Widget from scratch using pImpl, how to Q_PRIVATE_SLOT?

[Pl45m4]

Hi,

as the title already says, I'm working on a widget, which I'm building from scratch.
I followed Qt's pImpl design, therefore I have my own WidgetPrivate class (+ d_ptr) and make use of the Q_xxxxx macros as much as I can (Q_D, Q_Q, etc.).

The way my widget should work requires it to have a QTimer.
This timer and the to timeout() connected function I want to hide so that they are not visible/accessible from the "public" API (Widget.h). They are both part of my WidgetPrivate class.

So now my questions:

How to make this work?
I've read about Q_PRIVATE_SLOT but at the same time everywhere is written that it is only there because of compatibility to Qt4 and C++ Standard below C++11. In Qt5 and above it's allegedly not needed...

I also found this pretty old guide:

• http://coffeeandcode.blogspot.com/2008/01/private-implementation-slots.html

which works, when built exactly like it's written there.
I see

bob!
bob!
bob!
bob!
bob!

every second on timeout in my output pane.

However, adapting it to my project does not seem to work.
I don't use Qt4 (obviously) and I don't have inline functions.
My structure looks like:

• widget.h
• widget_p.h
• widget.cpp (Widget and WidgetPrivate both share this code file)

In addition I don't use the String-based connections...

This altogether results in a bunch of errors, which make my code not compile :(

Any help is appreciated :)

H:/dev/Qt/6.7.0/mingw_64/include/QtCore/qobject.h::215::-1">H:/dev/Qt/6.7.0/mingw_64/include/QtCore/qobject.h</a>:215:9: note:   template argument deduction/substitution failed:
H:/dev/Qt/6.7.0/mingw_64/include/QtCore/qobject.h: In substitution of 'template&lt;class Func1, class Func2&gt; static QMetaObject::Connection QObject::connect(const typename QtPrivate::FunctionPointer&lt;Prototype&gt;::Object*, Func1, const typename QtPrivate::ContextTypeForFunctor&lt;Func2&gt;::ContextType*, Func2&amp;&amp;, Qt::ConnectionType) [with Func1 = void (QTimer::*)(QTimer::QPrivateSignal); Func2 = void (WidgetPrivate::*)()]':
H:/dev/Qt/6.7.0/mingw_64/include/QtCore/qobject.h</a>:215:9: error: no type named 'ContextType' in 'struct QtPrivate::ContextTypeForFunctor&lt;void (WidgetPrivate::*)(), void&gt;'
H:/dev/Qt/6.7.0/mingw_64/include/QtCore/qobject.h</a>:264:9: note: candidate: 'template&lt;class Func1, class Func2&gt; static QMetaObject::Connection QObject::connect(const typename QtPrivate::FunctionPointer&lt;Prototype&gt;::Object*, Func1, Func2&amp;&amp;)'
  264 |         connect(const typename QtPrivate::FunctionPointer&lt;Func1&gt;::Object *sender, Func1 signal, Func2 &amp;&amp;slot)
      |         ^~~~~~~

[Christian Ehrlicher]

You can not use the pmf connect syntax with the pImpl idiom. One way is to derive the private class from QObject or use a lambda.

Maybe you can also take a look on QObjectPrivate::connect() in qobject_p.h (when someone asks - I don't know this function... 🙂)

[SGaist]

Hi,

Can you share a minimal reproducer ?
That would make things easier to spot the issue.

[Christian Ehrlicher]

You can not use the pmf connect syntax with the pImpl idiom. One way is to derive the private class from QObject or use a lambda.

Maybe you can also take a look on QObjectPrivate::connect() in qobject_p.h (when someone asks - I don't know this function... 🙂)

[Pl45m4]

Here is a stripped-down example to show what I'm doing:

widget.h
(Public API)

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QScopedPointer>


class WidgetPrivate;
class Widget: public QWidget
{
    Q_OBJECT

public:


    explicit Widget(QWidget *parent = nullptr);
    ~Widget();

Q_SIGNALS:
 
    // public signal which should fire when background logic
    // and timer timeout occur
    void timespanPassed();

protected:

    void mousePressEvent(QMouseEvent *ev);
    void mouseReleaseEvent(QMouseEvent *ev);

protected:

    Widget(WidgetPrivate &dd, QWidget *parent = nullptr);


protected:

    /*const*/ QScopedPointer<WidgetPrivate> /*const*/ d_ptr;


private:

    Q_DECLARE_PRIVATE(Widget)
    Q_DISABLE_COPY(Widget)

    // Is this needed?! If not, how else to do this
    Q_PRIVATE_SLOT(d_func(), void foo())

};

#endif // WIDGET_H

widget_p.h

#ifndef WIDGET_P_H
#define WIDGET_P_H


#include <QDebug>

class Widget;
class QTimer;

class WidgetPrivate
{
    Q_DECLARE_PUBLIC(Widget)
    Q_DISABLE_COPY(WidgetPrivate)

public:

    WidgetPrivate(Widget *q);
    virtual ~WidgetPrivate();

    void init();

    Widget *const q_ptr;

    // this is the timer I want to use internally
    QTimer *timer;

    // internal function which causes a signal in public API to fire
    // (after some processing) 
    void emitTimespanPassed();

private:

    void foo(); // private internal slot?!


