How to bypass Eclipse C++ glitch?

Question

Eclipse

I work in Eclipse C++...
I made two projects. Your Libu and your own project.
Everything compiled fine.
Now I added a class to lib. I use it in my project. Compile, you get a startup file that works. But in the eclipse itself it gives out as if there is no such file.
What to do?
PS Error "could not be resolved"

Answer 1

found:
in menu project->c/c++ index/freshen all files

Answer 2

Months of struggle with Eclipse CDT, including with the "could not be resolved" error, ended for me personally with a tactical retreat to QtCreator.
Eclipse is one of the most awkward C++ IDEs I've ever seen. Precisely because of the fact that you will never understand whether something will work after setting it up or not. And, moreover, will it work after the next update.