SIGNAL & SLOT

#include <QApplication>
#include <QHBoxLayout>
#include <QSlider>
#include <QSpinBox>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QWidget *window = new QWidget;
    window->setWindowTitle("Enter Your Age");

    QSpinBox *spinBox = new QSpinBox;
    QSlider *slider = new QSlider(Qt::Horizontal);
    
    spinBox->setRange(0,130);
    slider->setRange(0,130);

    QObject::connect(spinBox,SIGNAL(valueChanged(int)),slider,SLOT(setValue(int)));
    QObject::connect(slider,SIGNAL(valueChanged(int)),spinBox,SLOT(setValue(int)));

    spinBox->setValue(35);

    QHBoxLayout *layout = new QHBoxLayout;
    layout->addWidget(spinBox);
    layout->addWidget(slider);
    window->setLayout(layout);

    window->show();

    return app.exec();
}