Correct us of d_ptr pattern

GrahamL

Hi
I am developing a library and in Qt Tradition I want to use the d_ptr pattern.
I have no previous experience of using this pattern and have a simple example working but wanted to check that my implementation is correct.
To that end I have set out my classes below and would be grateful if anyone could confirm that my approach is correct(or not)

Thanks

Header file
@
#ifndef DISPLAYWIDGET_H
#define DISPLAYWIDGET_H
#include "DisplayWidgetsGlobal.h"
#include <QWidget>

class DisplayWidgetPrivate;

class DISPLAYWIDGETS_EXPORT DisplayWidget : public QWidget
{
Q_OBJECT

public:
DisplayWidget(QWidget *parent);
~DisplayWidget();

private:
DisplayWidgetPrivate* d_ptr;
Q_DECLARE_PRIVATE(DisplayWidget)
};

#endif // DISPLAYWIDGET_H
@

Private Header file
@
#include "DisplayWidget.h"

class QLabel;
class DisplayWidgetPrivate
{
public:
DisplayWidgetPrivate (DisplayWidget* parent);
void init();

QLabel* m_label;
DisplayWidget* const q_ptr;
Q_DECLARE_PUBLIC(DisplayWidget)

public:
DisplayWidgetPrivate();
};
@

Implementation file
@
#include <QLabel>
#include "DisplayWidget.h"
#include "DisplayWidget_p.h"

DisplayWidgetPrivate::DisplayWidgetPrivate(DisplayWidget* parent)
: q_ptr(parent)
{

}

void DisplayWidgetPrivate::init()
{
m_label = new QLabel("This is a label",q_ptr);
}

DisplayWidget::DisplayWidget(QWidget *parent)
: QWidget(parent),
d_ptr(new DisplayWidgetPrivate(this))
{
Q_D(DisplayWidget);
d->init();
}

DisplayWidget::~DisplayWidget()
{

}
@

sierdzio

On a quick look, it looks good.

GrahamL

Thanks - good to hear

mcosta

Hi,

to read more about d-pointer you can see "here":qt-project.org/wiki/Dpointer.

In your code I found only a problem, you don't have to define d_ptr and q_ptr because they are defined in QObject and QObjectPrivate classes; if you class inherits (directly or not) from QObject you don't have to define them.

GrahamL

Thanks mcosta
But if I remove the declarations for q_ptr and d_ptr the compilation fails

@
DisplayWidget.cpp(11): error C2614: 'DisplayWidgetPrivate' : illegal member initialization: 'q_ptr' is not a base or member
@

mcosta

Hi,

sorry MY ERROR , q_ptr is avalable only if your *Private Class inherits from QObjectPrivate.