Re: Writing unsigned char to std::ostream

From:

David Wilkinson <no-reply@effisols.com>

Newsgroups:

microsoft.public.vc.language

Date:

Sun, 02 Sep 2007 13:13:28 -0400

Message-ID:

<Ojk$USY7HHA.5160@TK2MSFTNGP05.phx.gbl>

David Webber wrote:

"David Wilkinson" <no-reply@effisols.com> wrote in message
news:OLC3W%23X7HHA.4660@TK2MSFTNGP02.phx.gbl...

...
ostrm.put(unsigned char(0xEF)); // 1
...
It fails due to the use of unsigned char as a type in this context.
Both of the following compile correctly (not sure about running):

ostrm.put(unsigned(0xEF)); // 2
ostrm.put((unsigned char)0xEF); // 3

My 2d-worth: I'd have used the last, or maybe even

ostrm.put( (unsigned char)(0xEF) );

just because it looks like stretching things to destruction to have a
function-style cast with an apparent "function name" containing a space.

1. Is this a parsing bug in g++/Comeau?

If it is, it's not one which surprises me!

> 2. Do you think the following is correct and portable?

void XMLHelper::WriteHeader(std::ostream& ostrm)
{
  ostrm << static_cast<unsigned char>(0xEFU);
  ostrm << static_cast<unsigned char>(0xBBU);
  ostrm << static_cast<unsigned char>(0xBFU);
  ostrm << "<?xml version=\"1.0\" encoding=\"UTF-8\" ?>\n";
}

Not sure - but it looks ugly with all those <<<< :-)

My instinct for clarity and security would be

ostrm.put( (unsigned char)(0xEF) );

Alternatively, how about

typedef unsigned char __uint8;

ostrm.put( __uint8(0xEF) );

Dave:

I think the trouble with your suggestions (and my original code) is that
ostream::put() takes a char argument, and conversion from unsigned char
to char is undefined behavior.

My immediate instinct was to change the "function-style cast" to a
"C-style cast". This certainly compiles, but may not run as intended on
some systems (in VC it works I think).

--
David Wilkinson
Visual C++ MVP