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.
paul
