01-two-windows.cpp

#include <QtWidgets>
#include "Counter.h"

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

    QLabel      lbl("0");
    QPushButton cmd("ADD");
    Counter     counter;

    lbl.show();
    cmd.show();

    QObject::connect(&cmd, SIGNAL(clicked()),
                     &counter, SLOT(slotInc()));

    QObject::connect(&counter, SIGNAL(counterChanged(int)),
                     &lbl, SLOT(setNum(int)));

    QObject::connect(&counter, SIGNAL(goodbye()),
                     &app, SLOT(quit()));

    return app.exec();
}