Are Undefined Constants Evaluated as 0 in C and C `#if` Macros?

Is an Undefined Constant Evaluated as 0 in C and C #if Macros?

A common assumption is that preprocessors interpret undefined constants as 0 when used in #if conditions. But can this be consistently relied upon, or does it yield unpredictable results?

The answer lies in the C99 and C 17 standards.

C99 Standard

The C99 standard specifies in §6.10.1 ¶3 that:

"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."

C 17 Standard

Similarly, the C 17 standard states in §16.1 ¶4:

"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."

Conclusion

Therefore, it is indeed reliable in both C and C to assume that undefined constants are evaluated as 0 in #if conditions. This behavior is explicitly defined in the language standards, ensuring consistent execution across different compilers and platforms.

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