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

From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2007-10-04 18:40:24

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