On 8 February 2016 at 18:31, H.J. Lu <hjl.to...@gmail.com> wrote:
> On Mon, Feb 8, 2016 at 10:30 AM, Jonathan Wakely <jwakely....@gmail.com>
> wrote:
>> On 8 February 2016 at 18:26, Jonathan Wakely <jwakely....@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....@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?
I don't know what the definition of "POD for the purposes of layout"
is, but if I was trying to define a better name for it I would start
by trying to understand how it is specified, instead of just throwing
existing terms together.
> 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?
Yes :-)
But that was already confusing when you called it an "empty
collection" because an array isn't a collection.
If I understand correctly, your proposal says that given:
struct A { }; // empty class type
typedef A A2[2]; // array of empty class types
struct B { A a[2]; }; // empty record?
struct B is an empty record ... is that right?
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