That isn't strictly true. You do not need to do garbage collection to compact memory. The data needed to do memory compaction and garbage collection largely overlaps, so you may as well do both, but you don't have to. To implement either you need to do a scan of all pointers in memory - for a garbage collection you save a currently in use list, for compaction you need to a map of where they are so you can change them latter. What you do with that data is different, but only the first step is the same. (though garbage collection can feed the empty places into the compactor which makes that work easier)
I've actually been thinking about how to do compaction in C++. The reason you cannot do it in general is code like "socket.send(new object, sizeof(void*)". The remote is then expected to send back that pointer which you cast to object and reference. Evil code for sure with a lot of failure cases that result in leaks or security holes, but possible.
