How can i link an external function to another static library?

Question

I am programming two static libraries with extern functions in C++.

In my first library I have this line of code:

EntryPoint.hpp:

extern Application* createApplication();

In my second library i want to define this function.

WindowEntryPoint.cpp:

extern Application* createApplication() {
...
}

In my test project I want to link both libraries. That causes a linker error:

EntryPoint.cpp:(.text+0x19): undefined reference to `createApplication()'
collect2: error: ld returned 1 exit status

but when I implement this function in my test executable project, it works! In other words, after implementing this function in my second static library, I get this linker error!