[Ns-developers] Default Values

[Tom Henderson]

Mathieu Lacage wrote:
> On Fri, 2008-01-18 at 21:27 -0500, Joseph Kopena wrote:
>> Hi all,
>>
>> First, a snippet from the Seattle meeting notes:
>>
>>  > - Two conventions regarding default values will be adopted:
>>  > -- Default values used to set values on an object should, in general,
>>  >  also have getters and setters to access the value through the
>>  >  object's interface.
>>  > -- Default values may be used for initialization only, and can either
>>  >  be global, static values or global but changeable per node.
>>
>> My take on this was the same as Tom's---that this was talking about
>> when objects are created, not necessarily restricting the use of
>> default values to scenario construction.  The former seems reasonable
>> to me, that default values should only be used to initialize objects
>> and should not be referenced throughout its lifetime.  A related point
>> is what I was talking about in regard to Craig's Set() function, it
>> should be made clear that changing the default values does not effect
>> already created objects.  To me that just sounds too chaotic.
> 
> What sounds chaotic ? Craig's generic Set ("path", value) proposal was
> not related in any way to DefaultValues: it was related to an extension
> of the Parameter proposal.

Mathieu,
I did not read Craig's proposal as an extension to the Parameter 
proposal.  I read it as suggesting to set up a parallel configuration 
registry that reuses the tracing namespace.

Can you describe a use case where the above definition:  "Default values 
should be only used to initialize objects and should not be referenced 
throughout its lifetime" will cause problems?  A use case that does not 
involve a user abusing or ignoring the above guidance?

Another helpful example would be to describe a case where you have a 
default value that needs to be overridden during some portion of the 
topology phase, and this cannot be accomplished by manipulating the 
value of the DefaultValue.  That is what I understand to be a motivation 
for your Parameter proposal (to send a configuration object through the 
topology API, instead of manipulating a default value).

> 
>> As I understand it, one of the issues ostensibly being addressed by
>> limiting default value use to the construction phase is that code may
>> change values without the user being aware, and cause hard to debug
>> misbehavior.  However, it's not obvious to me that the factory
>> getters/setters approach resolves that.  Wouldn't that code just as
>> easily interact with the factory as change a default value?  It seems
> 
> Sure. However, it seems pretty obvious to me that calling a Setter on a
> factory is not to going to influence in any way the behavior of an
> object which has already been created by that factory. Or are you
> referring to another problem ?

I also fail to see the distinction.  If we establish the above policy on 
DefaultValue usage, and audit the usage in ns-3-dev to conform to the 
above guideline, won't that be the same?

Tom