c++ constructor question

Question

#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();
}

For this line : MyWidget(QWidget *parent = 0); why we need to put = 0 here??

Stormenet · Accepted Answer · 2011-03-12 08:37:04Z

3

It is called an Default parameter

Basically you are saying unless you pass another value, the function (or constructor in this case) will be called with parent as 0.

When you'd had MyWidget(QWidget *parent); as constructor, you'd had to call it like MyWidget widget(0);

answered Mar 12, 2011 at 8:37

Stormenet

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Comments

Cody Gray · Accepted Answer · 2011-03-12 08:43:43Z

1

You do not have to put zero there. C++ allows you to put default value for a parameter. In this case, parameter parent will default to 0 if the constructor is invoked without specifying an argument.

edited Mar 12, 2011 at 8:43

Cody Gray♦

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answered Mar 12, 2011 at 8:38

Srdjan Jovcic

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Comments

Evil Activity · Accepted Answer · 2011-03-12 08:37:04Z

0

Don't need to put it there, but it's a default value. If you don't pass any value to the constructor, it will take the '0' as value. It's making things a bit easier in some cases for the programmer.

answered Mar 12, 2011 at 8:37

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