How to test if preprocessor symbol is #define'd but has no value?

Question

Using C++ preprocessor directives, is it possible to test if a preprocessor symbol has been defined but has no value? Something like that:

#define MYVARIABLE         


        

Answer 1

#if MYVARIABLE==0 My answer must be at least 30 chars, so that should do it!

Answer 2

I haven't seen this solution to the problem but am surprised it is not in common use . It seems to work in Xcode Objc. Distinguish between "defined with no value" and "defined set 0"

#define TRACE
#if defined(TRACE) && (7-TRACE-7 == 14)
#error TRACE is defined with no value
#endif

Answer 3

You can't since the preprocessor can only check for a numeric value. Your string compare is not covered by preprocessor syntax.