On 10/07/10 06:22, Stewart, Robert wrote: [snip]
I was suggesting that this:
struct P { short s; char c; short t; char d; };
could occupy less space than if the chars and shorts were reversed. That is, that P::c could occupy padding between P::s and P::t and P::d could occupy padding between a P instance and something following it in another composite. Doing so would not violate an ABI, unless the ABI specifically disallowed it, because it can be established as the expected layout in those cases.
The code: https://svn.boost.org/trac/boost/browser/sandbox /variadic_templates/libs/composite_storage /sandbox/layout_all.1.cpp produces output shown in the attachment. tagPs is with char's first, Ptags is iwht char's 2nd. Both sizes are same: sizeof(tagPs)=8 sizeof(Ptags)=8 the other output shows the alignments and offsets of each component was well as the padding. The offsets are listed starting from last to first element in the structs. the output prefixed with layout0 shows the layout for an empty struct. HTH. -Larry