qt example from the book

Question

hello, 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++.

Answer 2

Its a 0 pointer (think NULL without the type), or in Qt terms, "no parent".

Answer 3

The parent argument is for giving parents to new widgets. When given, it is useful for Qt to manage the object tree. (To automatically delete child objects.) It also has the concrete visible effect of "attaching" a new widget to another widget (ie, the parent). In your code however, the parent argument is not ever given, causing the widget to appear as a top level window and not to be deleted by Qt automatically. (It would not require deletion by Qt anyway in that code though.)