Inheriting a MainWindow class from QMainWindow and Ui_MainWindow

Question

I am learning Qt, Qt 5. When I start Qt Creator and create a project with ALL the default settings, I get these 2 files generated, (I am excluding main.cpp and the .pro file)

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

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

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

Now, I prefer to do it this way,

my_mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include "ui_mainwindow.h"


class MainWindow : public QMainWindow, private Ui_MainWindow
{
    Q_OBJECT

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

#endif // MAINWINDOW_H

my_mainwindow.cpp

#include "my_mainwindow.h"
#include <QMessageBox>

MainWindow::MainWindow(QWidget *parent) :   QMainWindow(parent)
{
    setupUi(this);
}

MainWindow::~MainWindow()
{

}

Here are the main differences between my code and Qt Creator's code:

No namespace Ui in my code. (Could anyone explain me the use of this namespace here ?)
I inherit the MainWindow class from both QMainWindow and Ui_MainWindow whereas the Qt Creator's code inherits it only from the QMainWindow class.

My question is, is there ANY disadvantage of using my approach or is there any advantage of using the Qt Creator's approach ? Please give a detailed answer.

@Rage In the Qt 4 days the meta object compiler (MOC) did not like multiple inheritance with QObjects or any derived classes. — Ralph Tandetzky, Jun 21 '13 at 14:05
@Rage Possibly because of the memory layout in the class. Possibly because inheriting from two classes derived from `QObjects` doesn't work. It also didn't like virtual inheritance. I do not know why. But it didn't work. — Ralph Tandetzky, Jun 21 '13 at 14:10
@RalphTandetzky Well, but I it works just **FINE** with Qt 5.0.2 — Shravan, Jun 21 '13 at 14:11

score 4 · Accepted Answer · answered Jun 21 '13 at 14:40

An advantage of the namespace is that it prevents naming clashes. All the automatically generated names from QtDesigner stay in their own namespace.
An advantage of making the Ui class a member, instead of using multiple inheritance, is that the Ui class only needs to be forward-declarded in the header. In your code, you have a #include "ui_mainwindow.h", and that in turn drags in a lot of includes from QtWidgets like <QLabel>, <QPushButton> etc. This decreases compilation speed significantly, as everyone who includes mainwindow.h now also includes those QtWidgets includes. When using the Ui class as a member and forward-declaring it, all these includes only need to be compiled when compiling mainwindow.cpp, but not when including mainwindow.h from somewhere else.

Inheriting a MainWindow class from QMainWindow and Ui_MainWindow

1 Answers1