In C/C++ Standard, the block comment can't nest.
But some old code has nested block comment.
2.7 Comments [lex.comment]
1 The characters /* start a comment, which terminates with the characters */.
These comments do not nest.
The characters // start a comment, which terminates with the next new-line
character. If there is a formfeed
or a vertical-tab character in such a comment, only white-space characters
shall appear between it and
the new-line that terminates the comment; no diagnostic is required. [Note: 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. ]