Qt - Compiler complains when invoking setLayout() on my MainWindow

Question

I wanna learn how to create a gui by hand without the designer. I tried to add a layout to my MainWindow but when running it says

QWidget::setLayout: Attempting to set QLayout "" on MainWindow "", which already has a layout

This is my code :

//Header
class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow();
private:
    QHBoxLayout *layout;
};

//Constructor in my *.cpp
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    layout = new QHBoxLayout;
    this->setLayout(layout);
}

//The usual main function
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

What is wrong? I did what my book said. I even looked up some code on the internet which was really hard to find somehow and it was still the same. I just cannot add a layout to my window.

Answer 1

There's a similar question which helped me find out what's wrong. Thanks to Mat for his link to that question.

What every QMainWindow needs is a QWidget as central widget. I also created a new Project with the designer, compiled it and looked the ui_*.h files up.

So every QMainWindow should look similar to this :

//Header
class MainWindow : public QMainWindow
{
    Q_OBJECT
    QWidget *centralWidget;
    QGridLayout* gridLayout;

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

};

//*.cpp
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    centralWidget = new QWidget(this);
    this->setCentralWidget( centralWidget );
    gridLayout = new QGridLayout( centralWidget );
}

Now you don't add / set the layout to the MainWindow. You add / set it to the centralWidget.