[boost] How do I specify no padding bytes within a struct/class?

I've never done this before, so I'm asking first: how do I specify that a struct/class should have no padding at all, at the start, end, or middle? My classes have internal arrays, and specifying no padding lets users use an array of classX<T> as an array of T (just like C++11 std::complex). Complier-specific answers are OK, as long as I can use preprocessor flags to isolate them. I have a preprocessor flag that's currently set to 0 to indicate that there's no packing. I'll change it to 1 whenever I get a compiler's packing support. Daryle W.