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 may switch the definition of DO_THIS between 0 and 1.

Recently I had to rearrange my source code and copy some code from one file to another. But I found that I had made a mistake and the two parts had become separated like so:

// file1.cpp #define DO_THIS 1 

and

// file2.cpp #if DO_THIS     // stuff #endif 

Obviously I fixed the error, but then thought to myself, why didn't the compiler warn me? I have the warning level set to 4. Why isn't #if X suspicious when X is not defined?

One more question: is there any systematic way I could find out if I've made the same mistake elsewhere? The project is huge.

EDIT: I can understand having no warning with #ifdef that makes perfect sense. But surely #if is different.

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.