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Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct


On Mon, Feb 8, 2016 at 10:30 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> On 8 February 2016 at 18:26, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>> On 8 February 2016 at 17:58, H.J. Lu wrote:
>>> On Mon, Feb 8, 2016 at 7:59 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>>>>>> A type is a standard-layout type, or it isn't.
>>>>>
>>>>> How about "An empty record is standard-layout Plain Old Data (POD)
>>>>> type and ..."?
>>>>
>>>> That's redundant, all POD types are standard-layout types.
>>>>
>>>
>>> Apparently, not all standard-layout types are POD types.  GCC has
>>>
>>> /* Nonzero means that this class type is not POD for the purpose of layout
>>>    (as defined in the ABI).  This is different from the language's POD.  */
>>> CLASSTYPE_NON_LAYOUT_POD_P
>>>
>>> and
>>>
>>> /* Nonzero means that this class type is a non-standard-layout class.  */
>>> #define CLASSTYPE_NON_STD_LAYOUT
>>>
>>> They aren't the same.
>>>
>>> struct A { };
>>> struct B { };
>>> struct C : A, B { };
>>>
>>> C is a standard-layout type, but not a standard-layout POD type.
>>
>> As the comment says, "POD for the purposes of layout" is different
>> from the language's POD. All standard-layout types are POD types
>> according to the language.
>>
>> So when you previously had "POD for the purposes of layout" that was
>> at least partially clear that you meant something other than what the
>> language means. But as pointed out, using a GCC-specific term is not
>> ideal.
>>
>> When you changed it to "POD for the purpose of standard-layout" that
>> became a completely meaningless term. Where is that defined?
>>
>> Your next suggestion was "standard-layout Plain Old Data (POD)" which
>> is even worse, now you're using two terms defined by the C++ language,
>> but you mean something different.
>>
>> When you mean something that is the same as the language (like "class
>> type") it makes sense to use the same term.
>>
>> When you mean something that is not the same as the language (like
>> "POD") it makes sense to use a different term, or clearly define how
>> you are using it.
>
> To be clear: it's really confusing to take two terms defined by the
> language, "POD" and "standard-layout", and smash them together to mean
> something new.
>
> According to your proposal, struct C is a POD type, and  a
> standard-layout type, but not a "standard-layout POD type". That's
> just crazy.

Can you suggest a better wording?

Another issue, if I define

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
   c. An array of empty class types.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Will it confuse people?


-- 
H.J.


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