----Original Message---- From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Andy Little Sent: 07 March 2006 15:14 To: boost@lists.boost.org Subject: Re: [boost] [bitfield] Initial bitfield proposal available inthevault
"Martin Bonner" <martin.bonner@pitechnology.com> wrote in message news:D997BF79D1E92C4793B7FCC04B4F90A51D79B6@pigeon.pi.local...
----Original Message---- From: Emile Cormier
The bitfield mechanism relies on this assumption: Unions of non-polymorphic, non-derived objects, having the exact same underlying data member type, will have the same size as this underlying data member type. I'm no language lawyer, so please let me know if this is a safe and portable assumption.
I'm not quite sure what you mean, but given: struct a { unsigned char ch; }; struct b { unsigned char ch; }; union u { a theA; b theB }; then you are not guaranteed that sizeof(u) == sizeof(unsigned char).
Though in practise you can use:
BOOST_STATIC_ASSERT(sizeof(u) == sizeof(unsigned char))
My point was exactly that you CANNOT use that. (On a certain class of machine).
On word addressed machines (which /are/ still being built), it is almost certain that the minimum size for a struct is a complete word. This is because the C and C++ standards effectively promise that pointers to structs are all of the same size (the size of a pointer-to-struct does not depend on the contents of the struct). It is desirable that a pointer-to-struct be the smaller, cheaper-to-dereference pointer to word (rather than the larger more-expensive-to-dereference pointer to char), so the smallest struct has to occupy a whole word.
I dont see why the size of a pointer to a struct affects the size of a struct which in the case of an empty struct is often 1 byte?
I don't think you have understood what a word addressed machine is! On most modern archictectures there are 8 bits stored at (for example) 0x100 and another 8 bits at 0x101. The 32 bits at 0x100 cover 0x100, 0x101, 0x102, and 0x103. On a word addressed machine, there may be 36 bits stored at 02000 and another (different) 36 bits stored at 02001. A simple 36-bit pointer can address individual words, but not sub-units within those words. To address individual bytes, you need a double-word pointer. One word identifies the word, and a few bits within the second word identifies which byte you are addressing. On such a machine, it makes sense for an empty struct to occupy a whole word (which is four nine-bit bytes), so that a pointer to struct can (always) be a single word pointer. -- Martin Bonner Martin.Bonner@Pitechnology.com Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ, ENGLAND Tel: +44 (0)1223 203894