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 I'm thinking about this correctly.

Preprocessor directives are handled before any source code is compiled. During that phase(s) of translation in which this occurs all preprocessor directives, macros, etc are handled and then the actual source code is compiled.

Since #if is used to determine if X has been defined and carry out some action if it has or has not been defined. The #if in the code snippet would compile without any errors because there aren't any errors as far as the compiler is concerned. You could always create a header file with specific #defines that your application would need and then include that header.