Re: [cxx-abi-dev] What is a POD? TC1 or first C++ Standard

[Nathan Sidwell <nathan@xxxxxxxxxxxxxxxx>]

Mark Mitchell wrote:
> Dennis Handly wrote:
>
> > > GCC is not going to change back to the pre-TC1 definition, even for
> > > return values
> > >   
> >
> > What do you mean by "return values"?  I said it only affects layout.
> > I first thought it also affects them but 3.1.4 Return Values, says 
> > nothing
> > about PODs.
> >  
> You are correct.  I thought it said that non-PODs could not be 
> passed/returned in registers, but, in fact, it is more specific: it 
> talks about non-trivial copy constructors and destructors.
> That makes things somewhat simpler.
>
> It seems to me that a POD with a pointer-to-member data member should 
> not be a "POD for purpose of layout" because the layout of a "POD for 
> the purpose of layout" is supposed to be whatever the C ABI would 
> require -- and the C ABI does not specify the layout of a type 
> containing a pointer-to-member.
> Now, the question is, when would this make a difference?  Dennis, can 
> you post a small test case showing where the layout is different 
> depending on whether you use the TC1 or pre-TC1 definition of POD?
I would have thought it'd change when the tail padding of such a class could
be used by a derived class. Something like,

struct A {
  int A::*ptr;
  char f;
};

struct B : A {
  char g;
};

Where is B::g placed?

nathan

--
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
nathan@xxxxxxxxxxxxxxxx    ::     http://www.planetfall.pwp.blueyonder.co.uk