QLabel does not display in QWidget

Question

I have the following hierarchy in my Qt Application: QMainWindow > QWidget (centralWidget) > QWidget (subclassed) > QLabel

Initialization code in my QMainWindow code

Answer 1

We need to show the label created by button click, cause centralwidget was already painted. Here is a working example, I added this as answer also I noticed better adding chatWidget as child to centralWidget where in your original code its added to the UI .. this is your choice.

Mainwindow:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    //
    ui->setupUi(this);
    centralWidget = new QWidget();
    centralWidget->setGeometry(width,height);
    chatWidget=new ChatWidget(centralWidget); // the subclassed QWidget
    setCentralWidget(centralWidget);

    // added testing
    QPushButton *btn = new QPushButton("MyButton",centralWidget);
    btn->setGeometry(100,100,100,100);
    btn->setMaximumSize(100,100);
    connect(btn,&QPushButton::clicked, chatWidget, &ChatWidget::displayChatAfterButtonPressed);
 }

and chatWidget:

ChatWidget::ChatWidget(QWidget *parent):QWidget(parent)
{
    QLabel  *lbl;
    lbl=new QLabel(this);
    lbl->setText("Hello World 1");
}

void ChatWidget::displayChatAfterButtonPressed()
{
    QLabel *lbl;
    lbl=new QLabel(this);
    lbl->setText("Hello World 2");
    lbl->show();
}

Answer 2

Widgets when they are visible for the first time call to be visible to their children, but since you are creating it afterwards they probably are not calling that method, a possible solution is to call the show method.

void ChatWidget::displayChatAfterButtonPressed()
{
    QLabel *lbl;
    lbl=new QLabel(this);
    lbl->setText("Hello World 2");
    lbl->show();
}

comment: it seems strange to me that the QMainWindow you set a central widget and then create the chatWidget as a parent to the QMainWindow, it is generally not recommended to add children to the QMainWindow because it has a given structure, what should be done is to place it inside the centralwidget.