[cxx-abi-dev] Empty Classes and data layout

[Richard Earnshaw]

On 09/05/13 19:10, John McCall wrote:
> On May 9, 2013, at 9:35 AM, Richard Earnshaw <rearnsha at arm.com> wrote:
>> We've been looking into a defect that's been raised on our C++ Binding
>> for ARM relating to the handling of empty classes, that is, something like
>>
>> struct S {};
>>
>> In C this is not legal, and our C ABI defines no mechanism for passing
>> such an object as a function parameter.  However, in C++ this is valid
>> and at least at first reading matches the definition of a POD class.
>>
>> However, the C++(98) states (clause 9 [class] para 3) that complete
>> objects and member sub-objects of class type have non-zero size; which
>> means that such a class, despite matching the GC++ABI rule for a POD for
>> the Purposes of Layout definition fails to meet the condition in clause
>> 2.2 that "All of these types have data size and non-virtual size equal
>> to their size", since the size is one, but the data-size is zero.
>
> This isn't a condition, it's a statement:  it specifies that the data size and
> non-virtual size of these types is equal to their size.  This is necessary
> because those values are otherwise computed by the layout algorithm,
> and, well, we don't run the layout algorithm on types that are POD for
> the purposes of layout.
>

But that's my point, an Empty class doesn't have it's size equal to it's 
data size, since the latter is zero, and the former must be non-zero. 
However, it does currently fit the rule of being POD for the purpose of 
Layout, so there's a contradiction in the specification.

R.

> The data size and non-virtual size of an empty class are ignored by the
> layout algorithm.  When laying out a data member, only sizeof(D) and
> alignof(D) are ever considered.  When laying out an *empty* base class,
> only nvalign(D) (assumed to be 1) is ever considered.
>
> The empty base class optimization applies to types regardless of whether
> they're POD for the purposes of layout.  Being POD only affects the sizeof
> vs. dsize/nvsize distinction.  This is permitted because base class
> subobjects are explicitly permitted to have zero size, and the rules for
> copying in and out of them are different.
>
> I agree that a clarification is in order because the base C ABI doesn't
> necessarily specify a layout for empty classes.  We should specify that
> empty classes have a size of 1 by definition.
>
> John.
>
>