Why no warning with “#if X” when X undefined?

Question

I occasionally write code something like this:

// file1.cpp
#define DO_THIS 1

#if DO_THIS
    // stuff
#endif

During the code development I ma

Answer 1

There is recursive issue. In case you have

#define MODEL  MODEL_A

#if (MODEL == MODEL_B)
 // Surprise, this is compiled!
#endif

where definition of MODEL_A and MODEL_B are missing, then it will compile.

#ifdef MODEL
#error Sorry, MODEL Not Defined
// Surprise, this error is never reached (MODEL was defined by undefined symbol!)
#endif 

#ifdef MODEL_B
#error Sorry, MODEL_B Not Defined
// This error is reached
#endif