"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))
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? regards Andy Little