Qt C++ How to connect a class's instance from other class - c++

Question

I have three C++ classes: DB, MainWindow and Login. The instance of DB resides as database in MainWindow, and a button calls the instance of Login called loginDialog. Now I want to connect loginDialog's accept() with database's (DB's) slot, but since the instance database resides in MainWindow, I'm confused as to how to access it.
DB.h
public:
void login(QString name,QString id,QString pass);
DB.cpp
void DB::login(QString name, QString id,QString pass){
qInfo()<<"Login approved"; //placeholder
}
MainWindow.h
private:
DB database;
public slots:
void dblogin(QString name,QString id,QString pass);
private slots:
void showLoginPage();
MainWindow.cpp
#include "Login.h"
MainWindow::MainWindow(QWidget *parent):QDialog(parent),ui(new Ui::MainWindow){
ui->setupUi(this);
connect(ui->pushButton,&QPushButton::clicked,this,&MainWindow::showLoginPage);
}
void MainWindow::dblogin(QString name, QString id, QString pass){
database.login(name,id,pass);
}
void MainWindow::showLoginPage(){
Login loginDialog;
loginDialog.exec();
}
Login.h
signals:
void makeLogin(QString name,QString id,QString pass);
private slots:
void startLogin();
Login.cpp
Login::Login(QWidget *parent):QDialog(parent),ui(new Ui::Login){
ui->setupUi(this);
connect(this,&QDialog::accept,this,&Login::startLogin);
connect(this,&Login::makeLogin,&MainWindow,&MainWindow::dblogin); //Error
}
void Login::startLogin(){
emit makeLogin(ui->f1->text(),ui->f2->text(),ui->f3->text());
}

Answer 1

As mentioned in the comments, make MainWindow the parent of Login and make the connection.
I suggest also that your IOController should be a member of MainWindow so that everything would be centered.
If things get complicated with connections, you can access to public members of MainWindow like that :
auto theParent = qobject_cast<MainWindow*>(this->parent());
if(theParent)
{
// access to any member of MainWindow
}