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

If you're desperate to prevent this kind of error, try the following which uses preprocessor token-pasting magic and expression evaluation to enforce that a macro is defined (and 0 in this example):

#define DEFINED_VALUE(x,y) (defined (y##x) ? x : 1/x)
#if DEFINED_VALUE(FEATURE1,) == 0

Answer 2

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.