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@gmail.com> wrote:
On 10/4/07, Phil Endecott <spam_from_boost_dev@chezphil.org> 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:
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