[SOLVED] unclear code snippet , seeking the help of c++ people

[knowledge_Seeker1]

Dear All,

I am seeking the help of people who know c++ .

Could anybody explain to me what is going on here ?

( i am tryin to learn ns3 and these codes are taken from ns3 tutorial chap7 )

class MyObject : public Object
{
public:
static TypeId GetTypeId (void)
{
static TypeId tid = TypeId ("MyObject")
.SetParent (Object::GetTypeId ())
.AddConstructor<MyObject> ()
.AddTraceSource ("MyInteger",
"An integer value to trace.",
MakeTraceSourceAccessor (&MyObject::m_myInt))
;
return tid;
}
MyObject () {}
TracedValue<int32_t> m_myInt;
};


void
IntTrace (int32_t oldValue, int32_t newValue)
{
std::cout << "Traced " << oldValue << " to " << newValue << std::endl;
}


int
main (int argc, char *argv[])
{
Ptr<MyObject> myObject = CreateObject<MyObject> ();
myObject->TraceConnectWithoutContext ("MyInteger", MakeCallback(&IntTrace));
myObject->m_myInt = 1234;
}

your help is HIGHLY appreciated

 

[bigdogguru]

Could anybody explain to me what is going on here ?

What parts of the posted code are you having trouble understanding?

What is your level of experience with C++ coding?

BigDog

 

[knowledge_Seeker1]

What parts of the posted code are you having trouble understanding?

What is your level of experience with C++ coding?

BigDog

Thank you for your reply.

I know java, I never progammed with c++ .

---------- Post added at 00:38 ---------- Previous post was at 00:34 ----------

i have problem understanding the whole thing !

---------- Post added at 00:41 ---------- Previous post was at 00:38 ----------

Let's take the first snippet :

class MyObject : public Object    //[COLOR="#FF0000"] here they are extending the Object class [/COLOR]
{
public:
static TypeId GetTypeId (void)    /[COLOR="#FF0000"]/ here they are creating a function that returns an object of type TypeId[/COLOR]
{
static TypeId tid = TypeId ("MyObject")    // they lost me from this point downward !
.SetParent (Object::GetTypeId ())
.AddConstructor<MyObject> ()
.AddTraceSource ("MyInteger",
"An integer value to trace.",
MakeTraceSourceAccessor (&MyObject::m_myInt))
;
return tid;
}
MyObject () {}
TracedValue<int32_t> m_myInt;
};

Please use CODE tags when posting your code

 

[bigdogguru]

It appears you would greatly benefit from the study a C++ tutorial.

While both Java and C++ are both OOP languages, there are many significant differences.

Some of the differences in nomenclature:

Methods = Class Member Functions

Properties/Attributes = Class Data Members

I'll attempt to clarify a few points of the code snippet:

        class MyObject : public Object  // [COLOR="#FF0000"]Class Declaration indicating the class MyObject inherits the Member Functions and Data Members from class Object[/COLOR]
        {
           public:  // [COLOR="#FF0000"]Indicates the following are publicly accessible Member Functions and Data Members[/COLOR]
 
           static TypeId GetTypeId (void) // [COLOR="#FF0000"]Static Member Function which can be invoked without the instantiation of an object[/COLOR]
           {
               static TypeId tid = TypeId ("MyObject")
              .SetParent (Object::GetTypeId ())
              .AddConstructor<MyObject> ()
              .AddTraceSource ("MyInteger",
                                      "An integer value to trace.",
                                      MakeTraceSourceAccessor (&MyObject::m_myInt));
              return tid;
           }

            MyObject () {} // [COLOR="#FF0000"]Class Constructor [/COLOR]

            TracedValue<int32_t> m_myInt; // [COLOR="#FF0000"]Data Member[/COLOR]
        };

I would recommend studying some of the following C++ online tutorials:

The C++ Tutorial

C++ Language Tutorial

Programming Tutorials - C, C++, OpenGL, STL

You may also find the following group of interest:

Embedded C/C++ Programming

Having study Java, you should be able to quickly understand much of the C++ syntax and methodologies after studying one of the tutorials posted above

BigDog

 

[knowledge_Seeker1]

That was EXTREMELY helpful ! thanks !

I will definitely read one of those tutorials

could you clarify few more things ?

int
main (int argc, char *argv[])
{
[COLOR="#FF0000"]
  //they creating an array called myObject of type "MyObject", but what is the Ptr for ?  [/COLOR]                  
Ptr<MyObject> myObject = CreateObject<MyObject> ();     

// what does the[COLOR="#FF0000"]  ->, and  &IntTrace[/COLOR] mean ?
myObject->TraceConnectWithoutContext ("MyInteger", MakeCallback(&IntTrace));
myObject->m_myInt = 1234;
}

It appears you would greatly benefit from the study a C++ tutorial.

