What is the value of an undefined constant used in #if?

c++ c-preprocessor
Tony Park picture Tony Park · Feb 23, 2011 · Viewed 8.7k times · Source

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?

Answer

Matteo Italia picture Matteo Italia · Feb 23, 2011

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.

Related questions

What is the difference between #include <filename> and #include "filename"?

In the C and C++ programming languages, what is the difference between using angle brackets and using quotes in an include statement, as follows? #include <filename> #include "filename"

Read More
Why are #ifndef and #define used in C++ header files?

I have been seeing code like this usually in the start of header files: #ifndef HEADERFILE_H #define HEADERFILE_H And at the end of the file is #endif What is the purpose of this?

Read More
__FILE__, __LINE__, and __FUNCTION__ usage in C++

Presuming that your C++ compiler supports them, is there any particular reason not to use __FILE__, __LINE__ and __FUNCTION__ for logging and debugging purposes? I'm primarily concerned with giving the user misleading data—for example, reporting the incorrect line number …

Read More