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Re: static_cast signed to unsigned

 
 
Siemel Naran
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      04-01-2004
"Kevin Goodsell" <(E-Mail Removed)> wrote in message
news:_sEac.9204

> > To make it safe, I think you can: make the function take pointer types

so
> > that Src is deduced to char rather than char*, use Andrei's trick to

enforce
> > that a conversion from Src to Dest exists.
> >
> > template<class Dest, class Src>
> > Dest * pointer_cast(Src * src)
> > {
> > typedef char[sizeof(Overload<Dest>::conversion(Src()))-1]
> > AssertConversion;
> > return static_cast<Dest *>(static_cast<void *>(src));
> > }
> >
> > where
> >
> > template <class Dest>
> > struct Overload
> > {
> > char (&conversion(...))[1];
> > char (&conversion(Dest))[2];
> > };


Josh's way is more succint.


> Would this actually be safe?
>
> Just so we're clear, the purpose that I had in mind for this cast
> operator is to allow conversion of a pointer of type Src* to Dest* if
> and only if that conversion is reasonable. How we define "reasonable"
> may be different, but what I had in mind was pointer conversions that
> are guaranteed to give well-defined results -- for example, Any* to
> void*, Any* to unsigned char*, maybe Any* to char*. Also T* to T* would
> probably be good to allow. Same with Derived* to Base*. But the main
> purpose I had in mind was for situations where you have a buffer that
> needs to be passed to a function, and that function expects a different
> type buffer (like char when you have unsigned char).


I don't think Any* to unsigned char * is reasonable. The buffer thing you
mention is one I use myself, and I used reinterpret_cast (there were public
functions for short&, bool&, double&, etc, which called a private function
to do the reinterpret_cast, so I think it's pretty safe). I guess the
newsgroup is the place to try new things .


> Anyway, isn't it a mistake to assume that the existence of a conversion
> from Src to Dest implies a conversion from Src* to Dest*? For example,
> wouldn't this allow int* to be converted to double*?


Right, it is incorrect. Can't think of the correct way, or if there is one,
short of specialization.


 
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Kevin Goodsell
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      04-01-2004
Siemel Naran wrote:

>
> I don't think Any* to unsigned char * is reasonable.


Forgive me if I'm incorrectly applying a C rule, but I believe the
standard allows inspection of any object as an array of unsigned char.
unsigned char has no trap representations (not the term used for it in
the C++ standard, but a convenient term nonetheless), so it can't give
undefined behavior just by "looking". Modifying the bit-sequence then
attempting to use it as the original type is a different matter, though.

But since you mentioned it, it occurs to me that doing this kind of
thing for a non-POD type might not be very useful, and probably not a
very good idea in most circumstances.

Even so, what I have in mind could replace reinterpret_cast for its most
commons uses, and should be safer than reinterpret_cast.

Of course you can just use reinterpret_cast instead... I'm just not
comfortable with it at all.

-Kevin
--
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