While both Java and C++ are both OOP languages, there are many significant differences.

Some of the differences in nomenclature:

Methods = Class Member Functions

Properties/Attributes = Class Data Members

I'll attempt to clarify a few points of the code snippet:

        class MyObject : public Object  // [COLOR="#FF0000"]Class Declaration indicating the class MyObject inherits the Member Functions and Data Members from class Object[/COLOR]
        {
           public:  // [COLOR="#FF0000"]Indicates the following are publicly accessible Member Functions and Data Members[/COLOR]
 
           static TypeId GetTypeId (void) // [COLOR="#FF0000"]Static Member Function which can be invoked without the instantiation of an object[/COLOR]
           {
               static TypeId tid = TypeId ("MyObject")
              .SetParent (Object::GetTypeId ())
              .AddConstructor<MyObject> ()
              .AddTraceSource ("MyInteger",
                                      "An integer value to trace.",
                                      MakeTraceSourceAccessor (&MyObject::m_myInt));
              return tid;
           }

            MyObject () {} // [COLOR="#FF0000"]Class Constructor [/COLOR]

            TracedValue<int32_t> m_myInt; // [COLOR="#FF0000"]Data Member[/COLOR]
        };

I would recommend studying some of the following C++ online tutorials:

The C++ Tutorial

C++ Language Tutorial

Programming Tutorials - C, C++, OpenGL, STL

You may also find the following group of interest:

Embedded C/C++ Programming

Having study Java, you should be able to quickly understand much of the C++ syntax and methodologies after studying one of the tutorials posted above

BigDog

 

[bigdogguru]

could you clarify few more things ?

int
main (int argc, char *argv[])
{
[COLOR="#FF0000"]
  //they creating an array called myObject of type "MyObject", but what is the Ptr for ?  [/COLOR]                  
Ptr<MyObject> myObject = CreateObject<MyObject> ();     

// what does the[COLOR="#FF0000"]  ->, and  &IntTrace[/COLOR] mean ?
myObject->TraceConnectWithoutContext ("MyInteger", MakeCallback(&IntTrace));
myObject->m_myInt = 1234;
}

The following statement neither creates an array nor is it part of the Standard C++ Language:

Ptr<MyObject> myObject = CreateObject<MyObject> ();

Normally, the "new" operator is used to instantiate an Object from a Class in C++.

However, according to the NS-3 documentation all reference counted objects must be instantiate/created by using the Template Methods, Create<> or CreateObject<>.

Objects derived/inherited from the Object Class must use the CreateObject<> Template Method.

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

Objects NOT derived/inherited from the Object Class must use the Create<> Template Method.

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

You'll notice both these Template Methods make use of the "Smart Pointer Class" feature of NS-3, Ptr<>.

The Ptr<> Class provides a degree of memory management for dynamically created objects. Garbage Collection or Memory Management is not inherently included in C++ for dynamically created objects, therefore it must be dealt with manually, within the your code. The NS-3 Ptr<> takes care of Garbage Collection for objects created by both the CreateObject<> and Create<> Template Methods.

The concept of Pointers is a common stumbling block for novice programmers not familiar with C/C++.

C++ was in fact born from C. C has a particularly power feature, Pointers, which enables virtually any entity to be referenced/dereferenced directly or indirectly by its address in memory.

While this feature does exist, in a fashion, in other programming languages, the mechanism is concealed by a level of abstraction which prevents programmers from dealing directly and fully understanding it.

Which brings me to the second statement in question:

myObject->TraceConnectWithoutContext ("MyInteger", MakeCallback([COLOR="#FF0000"]&IntTrace[/COLOR]));

The "&" operator is referred to as the "reference" or "address of" operator.

Its complement, the "*" operator is referred to as the "dereference" operator.

The "->" operator is referred to as the "indirect membership," "deferenced membership" or "structure pointer" operator.

A variable or object data member which contains the memory address of an entity is referred to as a Pointer.

The Pointer in effect, points to the entity of interest.

The following code demonstrates the basic use of pointers and associated operators:

// pointers
#include <iostream>
using namespace std;

int main ()
{
  int firstvalue = 5, secondvalue = 15;
  int * p1, * p2;          // declares two pointers to type integer

  p1 = &firstvalue;      // p1 = address of firstvalue
  p2 = &secondvalue; // p2 = address of secondvalue
  *p1 = 10;              // value pointed by p1 = 10
  *p2 = *p1;            // value pointed by p2 = value pointed by p1
  p1 = p2;               // p1 = p2 (value of pointer is copied)
  *p1 = 20;              // value pointed by p1 = 20
  
  cout << "firstvalue is " << firstvalue << endl;
  cout << "secondvalue is " << secondvalue << endl;
  return 0;
}

Code Output:

firstvalue is 10
secondvalue is 20

The topic of Pointers, their methodology and syntax, can be rather complex, far to complex to be adequately covered in even a lengthy forum posting.

