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

gcc can generate a warning for this, but its probably not required by the standard:

-Wundef
Warn if an undefined identifier is evaluated in an `#if' directive.