...
struct strct{QLabel * a;
QPushButton * b;};
strct * one; 
bool eventFilter(QObject *object, QEvent *event);
...

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
one = new strct;
one->a = new QLabel;
one->b = new QPushButton;
one->b->setVisible(false);
    installEventFilter(this);
}
bool MainWindow::eventFilter(QObject *object, QEvent *event){
if(object == one->a)
    if (event->type() == QEvent::FocusIn){
     one->a->setVisible(false);
     one->b->setVisible(true);}

    return false;
}

bool MainWindow::eventFilter(QObject *object, QEvent *event){
 
    if(object == usersList[i].bb)
    if (event->type() == QEvent::Enter){
        usersList[i].cc->setVisible(true);
        usersList[i].bb->setVisible(false);}
    if(object == usersList[i].cc)
    if(event->type() == QEvent::Leave){
        usersList[i].cc->setVisible(false);
        usersList[i].bb->setVisible(true);}


    return false;
}