This uses the Most Significant Bits to pack some bits into pointers..

Another technique is to use the Least Significant Bits: indeed since pointers are 64 bit aligned, the lowest 3 bits are always zero, therefore it is possible to pack 3 bits of information there.

And there's a third technique in C++, which is to use placement new to morph an object in-place. This can be use to toggle between 2 classes with a boolean method, where one has this method return true and the other class has this method return false. This creates per-object state that really uses bits in the vptr to store this boolean state. Obviously this can be used with a whole set of classes to stores more bits. I have used this successfully to store values of refcount (each AddRef/Release using placement new to morph from RefCounted1/RefCounted2/../RefCountedN classes) to implement reference counting without a counter.