Weird error in Keil compiler

Question

As you see in the following image, when I comment one of the include "gui.cpp" or include <gui.cpp>, I have a red cross error in the right side of using namespace GUI. but when I include both of them, the error removed. Does anyone have any idea?

Answer 1

Don't include .cpp files, or any source files. You should only include header files. Let the linker deal with source files.

You should create a gui.h and include that instead.