Sounds fragile, C++ compilers are permitted to do struct padding essentially as they please. A change in compiler could break the SSD<-->struct mapping (i.e. the member offsets).

C++ arrays, on the other hand, are guaranteed not to have padding. That's essentially what memory-mapped IO gives you out of the box.

https://stackoverflow.com/a/5398498

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