Using one variable in another class

[ronyNS]

I want to use QString username from mainwindow in the employee.cpp file and display it on a label. However when i declare it in the employeeinfo.cpp file i get this error:
Invalid use of non static member.
How can i access username variable in employeeinfo.cpp from mainwindow.h

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include<QLabel>
#include <QMainWindow>
#include<QPushButton>
#include<QSqlDatabase>
#include "employeeinfo.h"

namespace Ui {
class MainWindow;
}


class employeeinfo;
class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

    QString username,password; <<< this variable

private slots:
    

private:
    Ui::MainWindow *ui;
    
};


#endif // MAINWINDOW_H

employeeinfo.h

#ifndef EMPLOYEEINFO_H
#define EMPLOYEEINFO_H
#include <QMainWindow>
#include "mainwindow.h"

namespace Ui {
class employeeinfo;
}

class MainWindow;

class employeeinfo : public QMainWindow
{
    Q_OBJECT

public:
    explicit employeeinfo(QWidget *parent = 0);
    ~employeeinfo();

private slots:
    
private:
    Ui::employeeinfo *ui;
};

#endif // EMPLOYEEINFO_H

employeeinfo.cpp

#include "employeeinfo.h"
#include "ui_employeeinfo.h"

employeeinfo::employeeinfo(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::employeeinfo)
{
ui->setupUi(this);

QString a = MainWindow::username; <<< i get the error here
}


employeeinfo::~employeeinfo()
{
    delete ui;
}

[VRonin]

good design practice require that employeeinfo should not even know MainWindow exists.

Pass the username as an argument to the employeeinfo constructor.

explicit employeeinfo(const QString& username, QWidget *parent = 0);

employeeinfo::employeeinfo(const QString& username, QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::employeeinfo)
{
ui->setupUi(this);

QString a = username; <<< i get the error here
}

[ronyNS]

@VRonin
Hey Thanks

But now i get this error : no matching function call to 'employeeinfo::employeeinfo()'

when i declare an object to show employeeinfo window from mainwindow
employeeinfo *emp = new employeeinfo();

i declared this is mainwindow.cpp

[VRonin]

that's because there is where you need to pass the username

employeeinfo *emp = new employeeinfo(username);

Given the doubts you are having my advice is to take a quick look to a general C++ tutorial like http://www.cplusplus.com/doc/tutorial/ before diving into Qt