Question

I tried to create a Mainwindow with an slot, which creates a Widget and loads it to the ScrollArea in the Mainwindow. This doesn’t work, so I tired to create the Widget in the constructor of the mainwindow and I always get errors and don’t know why.. so what’s the right declaration of the Widget?

#include <QtGui>


class Mainwindow : public QMainWindow
{
    Q_OBJECT

public:
    Mainwindow(QMainWindow *parent = 0);

public slots:

private:
    QScrollArea *List,*Sublist,*Overall,*Settings;
    QLabel *label_title;
    QPushButton *bn_exit,*bn_list,*bn_overall,*bn_settings;
};

//! ------------------------------------- Mainlist -------------------------------------
class Sublist : public QWidget{
 Q_OBJECT
private:
    QLabel *title;
public:
    Sublist(QWidget *parent = 0);
};

and .cpp

Mainwindow::Mainwindow(QMainWindow *parent) : QMainWindow(parent) {

    this->resize(1024,576);
    //this->setWindowFlags(Qt::Popup);
    QPalette palette;
    palette.setColor(QPalette::Background, QColor(16,16,16));
    this->setPalette(palette);

    Sublist SecondList;

    //! [Set ScrollAreas]
    List = new QScrollArea(this);
    List->setGeometry(0,60,200,456);
    List->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOn);

    Sublist = new QScrollArea(this);
    Sublist->setGeometry(200,60,824,456);
    Sublist->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOn);
    //Sublist->setWidget(SecondList)

}

//! ---------------------------------------- MainList------------------------------------------------------
Sublist::Sublist(QWidget *parent){
    resize(1200,1200);
    title = new QLabel("Title",this);
    title->setGeometry(1120,1120,40,90);
}

Answer 1

I have played around with your code a bit, noticed a few things:

In class Mainwindow you define your QScrollArea variables:

QScrollArea *List,*Sublist,*Overall,*Settings;

You define a variable named Sublist of type QScrollArea, but you also have a class of the same name:

class Sublist : public QWidget

Probably would be a good idea to change the variable names for your scroll areas:

QScrollArea *list, *subList, *overall, *settings;

Next, in the constructor for class Sublist you pass a pointer to the parent class but never assign it to your base class. You also have a QLabel widget that never is placed anywhere. What seems to be needed is a layout for your custom widget.

The Sublist class could be something like this:

//.h
class Sublist : public QWidget
{
    Q_OBJECT

public:
    Sublist(QWidget *parent = 0);

private:
    QLabel *title;
    QVBoxLayout *layout;
};

//.cpp
Sublist::Sublist(QWidget *parent) : QWidget(parent) {
    resize(1200,1200);
    title = new QLabel("Title");
    title->setGeometry(1120,1120,40,90);

    QVBoxLayout *layout = new QVBoxLayout;
    layout->addWidget(title);
    setLayout(layout);
}

The Mainwindow class something like this:

//.h
class Mainwindow : public QMainWindow
{
    Q_OBJECT

public:
    Mainwindow(QMainWindow *parent = 0);

private:
    Sublist *secondList;
    QScrollArea *list, *subList, *overall, *settings;
    QLabel *label_title;
    QPushButton *bn_exit,*bn_list,*bn_overall,*bn_settings;
};

//.cpp
Mainwindow::Mainwindow(QMainWindow *parent) : QMainWindow(parent) 
{
    this->resize(1024,576);
    QPalette palette;
    palette.setColor(QPalette::Background, QColor(16,16,16));
    palette.setColor(QPalette::Foreground, QColor(255,255,255));//set text to white
    this->setPalette(palette);

    secondList = new Sublist(this);

    //! [Set ScrollAreas]
    list = new QScrollArea(this);
    list->setGeometry(0,60,200,456);
    list->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOn);

    subList = new QScrollArea(this);
    subList->setGeometry(200,60,824,456);
    subList->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOn);
    subList->setWidget(secondList);
}

Im still not 100% sure this is what you where trying to achieve with this code, but I hope that I have helped you to resolve some of the issues in your current implementation.