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c++cpreprocessor

How do you do logical exclusive XOR in the C preprocessor with defines

What's the simplest way to ensure that only one of two names is defined, for example:

#define USE_OPTION1
#define USE_OPTION2

#if not(USE_OPTION1 ^ USE_OPTION2)
#error "You must use at least one option, but not both"
#endif

I know there's no logical XOR in C or C++, so what's the best way of doing this? It doesn't have to be this, does it:

#define USE_OPTION1
#define USE_OPTION2
        
#ifdef USE_OPTION1
  #ifdef USE_OPTION2
  #error "You can't use both"
  #endif
#endif

#ifdef USE_OPTION2
  #ifdef USE_OPTION1
  #error "You can't use both"
  #endif
#endif

#ifndef USE_OPTION1
  #ifndef USE_OPTION2
  #error "You must use at least one"
  #endif
#endif
Solution

  • you can solve this by checking if both are defined equally. so 0 0 and 1 1 will throw the error, while 0 1 and 1 0 are allowed.

    #if defined(USE_OPTION1) == defined(USE_OPTION2)
#error "You must use at least one option, but not both"
#endif