On Tue, Dec 21, 2010 at 8:46 AM, joan_perez <jnperez@iri.upc.edu> wrote:

Hi all,

I have a few questions related to node handles which I believe have not been
asked before. For what I have seen, there are many different ways of using
node handles. I could not find the answers in the
http://www.ros.org/wiki/roscpp/Overview/NodeHandles wiki nor in any
previous topic.

1- It is better to have a single node handle for all topics in a node, or it
is more appropriate to use a node handle per topic?

Generally, a single node handle is fine. The only time people typically want to create extra nodehandles is to push a group of topics into a namespace. e.g. all the outgoing image topics into a "camera" namespace. But even then, all topics in the same namespace can make use of the same nodehandle.

2- Related to these question, should I declare node handles as member class
attributes or just as function variables? I have even seen node handles
created on a function call.

In the grand scheme of things, nodehandles, are fairly cheap, but they are certainly not free to create. Member class attributes is almost certainly the way to go. I would avoid creating a new node handle in any function that is called in a loop. But if the function is only being called once by your main function, there's no reason the node handle has to be part of a class.

3- Should I better use node handle's namespace rather than node namespace?

I'm not sure what you mean by this. The default nodehandle is in the namespace of the node. Nodehandle namespaces should only be used to push things down further.

4- Which is the main purpose of the private node handle (the one with
private namespace "~")? It is meant to be used only for the parameter
server?

This has never been terribly well specified, and different people will give you different opinions. The rule of thumb has been to use the node private namespace for very specific things that are only relevant to your node. Configuration parameters is one of the best examples of this. You can, of course, publish topics there, but it doesn't fit well within the notion of "anonymous publish-subscribe" since it means any subscriber needs to know the name of your node to listen to it. The key here is that nodes should really never be looking inside the private namespace of another node.

5- Is there any additional feature (apart from queuing relative namespace)
for specifying a node handle as child from the other?

Managing relative namespaces is all that nodehandles are good for. When you make one nodehandle the child of another, all it does is append its namespace onto the namespace of its parent.

Hope that helps,

--Jeremy