How to update a subclassed variable - c++

Question

class Label : public QLabel
{
Q_OBJECT
public:
QString str;
Label(QWidget* parent, QString text) : QLabel(parent)
{
this->str = text;
}
void paintEvent(QPaintEvent* p)
{
QLabel::paintEvent(p);
QPainter paint(this);
QPixmap pixmap;
pixmap.load(":/pic.png");
paint.drawPixmap(QRect(0, 0, 32, 32), pixmap);
qDebug() << "str: " << str;
paint.drawText(QPoint(10, 10), "Hello World");
}
};
Main::Main(QWidget *parent)
: QMainWindow(parent)
{
ui.setupUi(this);
QLabel* label = new Label(ui.pushButton, "test");
label.str = "hello world"; // <-- error: expression must have a class type
}
How to update the value of str from QLabel* label?
I tried: label.str = "hello world"; but I got the error: expression must have a class type.
Does subclassing controls increases the app CPU usage?
Each time the paintEvent(QPaintEvent* p) function is called it creates a new pixmap and repaint the control? or the pixmap is created only once and kept on memory and then 'reused'?
There's is any 'best practices' when subclassing buttons? or is it desirable to avoid subclassing?