XCode use of undeclared identifier, only appears if the file is opened

Question

When I clean, build and run my XCode project all goes well, but if I open a file that calls the function, an Use of undeclared identifier 'func()' appears. This is the whole implementation:

The function is called:

func();

And 'func()' is decleared in a .h file like so:

#if __cplusplus
extern "C" {
#endif

extern void func();

#if __cplusplus
}
#endif

and func() is implemented is a cplusplus library.

Why would the error ONLY appears if the file is open, but if I don't open it, it runs and works just fine?

Answer 1

Xcode has two ways of generating error messages. Usually both of these generate the same messages, so you do not detect that there are two systems.

The first system is the editor which does syntax coloring and auto-completion. It will also show error messages almost instantly after you write an error.

The second system is errors shown in the build log.

I suspect that your project has a complicated include setting. This prevents the first system from finding the correct file to include and therefore it cannot find the definition of func().