The following functions are not consistent:
Ipv6InterfaceContainer::Add (Ipv6InterfaceContainer& c)
Ipv4InterfaceContainer::Add (Ipv4InterfaceContainer c)

I.e., one works with a reference, the other by copy.
Moreover, the v6 one raises a compilation error for code like:
 interfaceContainer.Add (ipv6.Assign (someDevices));
which should be legal.

Both functions signature should be changed to be const reference.