Generating an error if checked boolean macro is not defined

Question

I have several configuration files each one containing the definition of some boolean macro, to be set to 0 or 1. Then, in my code, I check the value of such a macro to deci

Answer 1

Macro CHECK(x) will:

• fail if macro x is undefined,
• evaluate to 00 if x is defined to 0
• evaluate to 01 if x is defined to 1

$ cat main.cpp
#define CAT(x, y) x##y
#define CHECK(x) CAT(0, x)

// usage

#define COND0 0
#define COND1 1

#if CHECK(COND)
#endif

#if CHECK(COND0)
#pragma message "defined 1"
#else
#pragma message "defined 0"
#endif

#if CHECK(COND1)
#pragma message "defined 1"
#else
#pragma message "defined 0"
#endif

$ g++ main.cpp
main.cpp:9:1: error: user-defined literal in preprocessor expression
    9 | #if CHECK(COND)
      | ^~~~~
main.cpp:15:17: note: ‘#pragma message: defined 0’
   15 | #pragma message "defined 0"
      |                 ^~~~~~~~~~~
main.cpp:19:17: note: ‘#pragma message: defined 1’
   19 | #pragma message "defined 1"
      |                 ^~~~~~~~~~~