enum while preprocessing

[Alex Vinokur]

A program below prints "Second".
A preprocessor doesn't know BBB,
but it seems that it (preprocessor) uses 0 unstead of BBB.
Why doesn't the preprocessor produce error/warning: "BBB is indefined"?


====== foo.c ======
#include <stdio.h>

#define AAA 1

enum
{
BBB = 2
};


int main ()
{
#if (BBB > AAA)
printf ("First\n");
#else
printf ("Second\n");
#endif
return 0;
}
===================

Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn

 

[Richard Bos]

A program below prints "Second".
A preprocessor doesn't know BBB,
but it seems that it (preprocessor) uses 0 unstead of BBB.
Why doesn't the preprocessor produce error/warning: "BBB is indefined"?

Because converting unknown preprocessing identifiers to 0 is what it's
supposed to do. RTFS. From C99, 6.10.1#3: "After all replacements due to
macro expansion ... all remaining identifiers are replaced with the
pp-number 0".

Richard