The 'struct hack' is when you leave the type of the last member of a struct undefined (effectively making structs variable-sized). This is actually not a proble

The 'struct hack' is when you leave the type of the last member of a struct undefined (effectively making structs variable-sized). This is actually not a problem for runtime type checking, and is C99 compliant. What causes problems is casting pointers to ints and back, and casting all other crap to chars. This is not standards compliant. Casting ints to pointers will never be type-safe, but one way to get around that is to just ignore the cast, and overload arithmetic operators to work correctly on pointers - the pointers will carry around their type info, and everything should work ok. Casting other crap to chars will never work because it interferes with the way the other crap has its type encoded. Luckily in most cases this casting is done to perform I/O, where you can also just ignore the cast, and specialize the lowest-level I/O functions to dispatch on the actual types. The moral of the story is that you should basically ignore all the line noise the programmer produces about types, and look at the actual objects. This is exactly how Java works, btw. WRT hardware tagging and type checks, there's really no reason to do it on a byte-addressed superscalar processor. If you look at 64-bit Common Lisp implementations today, you'll actually find that they use only about half the available tag bits in each word. The only thing that needs to be boxed is double-floats.