On Mon, Feb 8, 2016 at 2:54 PM, H.J. Lu <hjl.to...@gmail.com> wrote: > On Mon, Feb 8, 2016 at 2:51 PM, Richard Smith <rich...@metafoo.co.uk> wrote: >> On Mon, Feb 8, 2016 at 2:46 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >>> On Mon, Feb 8, 2016 at 2:35 PM, Richard Smith <rich...@metafoo.co.uk> wrote: >>>> 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. >>> >>> Like this? >>> >>> 1. "class type". A class type is a structure, union or C++ class. >>> 2. "empty type". An empty type is >>> a. A type where it and all of its subobjects are of class or array >>> type. And >>> b. Either an array of empty types or a class type where every member >>> is of empty type. >> >> You don't need (b). It's implied by (a). > > Does (a) cover empty type?
Yes, (a) is a complete definition of "empty type" by itself: the definition is, essentially, that a complete recursive walk of the type and all its subobjects never sees anything that would require any storage (that is, it only sees class types and array types). _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
