Add widget to layout programmatically

kweber

I have a layout added to form in desginer, then, in run time I try to add an object, which extends from QWidget, to central widget. The program crashes in the point where I add the widget, with no possibility for further debug.

Code in main window:

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

    m_canvas = new RenderArea(this);
    QWidget *q = static_cast<QWidget*>(m_canvas);
    this->centralWidget()->layout()->addWidget(q);

    m_canvas->setLevel(1);
}

The crash is in line this->centralWidget()->layout()->addWidget(q); with message:

The inferior stopped because it received a signal from the operating system.
Signal name : SIGSEGV
Signal meaning : Segmentation fault

Where RenderArea is this:

class RenderArea : public QWidget
{
    Q_OBJECT

public:
    explicit RenderArea(QWidget *parent = nullptr);

    void setLevel(qreal value);

protected:
    void paintEvent(QPaintEvent *event) override;

private:
    qreal m_level = 0;
    QPixmap m_pixmap;
};

and

RenderArea::RenderArea(QWidget *parent)
    : QWidget(parent)
{
    setBackgroundRole(QPalette::Base);
    setAutoFillBackground(true);

    setMinimumHeight(200);
    setMinimumWidth(30);
}

jsulm

@kweber Please check q before using it!
It's a pointer and then static_cast<QWidget*> fails it will be null.
If RenderArea is a QWidget there is no need for this cast at all.

SGaist

Hi,

Are you sure centralCentral() does return a QWidget ?

Calling setCentralWidget(m_canvas); will likely be enough.

By the way, there's no need for that static_cast at all.

J.Hilk

additionally to what @jsulm said,

you're trying to add a Qwidget the the centralWidget layout, but your central widget has no layout.
in the designer itself you created a new layout, that is part of the centralWidget but not the one you access via this->centralWidget()->layout() that one does not exist.

kweber

I have the same problem if I skip the cast and change for this:

this->centralWidget()->layout()->addWidget(m_canvas);

(actually I introduced the cast just to try, I forgot to take it out)

J.Hilk

@kweber

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

    m_canvas = new RenderArea(this);

   setLayout(ui->verticalLayout);
   ui->verticalLayout->addWidget(m_canvas);
}

jsulm

@kweber said in Add widget to layout programmatically:

I have the same problem if I skip the cast and change for this:

You should read answers more carefully...

kweber

@SGaist that does work, but what I want is to specify an area of the designer for the widget to be added. In the image I posted the layout is almost as big as the window, but what I want is it to be a vertical line at the right of the window, that's why I wanted to use a box layout and want to put the widget inside it.

(I'm sew user, I can only post every 10 minutes :-) )

@J-Hilk I'm new to Qt, I don't know how to add a layout to centralWidget or how to achieve this.

Thank you

kweber

@J.Hilk said in Add widget to layout programmatically:

@kweber

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

    m_canvas = new RenderArea(this);

   setLayout(ui->verticalLayout);
   ui->verticalLayout->addWidget(m_canvas);
}

Thank you for this, after trying, the code does not add my widget and I get message:

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

What I want to achieve is to set this widget in the right end of the Window. Sorry, I'm just starting with Qt.

J.Hilk

@kweber said in Add widget to layout programmatically:

setLayout(ui->verticalLayout);

my bad for posting untested code

 ui->centralWidget->setLayout(ui->verticalLayout);