
TCPAgent
C++
Agent/TCP
OTcl
bound
user
obj
create
c_obj
Auto. construction by NS2
shadow object

OutlineOutline
Introduction
Summary

Creating an OTcl ObjectCreating an OTcl Object
 An OTcl is created using the following syntax
<className> create <var> [<args>]
which creates an object of class <className> and store the
created object in the variable <var>. Note, [<args>]is optional
input argument for object construction.
 There are two key steps when invoking new
◦ Executing instproc “alloc” to allocate memory to hold the object
◦ Executing instproc “init” (i.e., the constructor) to initialize class
variables

Instproc “Instproc “initinit””
It initializes class variables
It is referred to as “the constructor”
OOP structure
◦ Call the constructor of the base class first
◦ Use the instproc next

Instproc “Instproc “nextnext””
Instproc “next” executes the instproc with the
same name located in the parent class.
E.g., Class Agent/TCP derives from class Agent
When we see
the statement “eval $self next” executes to
“Agent init {}”.
Agent/TCP instproc init
{} {
eval $self next
…
}

OutlineOutline
Introduction
Summary

Shadow Object ConstructionShadow Object Construction
The process consists of two part
◦ Part I: OTcl—Creating an object using new
◦ Part II: C++—Shadow object construction
Part I: OTcl Part II: C++

Shadow Object ConstructionShadow Object Construction
Let’s use the following example
◦ OTcl class: Agent/TCP
◦ C++ class: TcpAgent
In OTcl, create an object using
new Agent/TCP
In C++, create a shadow TcpAgent
object

Part I: OTclPart I: OTcl
Main steps of Part I:
The OTcl statement new <classname> [<args>]
The OTcl statement $classname create $o $args
The instproc alloc of the OTcl class <classname>
The instproc init of the OTcl class <classname>
The instproc init of the OTcl class SplitObject
The OTcl statement $self create-shadow $args

1. The OTcl statement new <classname>
[<args>]
proc new { className args } {
set o [SplitObject getid]
if [catch "$className create $o $args" msg] {
...
}
return $o
}
step 2

2.The OTcl statement $classname create
$o $args
Again, the instproc create invokes two
instprocs in Steps 3 and 4.
3.The instproc alloc of the OTcl class
<classname>: Memory allocation

4.The instproc init of the OTcl class
<classname>
A general structure of the the instproc init is to
invoke the instproc init of the base class until
reaching the top level class, e.g.,
The top level class is class SplitObject 
Step 5
Agent/TCP instproc init
{} {
eval $self next
…
}

5. The instproc init of the OTcl class SplitObject
6. The OTcl statement $self create-shadow
$args:
 This executes, for example, instproc create-
shadow of class Agent/TCP
SplitObject instproc init args {
$self next
if [catch "$self create-shadow $args"] {
error "__FAILED_SHADOW_OBJECT_" ""
}
}

Part II: C++Part II: C++
Part I completes with the OTcl command
create-shadow.
In Part II, we are moving to the C++ domain.
Part I: OTcl Part II: C++

Let’s use an example of class TcpAgent
Main steps of Part II:
1. Step 6 in Part I
2. The C++ function create-shadow(...) of
class TclClass
3. The C++ function create(...) of class
TcpClass
4. The C++ statement new TcpAgent()

1. Step 6 in Part I:
The OTcl command create-shadow is bound to
The C++ function create-shadow(...) of class
TclClass
2.The C++ function create-shadow(...) of
class TclClass
3.The C++ function create(...) of class
TcpClass

3.The C++ function create(...) of class
TcpClass
4. The C++ statement new TcpAgent(): Create
a shadow object
static class TcpClass : public TclClass {
public:
TcpClass() : TclClass("Agent/TCP") {}
TclObject* create(int , const char*const*) {
return (new TcpAgent());
}
} class_tcp;
Step 4

OutlineOutline
Introduction
Summary

SummarySummary
In NS2, a user creates an object from the OTcl
domain using a global procedure “new”
NS2 automatically creates a so-called shadow
object in the C++ domain.
The shadow object construction consists of 2
parts:
◦ Part 1: Execute OTcl constructors
◦ Part II: Create a shadow object in the C++ domain.

For more information aboutFor more information about
NS 2NS 2
Please see this book from Springer
T. Issaraiyakul and E. Hossain, “Introduction to
Network Simulator NS2”, Springer 2009

NS2 Shadow Object Construction

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