Summary
I come across a compiling error, in which it says class A was not declared. However, I actually include A.h.
Details
After some research, I find out the bugs. In utils.h, I want to use some static member varible in class A and thus I include A.h. However, since utils.h acts as its name, I also include utils.h in A.h.
Then say in foo.cpp, I include A.h since I want to use the class A. But A.h includes utils.h then utils.h tries to include A.h but the include guard blocks that, which causes that class A was not declared in utils.h.
As a solution, I make that utils.h as independent as possible, and it does not include other user-defined classes (that’s what utils mean).