It's marginally more likely that your C tutor hadn't noticed that //-style comments are now C-style too, and imagined that Visual Studio permits it as some kind

It's marginally more likely that your C tutor hadn't noticed that //-style comments are now C-style too, and imagined that Visual Studio permits it as some kind of non-standard extension. (Correctly arguing he is right, then reasonably rebutting this argument, is an exercise for the reader.) This feature has only been with us as part of the standard since 1999, which in geological terms is merely the blink of an eye. I imagine this is why so many people seem not to know about it.