Re: [ns] NS2 problem with Packet::datalen() function (packet.h)

[Brian Adamson <adamson@itd.nrl.navy.mil>]

At 7:34 PM -0700 5/17/01, Haobo Yu wrote:
>  > For anyone interested, in "packet.h", there is a method define 
>"datalen()" :
>>
>>  inline int Packet::datalen() const {return data_ ? data_->size() : 0}
>>
>>  Since "data_" is actually a pointer of type AppData (the base class
>>  of PacketData), this actually ends up returning the sizeof the
>>  AppData class instead of the actual data_'s datalen_ ...
>
>AppData is an abstract class, and its size() method is a virtual one so
>type checking that you mentioned below is done automatically. Typecasting
>it to 'PacketData' will ruin it.


The current problem I have is that the AppData base class 
implementation of returns

"sizeof(AppData)"

So the current "int Packet::datalen()" function always returns 
"sizeof(AppData)" instead of the packet's "datalen()" ... which I 
believe was the intention of the "datalen()" method of the "Packet" 
class.  There are probably alternative (better) ways to remedy this 
situation ... Perhaps the type of the "data_" member of the Packet 
class should be of type PacketData instead of AppData? ...  Any 
suggestions for a better fix ... My current work-around in my own 
code is to access the Packet "data_" pointer (via the "userdata() 
method), cast is as type "PacketData" and get the PacketData size() 
... This seems a faily non-intuitive approach as compared to using 
the Packet::datalen() method given some fix for it ...


>
>- Haobo
>
>>  A fix for this is to cast the "data_" member before calling the
>>  virtual size() method:
>>
>>  inline int Packet::datalen() const {return data_ ?
>>  ((PacketData*)data_)->size() : 0}
>>
>>
>>  (Note this assumption is that the "data_" member _is_ of type
>>  PACKET_DATA ... the datalen() function could be made safer with a
>>  check to verify the type is PACKET_DATA before accessing the size()
>>  method ...)
>>
>>  This is at least the case on ns compiled with gcc-2.96 under Redhat Linux
>>

-- 
Brian
__________________________________
Brian Adamson
<http://manimac.itd.nrl.navy.mil>
<mailto:adamson@itd.nrl.navy.mil>