Symbol #

# Symbol is the special case.

• The # symbol is used only for preprocessor directives.
• It must appear at the start of a line (possibly preceded by whitespace).
• It is treated as a complete token and cannot be split or embedded in other tokens.

✅ Correct:

#include <iostream>
#define PI 3.14

❌ Incorrect:

int#x = 5; // Invalid: '#' cannot be used inside expressions