Ok, thanks for your answers. I'm now using a type flag in my message, 
but I wanted to be sure I was doing it properly.

Cheers,

Ugo

Brian Gerkey wrote:
> On Tue, Mar 23, 2010 at 3:30 AM, Ugo Cupcic <ugo@shadowrobot.com> wrote:
>> I was wondering how I can achieve to have different callback functions
>> on the same topic depending on the message types.
> 
> hi Ugo,
> 
> In ROS, a topic can only carry messages of a single type.  We decided
> early on that it would be too confusing if a topic could be of
> multiple types. So roscpp doesn't support multi-type callback
> registration.
> 
> (Note that the type of a topic actually can change over time, as it's
> determined by the publisher that most recently advertised the topic.
> But in a well-behaved ROS system, topics are stably typed).
> 
> 	brian.
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