[Interest] correct d_ptr implementation

[andre at familiesomers.nl]

Ok, but if your 'private' class is designed to be derived from, then I 
guess it was not all that private after all. It is more like it is to be 
protected instead of private. If the class really is private (for 
instance by not using a displaywidget_p.h for declaring it, but by just 
forward-declaring it from the private section of DisplayWidgets 
declaration in displaywidget.h and keeping all the rest in 
displaywidget.cpp instead), then I fail to see the point of either. 
There is no need for a private constructor or assignment operator then, 
as nobody can access them anyway, right?

If you want to allow subclassing the private part (like Qt does), then I 
guess you're right. But IMHO that means that the d_ptr isn't really 
private, but rather it has become protected instead.

André

Constantin Makshin schreef op 11.11.2013 19:42:
> Right, but:
> 1) classes derived from DisplayWidget may want to derive their *Private
> counterparts from DisplayWidgetPrivate — in this case private section
> will make DisplayWidgetPrivate's internals accessible only to this 
> class
> itself and its friend DisplayWidget;
> 2) since not all compilers support "= delete" from C++11, placing an
> intentionally-unimplemented constructor[s] and/or assignment operator
> into the private section is the most common way to mark them as 
> unavailable.
> 
> On 11/11/2013 10:22 PM, andre at familiesomers.nl wrote:
>> Constantin Makshin schreef op 11.11.2013 18:33:
>>> 1) put your 'd_ptr' into a smart pointer of some kind (usually
>>> QScopedPointer), your example leaks memory;
>>> 2) placing 'q_ptr' in the public section doesn't make much sense
>>> because
>>> it's supposed to be used only by the DisplayWidgetPrivate instance to
>>> access its "owner" (DisplayWidget doesn't need any external 
>>> information
>>> to access itself :) );
>>> 3) I guess the second "public" section in DisplayWidgetPrivate was
>>> supposed to be "private" :) .
>>> 
>>> Other than that, your example looks OK to me.
>>> 
>> How much sense does it make to have a private section in the private
>> DisplayWidgetPrivate class at all? Everything in there is private for
>> DisplayWidgets use anyway, right?
>> 
>> André
>> 
>> 
>>> On 11/11/2013 06:40 PM, Graham Labdon wrote:
>>>> 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()
>>>> {
>>>> 
>>>> }
> 
> 
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