undefined reference of a function but already defined in header

Question

main cpp

#include "utility.h"
#include <windows.h>

int main()
{ 
    do
    {
        changeColor();
        system("pause");
    } while (true);
}

utility cpp

#include "utility.h"
#include "variables.h"
#include <windows.h>

namespace utility
{
    void changeColor()
    {
        if (var::colorCounter == 0)
        {
            system("color af");
        }
        else if (var::colorCounter == 1)
        {
            system("color cf");
        }
        else if (var::colorCounter == 2)
        {
            system("color df");
        }
        else if (var::colorCounter == 3)
        {
            system("color 6f");
        }
        else
        {
            system("color 9f");
            var::colorCounter = -1;
        }

        var::colorCounter++;
    }
}

utility header

#ifndef utility

#define utility

void changeColor();

#endif

variables

#ifndef variables
#define variables

namespace var
{
    inline int colorCounter{};
}

#endif

idk whats causing it to produce the changeColor() to not define what should i do? also is may coding right?

Answer 1

There are many problems with your code which are shown through the comments in the modified program below.

main.cpp

#include "utility.h"
int main()

{
        utility::changeColor();//used utility:: because you've to be in the scope of namespace utility to call function changeColor()
}

utility.h

#ifndef UTILITY_H  //USED INCLUDE GUARDS
#define UTILITY_H 

namespace utility {
void changeColor(); //this function declaration is now inside the utitliy namespace

}
#endif

variables.h

#ifndef VARIABLES_H //USED INCLUDE GUARDS
#define VARIABLES_H

namespace var
{
    //NOTE the extern keyword here instead of inline keyword
    extern int colorCounter; //this is a nondefininig declaraiton for colorCounter. 

}

#endif

variables.cpp

#include "variables.h"

namespace var 
{
    int colorCounter = 0; //this is definition of colorCounter
}

The output of the above program can be seen here.

Modifications

Some of the modifications that i made include:

1. In main.cpp, you have to be in the scope of the namespace utility to call function changeColor(). This is achieved using utility::.
2. In utility.h, header guards are used. This is a recommended practice.
3. In utility.h, the function declaration for changeColor is placed inside the namespace utility.
4. In variables.h, extern keyword is used instead of inline keyword to make the declaration of colorCounter a declaration that is not a definition. This essentially means, colorCounter has external linkage.
5. In variables.cpp, the variable colorCounter has been initialized with value 0.

Note

If you still want to use inline instead of extern you can do so from C++17 and onward as can be seen here. So if you use inline your program will work for C++17 and onwards. But if you use extern as in my above code, then your program will work in all C++ versions. You can choose whichever version you want.