> If I declare a value type with 32-bit floats x, y and z, the in-memory representation for that struct contains exactly 12 bytes, arranged in the obvious way. Nothing more, nothing less.

C doesn't even give you this. See: "The Lost Art Of Structure Packing"

http://www.catb.org/esr/structure-packing/

Previously submitted (22 points, 1 thoughtful comment): https://news.ycombinator.com/item?id=44013568