};

#endif // WIDGET_P_H

widget.cpp

#include "widget.h"
#include "widget_p.h"

#include <QPainter>
#include <QPointer>
#include <QMouseEvent>
#include <QTimer>

// needed?!
#include "moc_widget.cpp"

WidgetPrivate::WidgetPrivate(Widget* q)
    : q_ptr(q)
    , timer(nullptr)
{

}


WidgetPrivate::~WidgetPrivate()
{

}

void WidgetPrivate::init()
{
    Q_Q(Widget);
    // init public API settings here

    timer = new QTimer(q);
    timer->setSingleShot(true);

    // interval is changable in API
    timer->setInterval(3000); // 3s
 
     // connect here?!
    // if so, how?
    // QObject::connect(timer, &QTimer::timeout, this, [&](){ foo(); });

    q->setMouseTracking(true);

}

void WidgetPrivate::emitTimespanPassed()
{
    Q_Q(Widget);
    QPointer<Widget> guard(q);

    // some conditions and logic to emit public signal...
    // [ ... ]
    // emit widget signal
    if (guard)
        emit q->timespanPassed();
}

// function connected to QTimer::timeout
void WidgetPrivate::foo()
{

    // if (xyz) { // some logic
        // cause emit of public signal in Widget API
        emitTimespanPassed();

    // }

}

// END INTERNAL PRIVATE
// #######################################################################################
// #######################################################################################
// #######################################################################################
// BEGIN PUBLIC API

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , d_ptr(new WidgetPrivate(this))
{
    Q_D(Widget);
    d->init();

     // OR connect here?!?!?
     // better here or in WidgetPrivate::init()?
     // no error but signal not emitted
    QObject::connect(d->timer, SIGNAL(timeout()), this, SLOT(foo())); 
    // does NOT work, crashes with mentioned error
    QObject::connect(d->timer, &QTimer::timeout, d_func(), &WidgetPrivate::foo); 
}

Widget::Widget(WidgetPrivate &dd, QWidget *parent)
    : Widget(parent)
    , d_ptr(&dd)
{

    Q_D(Widget);
    d->init();

}

void Widget::mousePressEvent(QMouseEvent *ev)
{
    Q_D(Widget);
    d->timer->start();
    QWidget::mousePressEvent(ev);
}

void Widget::mouseReleaseEvent(QMouseEvent *ev)
{
    Q_D(Widget);
    d->timer->stop();
    QWidget::mouseReleaseEvent(ev);
}

// END PUBLIC API
// #######################################################################################
// #######################################################################################
// #######################################################################################

main.cpp

#include <QApplication>
#include "widget.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;

    QObject::connect(&w, &Widget::timespanPassed, &w, [&](){
        qDebug() << "Timespan passed";
    });

    w.show();
    return a.exec();
}

This is my code... I'm unsure about many things there.
Where to connect my internal function?
Is the #include "moc_widget.cpp" needed? Some sources state that it's needed to tell moc to "moc" the WidgetPrivate part?!

Very much appreciated if any of you can shed a light on this procedure for me :))
I hope you get what I'm trying to do... but maybe there are even some structural flaws?! Can't tell actually :(

[Christian Ehrlicher]

As I said the pmf syntax needs a QObject as sender and receiver.

[Pl45m4]

@Christian-Ehrlicher said in Widget from scratch using pImpl, how to Q_PRIVATE_SLOT?:

As I said the pmf syntax needs a QObject as sender and receiver.

Ah sorry, while I was editing my example code I missed your reply :)

Oh wow didn't know that :0
So the ( SIGNAL(), SLOT() ) connection stuff is not 100% identical in terms of use-case and compatibility with the Functor-based connections?
Good to know.

@Christian-Ehrlicher said in Widget from scratch using pImpl, how to Q_PRIVATE_SLOT?:

Maybe you can also take a look on QObjectPrivate::connect() in qobject_p.h (when someone asks - I don't know this function... 🙂)

Yeah, I checked the Qt Private sources here and there while I was building my widget to see how it's done. Haven't seen this exact part, but will visit it again ;-)
Since I don't want to include the Qt Private source completely (which would make my widget highly dependent on a single Qt version), I made my own WidgetPrivate based on QWidgetPrivate "style" but without inheriting it... so no QObjectData, QObjectPrivate and whatnot... for me ;-)

Edit:

@Christian-Ehrlicher said in Widget from scratch using pImpl, how to Q_PRIVATE_SLOT?:

Maybe you can also take a look on QObjectPrivate::connect() in qobject_p.h (when someone asks - I don't know this function... 🙂)

This function?
What is there to see? Other than QObjectPrivate::connect.
You mean the use of two QObject Functors?

when someone asks - I don't know this function

Nobody will know 🤐

One way is to derive the private class from QObject or use a lambda.

If one way to make it work is using the SIGNAL / SLOT keywords again, I will probably try it.
Tried lambda, did not work either :/ Or maybe I was doing something wrong.

Edit_II:

I made my actual code work, using String-based connections while I moved the connect statement to the WidgetPrivate::init() function and hid the QTimer also behind the WidgetPrivate data.
Not 100% pleased, tho :D
Is it a good idea to make the Impl class a QObject?!
What's your recommendation?

@SGaist oh lol even missed your comment too :D Was about to add the code anyway :)