3. 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.
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4. MotivationMotivation
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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.
5. 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?
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TCPAgent
C++
Agent/TCP
OTcl
bound
user
obj
create
c_obj
Auto. construction by NS2
shadow object
7. Creating an OTcl ObjectCreating an OTcl Object
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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
9. Instproc “Instproc “nextnext””
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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
…
}
11. 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
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Part I: OTcl Part II: C++
12. 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
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13. 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 $args
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14. Part I: OTclPart I: OTcl
1. The OTcl statement new <classname>
[<args>]
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proc new { className args } {
set o [SplitObject getid]
if [catch "$className create $o $args" msg] {
...
}
return $o
}
step 2
15. Part I: OTclPart I: OTcl
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
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16. Part I: OTclPart I: OTcl
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
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Agent/TCP instproc init
{} {
eval $self next
…
}
17. Part I: OTclPart I: OTcl
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
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SplitObject instproc init args {
$self next
if [catch "$self create-shadow $args"] {
error "__FAILED_SHADOW_OBJECT_" ""
}
}
18. Part II: C++Part II: C++
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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++
19. Part II: C++Part II: C++
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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()
20. Part II: C++Part II: C++
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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
21. Part II: C++Part II: C++
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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
23. 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.
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24. 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
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