This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
Class types may have padding, influencing the result of sizeof. It is unclear whether the placement and amount of padding is implementation-defined, unspecified, or something else. If it is unspecified, the limits of permissible behavior are unclear. Empty classes might need special consideration.
Change in 220.127.116.11 [expr.sizeof] paragraph 2
... When applied to a class, the result is the number of bytes in an object of that class including any padding required for placing objects of that type in an array. The result of applying sizeof to a potentially-overlapping subobject is the size of the type, not the size of the subobject. [ Footnote: ... ]