why would std::string s("??<") output a { instead of ??< as expected?

Question

std::string s("??<");
std::cout << s << std::endl;

Why does that output { instead of ??<

I'm using Visual Studio 2008. I'm assume it's encoding it but why and what is the encoding called if that is what's happening?

This little %#$^*! caused me to look for a bug in my (unit test) code for 30 minutes before I figured out my string was mangled!! :(

Answer 1

Because of trigraphs.

These are the supported trigraphs, from the Wikipedia page:

• ??= → #
• ??/ → \
• ??' → ^
• ??( → [
• ??) → ]
• ??! → |
• ??< → {
• ??> → }
• ??- → ~

For Visual Studio, according to the documentation trigraphs are turned off by default (sensibly enough), so check your project/makefiles.