On 09/05/13 19:10, John McCall wrote:
On May 9, 2013, at 9:35 AM, Richard Earnshaw <[email protected]> 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.
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