Using M-; in C++-mode

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I'm new to C++ mode but a long time user of Emacs
for coding other languages.

In C++ mode C-h c M-; always replys comment-dwim
(like it does in all my programming modes)
and M-; usually produces a // style comment.

However earlier today something changed.
M-; started producing /*  */ style comments.
Upon restarting Emacs it went back to // style.

What caused the switch.
There are times when I'd like to do this purposely!

Thanks