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

From: Marco (mrcekets_at_[hidden])
Date: 2007-10-04 18:13:31

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).

Regards,
Marco Cecchetti

On Thu, 04 Oct 2007 22:15:33 +0200, Michael Fawcett  
<michael.fawcett_at_[hidden]> wrote:

> On 10/4/07, Phil Endecott <spam_from_boost_dev_at_[hidden]> wrote:
>> Michael Marcin wrote:
>> >
>> > Have a look at:
>> >
>> > http://www.gamedev.net/community/forums/topic.asp?topic_id=261920
>>
>> Yes, a very good page showing several solutions:
>>
>> 1. A vector of member pointers (may have some overhead).
>> 2. Anonymous structs and unions (may be non-standard).
>> 3. x, y and z implemented as references (makes object bigger).
>> 4. Relying on (&x)[1] pointing to y.
>
> For actual asm output, see
> http://www.gamedev.net/community/forums/topic.asp?topic_id=230443&PageSize=25&WhichPage=2
>
> for the "overhead" of option 1.
>
>> _IF_ 4 really is guaranteed to work portably (as its author claims), it
>> would get my vote:
>>
>> template <typename T>
>> struct point2d {
>>    T x, y;
>>    T& operator[](int i) { return (&x)[i]; }
>> };
>>
>> ...what's the catch?
>
> I'm almost positive the author was wrong in claiming that it is
> portable.  Here's an old thread about the issue:
>
> http://groups.google.com/group/comp.lang.c++.moderated/browse_frm/thread/df5bc148a994d511/36c133bbb606bece?lnk=gst&q=point+operator%5B%5D+x+y+z&rnum=6#36c133bbb606bece
>
>
> --Michael Fawcett
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