On 27 August 2012 20:49 Paul Koning wrote: > > On Aug 27, 2012, at 3:33 PM, Jonathan Wakely wrote: > >> On 27 August 2012 19:48, Paul_Koningwrote: >>> I'm doing some checking of data structure layouts in different releases of >>> our code -- which were produced by different releases of GCC (3.3.3 vs. >>> 4.5.4). >>> >>> One difference I'm seeing that is puzzling is in the handling of base >>> classes. Specifically, the case where a base class has padding at the end >>> to fill it out to a multiple of the alignment. >>> >>> In GCC 3.3.3, when such a class is used as a base class, that padding is >>> omitted, and the first derived class data member starts right after the >>> last base class real (not pad) data member. In GCC 4.5.4, the base class >>> is used padding and all, the first derived class data member starts after >>> the padding of the base class. >> >> This depends on whether the base class is a POD or not. >> >> According to a note in the Itanium C++ ABI "the C++ standard requires >> that compilers not overlay the tail padding in a POD" (I don't know >> off the top of my head where that is stated in the standard.) >> >>> Which is correct? Or are both correct? This sort of thing is a potential >>> cause of trouble if such a class is used as a container for persistent data. >> >> GCC 3.4 and later conform to the Itanium C++ ABI, which specifies the >> behaviour you're seeing as required by the C++ standard, so 4.5 is >> correct. > > Interesting. What if the base class is not a POD? It doesn't seem to be, if > I remember the definition of POD correctly.
G++ 3.4 and later can, and will, reuse the tail padding in a non-POD.
