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 DO_THIS is yours definition then simple and working solution seems to be usage of Function-like Macro:

#define DO_THIS() 1

#if DO_THIS()
    //stuff
#endif

I tested this under Visual Studio 2008, 2015 and GCC v7.1.1. If DO_THIS() is undefined VS gererates:

warning C4067: unexpected tokens following preprocessor directive - expected a newline

and GCC generates

error: missing binary operator before token "("