Writing unsigned char to std::ostream

David Wilkinson <no-reply@effisols.com>
Sun, 02 Sep 2007 12:37:44 -0400
In my MFC project I have a piece of code like

void XMLHelper::WriteHeader(std::ostream& ostrm)
   ostrm.put(unsigned char(0xEF)); // 1
   ostrm.put(unsigned char(0xBB));
   ostrm.put(unsigned char(0xBF));
   ostrm << "<?xml version=\"1.0\" encoding=\"UTF-8\" ?>\n";

where (if it matters) ostrm is opened in text mode. This code compiles
and (I think) runs as I intended on VC6-VC9.

However, I have been playing around with VMWare and Linux (Ubuntu) and
got it into my head to compile the non-GUI classes in my app using g++
(version 4.1.2). It won't compile.

I now see that my code was wrong (undefined behavior), because it
requires conversion of unsigned char to char, which is
implementation-defined (I just learned).

But this is not why it fails to compile on g++. 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

The same thing happens on Comeau (1 fails, 2 and 3 compile).


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

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";


David Wilkinson
Visual C++ MVP

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