Re: char data to unsigned char

From:

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

Newsgroups:

comp.lang.c++

Date:

Sun, 02 May 2010 03:54:02 +0200

Message-ID:

<hrim3l$8si$1@news.eternal-september.org>

On 02.05.2010 03:43, * Jonathan Lee:

OK stupid question time.

What's the proper way to process char data as unsigned char? A lot of
the things I write are only defined on "raw" data. Like, huffman
decoding, or block ciphers. But a lot of this data comes from files or
strings which are char sources. The way I've been handling this until
now is to simply reinterpret_cast<> a pointer to a char buffer into an
unsigned char pointer. Like

   // convenience
   void encrypt(const char* data, std::size_t n) {
     encrypt(reinterpet_cast<const unsigned char*>(data, n);
   }

   // "real" function
   void encrypt(const unsigned char* data, std::size_t n) {
     ..
   }

I don't see any guarantee in the standard that this will work, and
it's bugging me.

Don't let it. There's no formal guarantee about assigning back to char, but (1)
you're probably not doing any assigning back to plain char, and (2) that lack of
formal guarantee is just in support of sign-and-magnitude char's on the ENIAC
(some member of the C++ committee fancies the ENIAC). Nobody uses the ENIAC any
more, and besides, there's no C++ compiler for that machine.

So... really what should I be doing?

Exactly what you're doing. :-)

Well, except that I'd prefer to use a signed size spec instead of unsigned, but
hey, that's not something that I can say you "should" be doing, just that
avoiding mixing signed and unsigned in expressions can save a lot of work.

Cheers & hth.,

- Alf