Re: Writing unsigned char to std::ostream

Ulrich Eckhardt <>
Mon, 03 Sep 2007 11:12:29 +0200
David Wilkinson wrote:

David Webber wrote:

"David Wilkinson" <> wrote in message

  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!

My personal gut feeling is that Comeau and g++ are both correct and MSC is
jumping through some loops for user-convenience.

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";

The static_cast is the same as the former function-style cast, but it works
correctly and portably even with the space in the type's name.


  unsigned char const utf8_bom[] = { 0xef, 0xbb, 0xbf, 0};
  ostrm << utf8_bom << header << std::endl;

Surprisingly, there are in fact overloads for signed and unsigned char in

My instinct for clarity and security would be

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

Well, that's the same as a static_cast, except that C-style casts are
frowned upon.

Alternatively, how about

typedef unsigned char __uint8;

ostrm.put( __uint8(0xEF) );

This is really bad advise. Anything with two consecutive underscores is
reserved and you should never create symbols in that namespace and always
be careful when using them, because their meaning is typically

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.

Wasn't that implementation-defined? Implementation-defined is something I'll
live with, but undefined is something I'd rather avoid...

My immediate instinct was to change the "function-style cast" to a
"C-style cast".

Never use C-style casts in C++, they only serve to hide broken code and move
the error detection from compile-time to runtime.

This certainly compiles, but may not run as intended on
some systems (in VC it works I think).

Well, that's what unittests are for.


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