Re: [boost] [GTL] - geometric template library - determining interest

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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