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Asked: 2026-05-25T12:19:45+00:00

I’m working on a proprietary unix-like os (I don’t know if that’s relevant though)

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I’m working on a proprietary unix-like os (I don’t know if that’s relevant though) and compiling with g++.

I noticed recently that if I put xml-like tags in my C++ comments I get compiler errors. I don’t particularly need to do this, but I thought it was strange and I’d like to know why it’s an issue for the compiler. For example:

// <debugoutput>
std::cerr << "I'm debugging!" << std::endl;
// </debugoutput>

would cause massive compiler errors if it were in the middle of my code somewhere. Changing the last comment line </debugoutput> to <debugoutput> makes it compile fine though.

Does anyone know why the compiler would be confused by that line being in a comment? The compiler errors generated when this happens don’t seem related at all – they’re more like what you’d see if you missed the semi colon on the end of a class, undefined references to well defined classes, etc. I can’t paste the output from my dev system, but trust me that it doesn’t look related to the issue – its more like the compiler got confused.

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1 Answer

  1. Editorial Team

    This sounds suspiciously like a digraph related issue, but without the actual error message or a small code sample that exhibits the problem it’s hard to tell for sure.

    Try changing the whitespacing between the <, / and actual text, as well as try it within a C-style comment to see if that provides additional insight.

    For information on C/C++ digraphs and trigraphs see http://en.wikipedia.org/wiki/C_trigraph#C and also Purpose of Trigraph sequences in C++? and Why are there digraphs in C and C++? from SO.

    It seems possible that there is some sequence being picked up (for example </ as a digraph and it’s throwing off the compiler).

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