Window Components in Qt

Graphical user interfaces (GUIs) in Qt are built using a hierarchy of windows and widgets. The <QtGui> module provides the foundational classes for creating these UI elements, and all visual components are represented as objects derived from the QWidget class.

The Role of QWidget

QWidget serves as the base class for all UI components in Qt. It inherits from both QObject (enabling signals and slots) and QPaintDevice (supporting painting operations). Key characteristics include:

• It can render itself and respond to user input events.
• Every visible UI element—buttons, labels, dialogs—is ultimately a QWidget.
• When instantiated without a parent, it becomes a top-level window.
• It commonly acts as a container (parent widget) for other child widgets.

Using QLabel

QLabel is a lightweight widget used to display static text or images. While it can technically exist as a standalone window (since it inherits from QWidget), it is typically embedded within a parent widget for practical use.

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QWidget>

class MainWindow : public QWidget
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
};

#endif // MAINWINDOW_H

#include "MainWindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QWidget(parent)
{
}

MainWindow::~MainWindow()
{
}

#include <QApplication>
#include <QWidget>
#include <QLabel>
#include "MainWindow.h"

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

    QWidget topLevel;
    QLabel infoLabel(&topLevel);

    topLevel.setWindowTitle("Example App");
    infoLabel.setText("This is a label");

    topLevel.show();

    return app.exec();
}

Customizing Window Types and Styles

Qt allows fine-grained control over window apearance and behavior through the Qt::WindowType flags, passed as the second argument to a widget’s constructor (the first being the parent).

Common Window Types

• Qt::Dialog: Creates a dialog-style window, often modal.
• Qt::Window: Standard top-level application window.
• Qt::SplashScreen: Used for splash screens during application startup.

Window Flags for Custom Behavior

• Qt::WindowStaysOnTopHint: Keeps the window above others.
• Qt::WindowContextHelpButtonHint: Replaces the minimize/maximize buttons with a context-sensitive help button (common on Windows).

#include <QApplication>
#include <QWidget>
#include <QLabel>

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

    QWidget window(nullptr, Qt::Window | Qt::WindowStaysOnTopHint);
    QLabel label(&window);
    label.setText("Always-on-top window");

    window.setWindowTitle("Custom Window");
    window.show();

    return app.exec();
}