4 ms·

Well there is one way to nest comments in C, and that's by using #if 0: #if 0 This is a #if 0 nested comment! #endif #endif

• fanf2 2y ago
  Except that text inside #if 0 still has to lex correctly. (unifdef has some evil code to support using C-style preprocessor directives with non-C source, which mostly boils down to ignoring comments. I don’t recommend it!)
  • dahart 2y ago
    > Except that text inside #if 0 still has to lex correctly. Are you sure? I just tried on godbolt and that’s not true with gcc 14.2. I’ve definitely put syntax errors intentionally into #if 0 blocks and had it compile. Are you thinking of some older version or something? I thought the pre-processor ran before the lexer since always…
    • fanf2 2y ago
      There are three (relevant) phases (see “translation phases” in section 5 of the standard): • program is lexed into preprocessing tokens; comments turn into whitespace • preprocessor does its thing • preprocessor tokens are turned into proper tokens; different kinds of number are disambiguated; keywords and identifiers are disambiguated If you put an unclosed comment inside #if 0 then it won’t work as you might expect.
      • dahart 2y ago
        Ah, I see. You’re right!