Communicating between Widgets without parent casting

[DerManu]

[quote author="TheDestroyer" date="1336489463"]Well, if this can never happen unless there's a bug in my code, how do you explain Andre's response? I think he would've suggested there's a problem in my code if there really would be, don't you think?[/quote]
I think "this post of his":http://qt-project.org/forums/viewreply/74751/ implies that, but I guess he may explain what he thinks the source of the problem is in follow-up posts.

For me this is a question of probabilities. Either there's a tremendous bug with catastrophic effects in GCC (think about it, this problem doesn't occur in one of a million runs but consistently!) and nobody else has noticed it yet. Or there's a glitch in your code that you've just overlooked.
When I understand you correctly, you have used the pointer injected observer pattern in projects before, and it worked, right?

[TheDestroyer]

The thing is that the way those GUI's work is different. That's why I could expect a different behaviour.

Yes, this approach works when I use it outside Qt. But with GUIs of Qt it won't work. No one knows how those meta files of the GUIs are managed..., and how they take information from other objects.

[DerManu]

[quote author="TheDestroyer" date="1336490799"]The thing is that the way those GUI's work is different. That's why I could expect a different behaviour.[/quote]Not in such a fundamental way. A QWidget subclass still is a normal C++ class after the moc run, and when it has a pointer member you can do with it whatever you can also do in a non GUI class.
[quote author="TheDestroyer" date="1336490799"]No one knows how those meta files of the GUIs are managed..., and how they take information from other objects.[/quote]That's not right, what the meta object compiler does is relatively transparent when you look at the outputted moc_ files. No magic going on, it just adds some homebrew RTTI class and boilerplate code to every QObject class to make sigs/slots work. Specifically, it does not touch your settings pointer member.

I've tried it. Here are three files: main.pro, main.h and main.cpp:
main.pro:
@QT += core gui
TEMPLATE = app
HEADERS += main.h
SOURCES += main.cpp@
main.h:
@#ifndef HEADER_H
#define HEADER_H
#include <QtGui>

class Settings
{
public:
Settings():intValue(0), boolValue(false) {};
~Settings() {}
int intValue;
bool boolValue;
};

class QTestWidget : public QWidget
{
Q_OBJECT
public:
explicit QTestWidget(QWidget *parent, Settings *settings);
protected:
virtual void paintEvent(QPaintEvent *event);
private:
Settings *mSettings;
QTimer mRenderTimer;
};

class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow() { delete mSettings; }
private slots:
void changeBoolSetting();
void changeIntSetting();
private:
Settings *mSettings;
};

#endif // HEADER_H@
and main.cpp:
@#include "main.h"

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}

// === Implementation of MainWindow, QTestWidget and Settings ===

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
mSettings(new Settings)
{
setGeometry(50, 50, 100, 110);
QTestWidget *tw = new QTestWidget(this, mSettings);
QPushButton *b1 = new QPushButton(this);
QPushButton *b2 = new QPushButton(this);
tw->setGeometry(5, 5, 90, 70);
b1->setGeometry(25, 75, 25, 25);
b2->setGeometry(50, 75, 25, 25);
connect(b1, SIGNAL(clicked()), SLOT(changeBoolSetting()));
connect(b2, SIGNAL(clicked()), SLOT(changeIntSetting()));
}

void MainWindow::changeBoolSetting()
{
mSettings->boolValue = !mSettings->boolValue;
}

void MainWindow::changeIntSetting()
{
mSettings->intValue += 1;
}

QTestWidget::QTestWidget(QWidget *parent, Settings *settings) :
QWidget(parent),
mSettings(settings)
{
connect(&mRenderTimer, SIGNAL(timeout()), this, SLOT(update()));
mRenderTimer.start(500);
}

void QTestWidget::paintEvent(QPaintEvent *event)
{
Q_UNUSED(event)
QPainter painter(this);
painter.drawRect(rect().adjusted(0, 0, -1, -1));
painter.drawText(rect(),
Qt::AlignCenter,
QString::number(mSettings->intValue)+"\n"+
QString(mSettings->boolValue ? "true" : "false"));
painter.drawText(0, 13, QDateTime::currentDateTime().toString("hh:mm:ss:zzz"));
}@

run
qmake; make; ./main
and you'll see that it works as expected. Click the two buttons and see the QTestWidget update its display (in 500ms render-intervals), depending on it's injected Settings pointer.