Is the backslash acceptable in C and C++ #include directives?

Question

There are two path separators in common use: the Unix forward-slash and the DOS backslash. _{^{Rest in peace, Classic Mac colon.}} If used in an #include dir

Answer 1

Blackslash is undefined behavior and even with a slash you have to be careful. The C99 standard states:

If the characters ', \, ", //, or /* occur in the sequence between the < and > delimiters, the behavior is undefined. Similarly, if the characters ', \, //, or /* occur in the sequence between the " delimiters, the behavior is undefined.