Re: pointer casts(newbie)

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Mon, 09 Feb 2009 17:01:55 +0100

Message-ID:

<gmpjts$r6m$1@news.motzarella.org>

* Kai-Uwe Bux:

James Kanze wrote:

On Feb 8, 10:25 pm, Kai-Uwe Bux <jkherci...@gmx.net> wrote:

eminha...@googlemail.com wrote:

would it be possible to cast pointers to any other type of pointer?
for example:
int * a cast to string * b?

Yes, but you could not do anything with the result except for
casting it back. More precisely, the standard guarantees that
when you cast back to the original type, you get the original
value. No other guarantees are made.

That's not quite true.

Right. As I said, in the part you snipped:

That is only an approximation. The precise rules depend on the pointer
types involved and on the cast used.

Note the dependence on the pointer type.

If you cast it to a character type, you
can access the individual bytes. Modifying them will result in
undefined behavior if you later try to use the object with its
original type, but something like:

    template< typename T >
    void
    dump(
        std::ostream& dest,
        T const& obj )
    {
        IOSave saver( dest ) ;
        dest.setf( std::ios::hex, std::ios::basefield ) ;
        dest.fill( '0' ) ;
        unsigned char const*current
            = reinterpret_cast< unsigned char const* >( &obj ) ;
        unsigned char const*end = current + sizeof( T ) ;
        while ( current != end ) {
            dest << std::setw( 2 ) << *current ;
            ++ current ;
            if ( current != end ) {
                dest << ' ' ;
            }
        }
    }

is sometimes useful (and has fully defined behavior).

Nice. I agree that it is sometimes useful to read the underlying bytes of an
object.

IOSave ???

Well, the most practically useful exception to the general rule (for
reinterpret_cast) is to cast from pointer to first member in POD struct, to
pointer to that POD struct, and vice versa.

This allows a kind of C-like simulation of inheritance.

I don't recall exactly how well-defined it is. Perhaps it was defined as
implementation defined but with guideline. But anyway it's practically
well-defined, due to the need to support C compatibility.

Cheers,

- Alf