------- Comment #14 from rguenther at suse dot de 2009-01-29 09:19 -------
Subject: Re: [4.4 regression] Unexplained "'<anonymous>' is
used uninitialized in this function" warning in cc1plus -m64
On Wed, 28 Jan 2009, mmitchel at gcc dot gnu dot org wrote:
> ------- Comment #13 from mmitchel at gcc dot gnu dot org 2009-01-28 23:56
> -------
> Actually, CLASSTYPE_EMPTY_P is probably a fine thing to use for C++. (It's of
> course C++ specific; you'd either need to access it via a hook, or promote to
> a
> language-independent bit.) CLASSTYPE_EMPTY_P will not capture an array of
> empty objects, but that's an extreme corner-case.
>
> Note that CLASSTYPE_EMPTY_P classes may have arbitrary size. That's because
> of
> things like:
>
> struct A{};
> struct B : public A {};
> struct C : public A, public B {};
>
> In C, you cannot put the B sub-object at the same address as the A sub-object
> since that would end up with two A sub-objects (the A-in-B-in-C subobject and
> A-in-C subobject) at the same address. So, C will be a two-byte structure.
> Obviously, you can generalize this to make arbitrarily huge empty classes.
Ok. But, as opposed to inheritance, inserting empty members seems to
make a class non-empty:
struct A {};
struct B { A x; };
even if in the single-member case I would have expected it to behave
like the single-inheritance case really. Note that cp_expr_size boils
down to checking CLASSTYPE_EMPTY_P already, but in the member case
doesn't seem to map to "does not need initialization".
Richard.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38908
