ByteBuffer has methods for reading primitive values of all sizes -- ints, floats, etc. -- from arbitrary offsets within the buffer. So you just need a wrapper object with nice generated methods for each field which in turn call the ByteBuffer getters.

None that the C++ code works this way too. We don't cleverly generate a struct that just happens to have the right layout; we generate a class that wraps a pointer and provides inline accessors that read/write values from the correct offset relative to that pointer. After inlining, it's just as fast.