However if I am sending a ConstPtr between nodes I shouldn't generate an extra copy of the data the ptr points to as well correct?
If you're asking whether publishing a ConstPtr allows nodes to also benefit from zero-copy message exchange, then no, that's (unfortunately) not how it works.
nodelets can exchange messages without copying anything as they share the same memory address space. This allows them to (essentially) exchange pointers to messages, instead of (copies of) the messages themselves.
Nodes are run in separate processes, each with their own memory address space. Exchanging pointers would be meaningless, as objects will not "live" at the same addresses for these different processes.
So even though you're publish(..)ing a ConstPtr, the middleware will have to make a copy, as there is no other way to reliably exchange message data between nodes (ie: processes).
See #q346374 for another recent question about this.
