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

You can use the BOOST_PP_IS_EMPTY macro like so:

#include 

#define MYVARIABLE
#if !defined(MYVARIABLE) || !BOOST_PP_IS_EMPTY(MYVARIABLE)
    // ... blablabla ...
#endif

That did the trick for me. I shall add this macro is undocumented, so use it with caution.

Source: preprocessor-missing-IS-EMPTY-documentation