V1119. Preprocessing directive is present within a macro argument. This leads to undefined behavior.

A token that looks like a preprocessing directive should not be used as a macro argument. Using preprocessing directives as macro arguments results in undefined behavior.

Look at the example:

#include <stdio.h

#define PRINT(arg) printf("%s", #arg)

int main(int argc, char *argv[])
{
  PRINT(My program prints:
    #ifndef FLAG
      hello.
    #else
      world.
    #endif 
  );
}

Depending on whether the FLAG macro is defined, the developer expects the program to print either My program prints: hello or My program prints: world to stdout. Because the behavior is undefined, the compiler can do anything in this situation.

If a program is compiled using GCC or Clang without defining the FLAG macro, the result will be one of the expected options:

My program prints: hello.

However, a program compiled using MSVC will print an unexpected result:

My program prints: #ifndef FLAG hello. #else world. #endif

The fixed code:

#include <stdio.h>

#define PRINT(arg) printf("%s", #arg)

int main(int argc, char *argv[])
{
#ifndef FLAG
  PRINT(My program prints: hello.);
#else
  PRINT(My program prints: world.);
#endif 
}

Now all programs compiled using different compilers print the same text depending on the FLAG macro definition.

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