As a matter of fact there have been entire texts written solely on the topic of Pointers.

I would strongly recommend you study the section on Pointers in one of the C/C++ tutorials previously posted.

Also study the section on C++ Templates, of which NS-3 appears to make frequent use.

I'm afraid there is no "easy" path to understanding C or C++ and therefore, the same can be said concerning the understanding and coding with NS-3.

Only through copious amounts of studying and practice coding can you achieve your goals.

BigDog

 

[knowledge_Seeker1]

Thank you Thank you thank you for all of your efforts, ! I FULLY understand the code now !

int
main (int argc, char *argv[])
{
[COLOR="#FF0000"]
  //they creating an array called myObject of type "MyObject", but what is the Ptr for ?  [/COLOR]                  
Ptr<MyObject> myObject = CreateObject<MyObject> ();     

// what does the[COLOR="#FF0000"]  ->, and  &IntTrace[/COLOR] mean ?
myObject->TraceConnectWithoutContext ("MyInteger", MakeCallback(&IntTrace));
myObject->m_myInt = 1234;
}

The following statement neither creates an array nor is it part of the Standard C++ Language:

Ptr<MyObject> myObject = CreateObject<MyObject> ();

Normally, the "new" operator is used to instantiate an Object from a Class in C++.

However, according to the NS-3 documentation all reference counted objects must be instantiate/created by using the Template Methods, Create<> or CreateObject<>.

Objects derived/inherited from the Object Class must use the CreateObject<> Template Method.

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

Objects NOT derived/inherited from the Object Class must use the Create<> Template Method.

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

You'll notice both these Template Methods make use of the "Smart Pointer Class" feature of NS-3, Ptr<>.

The Ptr<> Class provides a degree of memory management for dynamically created objects. Garbage Collection or Memory Management is not inherently included in C++ for dynamically created objects, therefore it must be dealt with manually, within the your code. The NS-3 Ptr<> takes care of Garbage Collection for objects created by both the CreateObject<> and Create<> Template Methods.

The concept of Pointers is a common stumbling block for novice programmers not familiar with C/C++.

C++ was in fact born from C. C has a particularly power feature, Pointers, which enables virtually any entity to be referenced/dereferenced directly or indirectly by its address in memory.

While this feature does exist, in a fashion, in other programming languages, the mechanism is concealed by a level of abstraction which prevents programmers from dealing directly and fully understanding it.

Which brings me to the second statement in question:

myObject->TraceConnectWithoutContext ("MyInteger", MakeCallback([COLOR="#FF0000"]&IntTrace[/COLOR]));

The "&" operator is referred to as the "reference" or "address of" operator.

Its complement, the "*" operator is referred to as the "dereference" operator.

The "->" operator is referred to as the "indirect membership," "deferenced membership" or "structure pointer" operator.

A variable or object data member which contains the memory address of an entity is referred to as a Pointer.

The Pointer in effect, points to the entity of interest.

The following code demonstrates the basic use of pointers and associated operators:

// pointers
#include <iostream>
using namespace std;

int main ()
{
  int firstvalue = 5, secondvalue = 15;
  int * p1, * p2;          // declares two pointers to type integer

  p1 = &firstvalue;      // p1 = address of firstvalue
  p2 = &secondvalue; // p2 = address of secondvalue
  *p1 = 10;              // value pointed by p1 = 10
  *p2 = *p1;            // value pointed by p2 = value pointed by p1
  p1 = p2;               // p1 = p2 (value of pointer is copied)
  *p1 = 20;              // value pointed by p1 = 20
  
  cout << "firstvalue is " << firstvalue << endl;
  cout << "secondvalue is " << secondvalue << endl;
  return 0;
}

Code Output:



The topic of Pointers, their methodology and syntax, can be rather complex, far to complex to be adequately covered in even a lengthy forum posting.

As a matter of fact there have been entire texts written solely on the topic of Pointers.

I would strongly recommend you study the section on Pointers in one of the C/C++ tutorials previously posted.

Also study the section on C++ Templates, of which NS-3 appears to make frequent use.

I'm afraid there is no "easy" path to understanding C or C++ and therefore, the same can be said concerning the understanding and coding with NS-3.

Only through copious amounts of studying and practice coding can you achieve your goals.

BigDog