iOS: Unknown classes in reflection

At runtime, when listing the classes in my main bundle (using objc_getClassList). I see things like :

_TtGCs18ManagedProtoBufferVs29_HashedContainerStorageHeaderVs5UInt8_
_TtGCs13ManagedBufferVs29_HashedContainerStorageHeaderVs5UInt8_
_TtGCs28_NativeDictionaryStorageImplCSo8NSObjectPs9AnyObject__
_TtGCs18_HeapBufferStorageVs18_StringBufferIVarsVs6UInt16_

Any ideas what those are??

Also, they do not always appear...

I also see NSKVONotifying_NSUserDefaults sometimes.