From: Marco (mrcekets_at_[hidden])
Date: 2007-10-05 08:08:56
On Fri, 05 Oct 2007 00:40:24 +0200, Giovanni Piero Deretta
<gpderetta_at_[hidden]> wrote:
> On 10/5/07, Marco <mrcekets_at_[hidden]> wrote:
>> About portability this is what the standard (9.2/12) says :
>>
>> > Nonstatic data members of a (non-union) class declared without an
>> > intervening /access-specifier/ are allocated so that later members
>> have
>> > higher addresses within a class object. The order of allocation of
>> > nonstatic data members separated by an /access-specifier/ is
>> > unspecified. Implementation alignment requirements might
>> > cause two adjacent members not to be allocated immediately after each
>> > other; so might requirements for space for managing virtual functions
>> > and virtual base classes.
>>
>> as recently pointed out to me by Sebastian Redl.
>>
>> > template <typename T>
>> > struct point2d {
>> > T x, y;
>> > T& operator[](int i) { return (&x)[i]; }
>> > };
>>
>> In the struct point2d there is no virtual functions,
>> between data members there is no intervening access specifier,
>> so if sizeof(T) is a multiple of the used alignment the standard
>> guarantes
>> that such code is valid,
>> and this condition is verified for T = int, long, float, double using
>> the
>> default alignment (4 bytes).
>>
>
> I do not think that the standard guarantees the lack of padding
> between 'x' and 'y'.
> IIRC You are only guaranteed that &x < &y, not that &x +1 == &y.
>
> gpd
IMO, it's a subtle question. Formally you're right about what standard
guarantees, but in the above struct members x, y have the same type T, so
data alignment require no padding at all. What should be put between x and
y ? IMO padding is required to manage data alignment. Are there other
cases where padding is needed ?
Regards,
Marco
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