What is the value of an undefined constant used in #if?
c++, c-preprocessor
Solution
Yes, it can be relied upon. The C99 standard specifies at §6.10.1 ¶3:
After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers are replaced with the pp-number `0`
Edit
Sorry, I thought it was a C question; still, no big deal, the equivalent section in the C++ standard (§16.1 ¶4) states:
After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers and keywords, except for `true` and `false`, are replaced with the pp-number `0`
The only difference is the different handling of `true` and `false`, which in C do not need special handling, while in C++ they have a special meaning even in the preprocessing phase.
Problem
My preprocessor appears to assume that undefined constants are 0 for the purpose of evaluating `#if` conditions. Can this be relied upon, or do undefined constants give undefined behaviour?