On Aug 27, 2012, at 4:05 PM, Gabriel Dos Reis wrote:
> On Mon, Aug 27, 2012 at 1:48 PM, <[email protected]> wrote:
>> 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.
>>
>> 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.
>>
>> paul
>>
>
> Is this message
>
> http://gcc.gnu.org/ml/gcc/2002-08/msg00874.html
>
> relevant to your case?
>
> -- Gaby
Yes, that looks like the exact case. And the mail thread seems to say that the
"3.3.3" behavior I'm seeing is what G++ was doing at that time, as was HP --
but not Intel. So now we have it done differently in later compilers.
I know this is changing data structure layouts in our code; I don't know yet if
that is a problem (i.e., if it applies to layouts used in persistent data or in
protocol messages). I assume there isn't some compiler switch I can use to
control this behavior?
paul
