On Mon, Feb 8, 2016 at 1:40 PM, H.J. Lu <hjl.to...@gmail.com> wrote: > > On Mon, Feb 8, 2016 at 12:38 PM, Richard Smith <rich...@metafoo.co.uk> wrote: > > On Mon, Feb 8, 2016 at 12:05 PM, H.J. Lu <hjl.to...@gmail.com> wrote: > >> > >> On Mon, Feb 8, 2016 at 11:33 AM, Jonathan Wakely <jwakely....@gmail.com> > >> wrote: > >> > On 8 February 2016 at 19:23, Richard Smith wrote: > >> >> "POD for the purpose of layout" is defined in the Itanium C++ ABI here: > >> >> > >> >> http://mentorembedded.github.io/cxx-abi/abi.html#definitions > >> > > >> > Thanks. So there's no problem using "POD for the purposes of layout", > >> > and the change to "POD for the purpose of standard-layout" was > >> > unnecessary and just confused things. > >> > >> Here is the revised proposal: > >> > >> 1. "class type". A class type is a structure, union or C++ class. > >> 2. "empty class type". An empty class type is: > >> a. A class type without member. Or > >> b. A class type with only members of empty class types. Or > > > > > > (a) is a special case of (b). > > > >> c. An array of empty class types. > > > > > > It seems confusing to call an array a class type. Instead, how about: > > > > 2. An empty type is either an array of empty types or a class type where > > every member is of empty type. > > > >> 3. "empty record". An empty record is Plain Old Data (POD) for the > >> purposes of layout and > >> a. A class type without member. Or > >> b. A class type with only members of empty class types. > > > > > > (a) is a special case of (b). > > > >> 4. No memory slot nor register should be used to pass or return an object > >> of empty record. > > > > > > Objects of array type are never passed or returned (but if through some > > language extension they were, we'd want this rule to apply). So you don't > > need rule 3 and this can be just: > > > > 3. No memory slot nor register should be used to pass or return an object > > of empty type. > > Thanks very much for your inputs. Here is the proposal: > > 1. "class type". A class type is a structure, union or C++ class. > 2. "empty type". An empty type is either an array of empty types or a > class type where every member is of empty type. > 3. No memory slot nor register should be used to pass or return an object > of empty type.
David Majnemer points out that we also need to say something about base classes. We could handle that case like this: 2. "empty type". An empty type is a type where it and all of its subobjects are of class or array type. Following the C++ rules, this also means that a class that contains only unnamed bitfields is empty, because unnamed bitfields are not subobjects, but might be worth explicitly stating for the C case. That also matches Clang's behavior.
