Confusion about namespaces and classes in QT

[etrusks]

Hi, I just started to learn QT5.5 and I kind of don't understand this simple thing

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {      //start of Ui namespace
class MainWindow;        
}                   //end of Ui namespace

//Rest is defined outside this namespace
class MainWindow : public QMainWindow
{
    Q_OBJECT

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

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

In this header MainWindow class is forward declared in this Ui namespace but after that the actual definition is written outside this namespace. It just doesn't make sense. As I see it these are 2 separate classes with same name but different namespaces right?

If that is so and I dont even have to write definition for Ui::MainWindow it means that its already written and it doesn't make sense to forward declare it.

I checked this out with simple class X that I wrote and indeed they were 2 different classes if I created header this way. I could obviously use only that class that's defined outside namespace.

If I remove

namespace Ui {
class MainWindow;
}

part than im getting error on line Ui::MainWindow *ui; that says

'Ui' does not name a type

Probably not the right place to ask for help but ill take my chances :D

[Moschops]

In this header MainWindow class is forward declared in this Ui namespace but after that the actual definition is written outside this namespace. It just doesn't make sense. As I see it these are 2 separate classes with same name but different namespaces right?

Correct. Two different classes.

If that is so and I dont even have to write definition for Ui::MainWindow it means that its already written and it doesn't make sense to forward declare it.

It does if we want the line Ui::MainWindow *ui; to be accepted by the compiler. Just because something has been defined in some other file somewhere else doesn't mean the code in this file magically knows about it.

[etrusks]

Ohh tnx man, I think I started to understand it tnx to you!

This is the corresponding mainwindow.cpp file

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

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


}

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

Here ui_mainwindow.h is included and thats where the definition is. Forward declaration was only because the author didnt want ui_mainwindow.h to be included together with mainwindow.h. At least now I think so. Thank you man - this actually seems very useful!

EDIT : This is like the happiest moment of my day :D