class Checker : public QWidget
{
    Q_OBJECT
public:
    int type, x, y, active;
    Checker(QWidget *parent, int _x, int _y, int _type);
    void paintEvent(QPaintEvent *event);
    void mousePressEvent(QMouseEvent *event);
};

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
//.....
            checkers[j][i] = new Checker(this, j, i, 1); //создание очередной шашки

}
Checker::Checker(QWidget *parent, int _x, int _y, int _type) :QWidget(parent)
{
    type = _type, x = _x, y = _y, active = 0;
    setGeometry(75 * x, 75 * y, 75, 75);
    show();
}


void Checker::mousePressEvent(QMouseEvent *event)
{
    cerr << "WIN" << endl;
}