Re: [boost] offset_cast proposal

From: Terje Slettebø (tslettebo_at_[hidden])
Date: 2002-11-24 10:59:45

>From: "Pavol Droba" <droba_at_[hidden]>

> On Sun, Nov 24, 2002 at 01:26:05PM +0100, Terje Sletteb? wrote:
> > >From: "Pavol Droba" <droba_at_[hidden]>
> >
> > > I have developed a simple cast function which I found very useful.
Here it
> > is:
> > >
> > > template< typename T >
> > > inline T offset_cast( void* p, unsigned int  offset=0 )
> > > {
> > > return reinterpret_cast< T >( static_cast<unsigned char*>(
p )+offset );
> > > }
> > >
> > > template< typename T >
> > > inline T offset_cast( const void* p, unsigned int offset=0 )
> > > {
> > > return reinterpret_cast< T >( static_cast<const unsigned char*>(
> > p )+offset );
> > > }
> > >
> > > Its purposes is to simplify a mapping of C-like structure onto binary
> > data. Here is
> > > the example: Assume thath you have a network packet in raw format, and
you
> > want
> > > to read IP-header information. You could do following:
> > >
> > > ip_header* ip=offset_cast<ip_header>( packet, ip_header_offset );
> > >
> > > instead of ugly pointer arithmetic.
> >
> > Uhm, I think pointer arithmetic could be preferrable to casts,
particularly
> > reinterpret_cast, as pointer arithmetic is at least not
> > implementation-dependent, unlike the reinterpret_cast.
>
> I don't see you point here. I'm not sure why whould be reinterpret_cast
> implemetation dependent and pointer aritmetic not?

Pointer arithmetic, in itself, isn't implementation-dependent. You said that
your example was an alternative to pointer arithmetic. However, is it
possible to do the above with pointer arithmetic, only, and no casts? In
other words, what is the alternative you compare to? [Addition: I see you
reply to this below here]

> > Also, I find nothing ugly about pointer arithmetic - in fact, STL is
based
> > on it, except that it's abstracted as "iterators". int array[10];
> > std::for_each(array,array+10,f) is pointer arithmetic. Do you find that
> > ugly, as well?
>
> I'm sorry, you are right, ugly is not a good term here. However the
purpose
> of the offset cast is not to avoid pointer arithemtic at all cost.
> Ist purpose is to simplify mapping of structures over memory buffers.
>
> Problem here is to shift a pointer n-bytes forward. You can do it
> with pointer arithmetic, but still you have to cast the pointer to
something
> what have size of 1 otherwise you will not shift it n-bytes but
n*sizeof(type).
>
> So offset_cast handles this for you. It is merely a shorcut, which could
be
> very handy if you are working with raw data.

Ok, I understand the purpose of it, now. The purpose is to encapsulate the
pointer arithmetic and casts, into a component. However, as also Dave
pointed out, it's implementation-dependent whether it would work or not. It
depends on things like alignment, and any padding in the class you try to
extract (such as ip_header, above here). As I understand, it's only safe as
long as the data was the original type to begin with, rather than a plain
char buffer.

Regards,

Terje