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copied!<p>As far as I understand, you have two classes: <code>MainWindow</code> and <code>Dialog</code>. As <code>MainWindow::myPosReg</code> is protected, you can only access it in <code>Dialog</code>, if <code>Dialog</code> derives from <code>MainWindow</code> or if it is a friend.</p> <p>The following design should work:</p> <pre><code>class MainWindow : public QMainWindow { [...] protected: DateReg *myPosReg; friend class Dialog; }; class Dialog { [...] MainWindow* myMainWindow; void someFunction() { myMainWindow->myPosReg->setValue([...]); } }; </code></pre> <p>As an alternative, I would suggest a more Qt-ish way by using signals and slots. When the spin-boxes in <code>Dialog</code> change, emit a signal to which <code>MainWindow</code> listens.</p> <p>We start with the dialog:</p> <pre><code>class Dialog : public QDialog { [...] Dialog() { [...init ui...] connect(ui->spinBoxA, SIGNAL(valueChanged(int)), this, SLOT(onSpinBoxValueChanged())); connect(ui->spinBoxB, SIGNAL(valueChanged(int)), this, SLOT(onSpinBoxValueChanged())); connect(ui->spinBoxC, SIGNAL(valueChanged(int)), this, SLOT(onSpinBoxValueChanged())); } signals: void valuesChanged(int a, int b, int c); private slots: void onSpinBoxValueChanged() { emit valuesChanged(ui->spinBoxA->value(), ui->spinBoxB->value(), ui->spinBoxC->value()); } }; </code></pre> <p>So the dialog has an internal slot <code>onSpinBoxValueChanged()</code>, which is called when one of the spinboxes changes its value. The sole purpose of this slot is to emit another signal <code>valuesChanged</code> to which we'll connect the main window (i.e. the dialog's internal slot combines three values into one signal).</p> <p>The main window could be the following:</p> <pre><code>class MainWindow : public QMainWindow { [...] DateReg *myPosReg; public slots: void setValues(int a, int b, int c) { myPosReg->setValues(a, b, c); } }; </code></pre> <p>And the code, which connects both</p> <pre><code>int main() { [...] MainWindow* mainWindow = new MainWindow; Dialog* dialog = new Dialog; connect(dialog, SIGNAL(valuesChanged(int,int,int)), mainWindow, SLOT(setValues(int,int,int))); } </code></pre> <p>When the user changes the value in a spinbox, the following chain-reaction happens:</p> <pre><code>QSpinBox::valueChanged(int) -> Dialog::onSpinBoxValueChanged() -> Dialog::valuesChanged() -> MainWindow::setValues() -> DateReg::setValues() </code></pre> <p>The advantage is that you decouple <code>MainWindow</code> and <code>Dialog</code>: <code>Dialog</code> just signals that the values have changed but does not care what happens with them and <code>MainWindow</code> changes its <code>myPosReg</code> but does not care, where the values are coming from.</p>

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