include file logic

Question

The following is the code I currently cannot resolve:

error.cpp - the main module

#include <iostream>
#include "a.h"
#include "k.h"
int main()
  {
  A a;
  std::cout << "Hello World!\n";
  func();
  }

a.h

 #pragma once
 struct A {
 };

k.h - just another module containing one function

 #pragma once
 void func();

k.cpp

 #include "a.h" // for struct A
 #include "k.h" // for function prototype

 void func() {  
 struct A a;
 }

What I did is the following:
I have soem MFC code, which was not compiling anymore. What I did was I created another Single Document application and dumped all the class modules into the newly created directory and added them to my project. Now I wanted to use the functionality of one of the classes I had included and in OnInitialUpdate I created an object from my class.

The error messages resembled the error messages I got from above code: The error message changes from the following ...

Code    Description Project File    Line
LNK2019 unresolved external symbol "void __cdecl func(void)" (? 
func@@YAXXZ) referenced in function main    error    
C:\Projects\C++Projects\error\error.obj 1

LNK1120 1 unresolved externals  error    
C:\Projects\C++Projects\error\x64\Debug\error.exe   1

to the following:

    C2079   'a' uses undefined struct 'main::A' error    
    C:\Projects\C++Projects\error\error.cpp 8
    E0070   incomplete type is not allowed  error    
    C:\Projects\C++Projects\error\k.cpp 5
    lnt-uninitialized-local Local variable is not initialized.  error           
    C:\Projects\C++Projects\error\k.cpp 5

    C2079   'a' uses undefined struct 'A'   error       
    C:\Projects\C++Projects\error\k.cpp 5
    lnt-uninitialized-local Local variable is not initialized.  error
    C:\Projects\C++Projects\error\error.cpp 8

after renaming function fct() to func in k.pp!!!

What is wrong?

and careful with terminology. C++ modules do exist, but there is none in your code — 463035818_is_not_an_ai, Aug 17 '23 at 16:06
Sorry, if I might have sounded confusing, but I am working with C++ modules and the error I am currently dealing with could be duplicated with the code I provided. I thought it was related to the way I used include files. And yes, it is a linker error. Thank you. — Michaelinscarbororough, Aug 17 '23 at 16:55
You don't define `func` anywhere ... Is `fct` in k.cpp a typo? — ChrisMM, Aug 17 '23 at 19:48
Yes, you are right. Please consider fct in k.pp func. I'll add the changes. Thank you. — Michaelinscarbororough, Aug 17 '23 at 22:43
@ πάντα ῥεῖ I changed my opinion after reading through this: What is an undefined reference/unresolved external symbol error and how do I fix it? (39 answers). I am not really sure, whether that is my answer, or not. — Michaelinscarbororough, Aug 17 '23 at 23:20

include file logic

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