here is my .H

#ifndef ELFENLIEDTOPFAN5RES_H
#define ELFENLIEDTOPFAN5RES_H
#include <QString>


class ELFENLIEDTOPFAN5RES
{
public:
ELFENLIEDTOPFAN5RES();
static QString BO3_Path;
};




#endif // ELFENLIEDTOPFAN5RES_H

and here is the cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QMessageBox>
#include <QStatusBar>
#include "elfenliedtopfan5Res.h"


ELFENLIEDTOPFAN5RES::ELFENLIEDTOPFAN5RES()
{
    ELFENLIEDTOPFAN5RES::BO3_Path = (QString(getenv("TA_GAME_PATH")).replace('\\', '/'));

}


MainWindow::MainWindow(QWidget *parent)

    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

}


MainWindow::~MainWindow()
{

    delete ui;
    //const QString elfenliedtopfan5Res::BO3_PATH = (QString(getenv("TA_GAME_PATH")).replace('\\', '/'));
}



void MainWindow::on_pushButton_clicked()
{
    QString elfen2 = ELFENLIEDTOPFAN5RES::BO3_Path;
   QMessageBox::information(this,"title",elfen2);
}

this is the error it throws,
mainwindow.cpp:-1: error: undefined reference to `ELFENLIEDTOPFAN5RES::BO3_Path'

but unsure of why its doing this i have tried multiple things and looked on loads of global string setups but still no avail
sorry if its a noob question
thank you for reading
elfenliedtopfan5