[Ns-developers] [ns3] trace path documentation

[Mathieu Lacage]

On Fri, 2008-04-11 at 01:28 +0100, Gustavo Carneiro wrote:


>         There are a couple of issues to consider:
>           - I had to make one API change to the attribute system: if
>         you want to
>         set or get a pointer of the form Ptr<T> as an attribute, you
>         must now
>         wrap that pointer in an instance of the Pointer class.
>         Ptr<Queue> queue;
>         queue = csma->GetAttribute ("TxQueue");
>         is now:
>         Ptr<Queue> queue;
>         Pointer q = csma->GetAttribute ("TxQueue");
>         queue = q.Get ();
> 
> Ugh!.. 
> 
> Can you at least rename it to PointerAttribute or something.  Having
> both Pointer and Ptr is rather confusing.
> 
> Moreover, generally I think all classes that only wrap attribute
> values and which have a Get() method should be renamed to FooAttribute
> (replace Attribute with a suffix of choice).  That would make it clear
> when a class is only a wrapper to an actual value.

Thinking more about this and other issues lead me to re-consider this
problem in light of the larger problem of trying to explain in a
sensible and consistent way how to set and get attributes in objects or
across the helper API. The issue we face in the current code and in what
you propose is that different attributes get a different treatment
because of implementation details (which make a lot of sense if you know
how the implementation works but which are pretty weird for a user). For
example, if I want to deal with a double:

object->SetAttribute ("MyDouble", Double (10.0));
Double v = object->GetAttribute ("MyDouble");
double value = v.Get ();

but, if I want to deal with a Vector:

Vector v = Vector (0.0, 0.0, 0.0);
object->SetAttribute ("MyVector", vector);
v = Object->GetAttribute ("MyVector");

The issue, here, is that you have to wrap the value you want to set or
get with a Double but not with a Vector.

The only way to make the two classes of attributes behave the same is to
_always_ request that you use a 'wrapper' class to set or get an
attribute:

object->SetAttribute ("MyDouble", DoubleAttribute (10.0));
object->SetAttribute ("MyVector", VectorAttribute (v));
DoubleAttribute v = object->GetAttribute ("MyDouble");
double doubleV = v.Get ();
VectorAttribute vec = object->GetAttribute ("MyVector);
Vector vectorV = vec.Get ();

The above code is certainly verbose but it is at least very consistent
and avoids the problem you originally complained about. Would there be
complaints if I implemented that ?

regards,
Mathieu