Question

I have this snippet of the code:

 #include <QApplication>
 #include <QFont>
 #include <QPushButton>
 #include <QWidget>

 class MyWidget : public QWidget
 {
 public:
     MyWidget(QWidget *parent = 0);
 };

 MyWidget::MyWidget(QWidget *parent)
     : QWidget(parent)
 {
     setFixedSize(200, 120);

     QPushButton *quit = new QPushButton(tr("Quit"), this);
     quit->setGeometry(62, 40, 75, 30);
     quit->setFont(QFont("Times", 18, QFont::Bold));

     connect(quit, SIGNAL(clicked()), qApp, SLOT(quit()));
 }

 int main(int argc, char *argv[])
 {
     QApplication app(argc, argv);
     MyWidget widget;
     widget.show();
     return app.exec();
 }

can somebody please explain what exactly is going on in this line

MyWidget(QWidget *parent = 0);

a little bit difficult understand what is this parent, thanks in advance

Answer 1

That is an argument to the constructor with a default argument (NULL since NULL is defined as 0 as per the c++ standard). Default meaning passing no parameter is the same as passing NULL.

Since Qt’s widgets are arranged in a hierarchal system (parent -> child relationships) parent is the widget which is the “owner” or “container” of the current one (NULL means no parent aka a root widget of sorts). For GUI items a widget will often have the widget it is contained in as its parent.

This is advantageous since when a parent is deleted, it will delete any children is has automatically removing the need for much of the memory management that comes with c++.