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.
