RE: C++ 3.4.5 packed reference warning

["Steven L. Zook" <SLZook at Qualstar dot com>]

I guess what I don't understand is why struct A isn't POD. A reference
to something is basically just a pointer. It has no alignment
restrictions that a pointer wouldn't.

-----Original Message-----
From: gcc-owner@gcc.gnu.org [mailto:gcc-owner@gcc.gnu.org] On Behalf Of
Nathan Sidwell
Sent: Wednesday, December 07, 2005 8:03 AM
To: Steven L. Zook
Cc: gcc@gcc.gnu.org
Subject: Re: C++ 3.4.5 packed reference warning

Steven L. Zook wrote:
> When I compile this using gcc rev 3.4.5 (m68k-elf):
> 
> struct A { char            B;
>            unsigned char & C; } __attribute__((packed)); unsigned char

> D; A E = { 'F', D };
> 
> I get:
> 
> testpp.cpp:2: warning: ignoring packed attribute on unpacked non-POD 
> field `unsigned char&A::C'
> 
> 
> sizeof( E ) == 6 (B, a byte of pad since m68k uses 16 bit alignment, 
> and C).
> 
> This construct compiled without warning under 3.3.3 and gave sizeof( E

> ) == 5.
> 
> Is this behavior under 3.4.5 (both the warning and the ignoring of the

> packed attribute on the reference member) proper (justified, 
> necessary, good)?

It is bad to pack non-pod structs, because of alignment assumptions of
member functions thereof.  The checking code just checks the non-podness
of the type, as non-pod is a well defined term.  This could be relaxed.
File a bug report if needed.

nathan

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