Undefined reference to 'Class::class' when using Class::staticclass.function();

Question

When I use a static member by itself, so:

window::gfx;

It compiles fine, but whenever I use that members function, so:

window::gfx.set_background(BLACK);

I get the following error:

undefined reference to `window::gfx'

I have wrote the files that involve this error, however all of my code is left at: https://github.com/averagedolphin/pong

main.cpp:

#include <iostream>
#include "raylib.h"
#include "window/window.h"


int main()
{
    window window;

    while (!WindowShouldClose())
    {
        // Graphics gui phase
        BeginDrawing();
        window::gfx.set_background(BLACK);
        EndDrawing();
    }
}

window.h:

#pragma once

#include "../macros.h"

#include "graphics/graphics.h"
#include "gui/gui.h"
#include "control/control.h"


class window
{
private:

public:


    static graphics gfx;
    static gui gfxui;
    window();

};

window.cpp:

#include "window.h"

#include "raylib.h"
#include "../macros.h"
#include "control/control.h"
#include "graphics/graphics.h"
#include "gui/gui.h"

window::window()
{
    InitWindow(WINDOW_WIDTH, WINDOW_HEIGHT, WINDOW_TITLE);

    // Initialize subclasses

}

graphics.h:

#pragma once

#include "raylib.h"

class graphics
{
public:
    void set_background(Color clr);
};

graphics.cpp:

#include "graphics.h"

#include "raylib.h"

void graphics::set_background(Color clr)
{
    ClearBackground(clr);
}

Thanks,