[lex.comment]

The characters /* start a comment, which terminates with the characters */.

These comments do not nest.

The characters // start a comment, which terminates immediately before the next new-line character.

[Note 1: 

The comment characters //, /*, and */ have no special meaning within a // comment and are treated just like other characters.

Similarly, the comment characters // and /* have no special meaning within a /* comment.

— end note]