gcc.info: C++ Comments

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C++ Style Comments

   In GNU C, you may use C++ style comments, which start with `//' and
continue until the end of the line.  Many other C implementations allow
such comments, and they are likely to be in a future C standard.
However, C++ style comments are not recognized if you specify `-ansi'
or `-traditional', since they are incompatible with traditional
constructs like `dividend//*comment*/divisor'.