Re: [boost] offset_cast proposal

From: Terje Slettebø (tslettebo_at_[hidden])
Date: 2002-11-24 17:18:10

> On Sun, Nov 24, 2002 at 07:27:42PM +0100, Terje Sletteb? wrote:
> > >From: "Pavol Droba" <droba_at_[hidden]>
> > >
> > > Reinterpret-cast could be avoided like this:
> > >
> > > template< typename T >
> > > inline T* offset_cast( void* p, unsigned int  offset=0 )
> > > {
> > >   return static_cast<T*>(
> > > static_cast< void* >( static_cast<unsigned char*> p )+offset )
> > > );
> > > }
> >
> > Yes, but that wouldn't make it any more safe, as it would still depend
on
> > the alignment requirements, etc. that you mention.
>
> Alignment dependency is quite natural here. I'm talking about analysis of
binary data.
> I don't see a way how to map a c-like structure onto binary data if you
don't know what's
> the bineary representation of the structure and if if does not match the
required
> pattern.

What you say here is one of the aspects, the layout in the struct, and any
internal padding (for internal alignment). However, that's not all. Even if
you have the same alignment between the components in the struct, as in the
buffer, you still have the "external alignment" to consider, that is, if the
struct, in the buffer, starts at the right alignment. If the struct was
written to the same buffer, then it will be correct. However, if the buffer
is read from an external source, like a stream, then that is far from
guaranteed.

For example, if you have a buffer like this (starting at an aligned
address):

+0 - One byte
+1 - Number 1
+5 - Number 2
+9 - Number 3

Even if you have a struct with three 32-bit ints, no internal padding, if
you try to map it to the above buffer (at offset 1), on a platform that
requires 4-byte alignments for ints, it won't work.

Regards,

Terje