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

When you can't use a compiler that has a warning message (like -Wundef in gcc), I've found one somewhat useful way to generate compiler errors.

You could of course always write:

#ifndef DO_THIS
    error
#endif
#if DO_THIS

But that is really annoying

A slightly less annoying method is:

#if (1/defined(DO_THIS) && DO_THIS)

This will generate a divide by zero error if DO_THIS is undefined. This method is not ideal because the identifier is spelled out twice and a misspelling in the second would put us back where we started. It looks weird too. It seems like there should be a cleaner way to accomplish this, like:

#define PREDEFINED(x) ((1/defined(x)) * x)
#if PREDEFINED(DO_THIS)

but that doesn't actually work.