Maybe he's referring to how identifiers with leading underscores followed by a capital letter are reserved in C++ (I'm not sure if they are in C)...

Oho. Followed by a capital letter? Interesting.

I always thought that "all identifiers with a leading underscore were reserved". I just consciously ignored it, and have never had a problem in years. But I was also always using member variable names like "_children", "_childCount", etc, not "_Children".

In fact, here's a reference: http://msdn.microsoft.com/en-us/library/e7f8y25b(v=vs.80).as... (It's in a "Microsoft Specific" block but says it's part of the ANSI C standard.) It applies to C, and it also applies to identifiers staring with two underscores.

The C Standard reserves identifiers starting with a leading underscore for the system implementation (C standard library, Posix libraries, etc). They aren't meant for use by user code (or rather, you can use them, but they might conflict with system defined identifiers).