This is preliminary, but I think debugging C++ in Emacs is the same process as you use to Debug C in Emacs. Except you may want to use some additional C++ features of GDB.
Amazingly, methods can be called on objects as normal, allowing you to inspect their contents:
Given
1 vector<int> l;
2 l.push_back(1);
3 l.push_back(2);
4 l.push_back(3);
p l[1] will print $1 = (int &) @0x100160: 2