C++ error code C2065: '<class name>' undeclared identifier, even though it should be declared in another .h-file

Question

I have a multifile program, and I can't figure out why my program says that "Customers" (in the registerNewUser() function) is an undeclared identifier.

proc.h

#ifndef PROC_H
#define PROC_H
#include <iostream>
#include "const.h"
#include "customers.h"
#include <fstream>
using namespace std;

void registerNewUser(Customers cBase); // Add new user.

#endif // !PROC_H

I have included the header file (customers.h) with the Customers class also.

customers.h

#ifndef CUSTOMERS_H
#define CUSTOMERS_H
#include <iostream>
#include "const.h"
#include "proc.h"
#include "customer.h"
using namespace std;

class Customers {
    private:
        char* current;
        List* customerList;     // List for customers.
    public:                        
        Customers();            // Constructor.
        ~Customers();           // Destructor.
        void handler();         // Customers handler/menu.
        void addNew(char username[]);
    };

#endif // !CUSTOMERS_H

Can anyone see what's wrong?

Answer 1

Basically including "customers.h" in "customers.h" wouldn't be a problem here, since you have a guard (plus point for that). Nevertheless it is not very nice.

As NathanOliver said it COULD be a problem with the order of the includes but it doesn't have to. If you include proc.h first everything is fine. If you include customers first, the compiler includes proc.h before he sees the customer class. proc then wont include customers.h (since its guard prevents it). Then he will find your function not knowing what "Customer" means. So depending on the include order of your header files it will or will not work.

If you want a hint: I normally first only include the necessary files for a forward declaration, then do a forward declaration. Then I include the files necessary files for the definition of the class (These will already know that the class exists). The complete class declaration (with member function declaration) follows. If you do it like this you can avoid many mistakes. In your case:

#ifndef CUSTOMERS_H
#define CUSTOMERS_H

class Customers;

#include "proc.h"
#include "ListTool2B.H"
using namespace std;

class Customers 
{
    private:
        char* current;
        List* customerList;     // List for customers.
    public:                        
        Customers();            // Constructor.
        ~Customers();           // Destructor.
        void handler();         // Customers handler/menu.
        void addNew(char username[]);
};

#endif