Every developer has faced this moment: the interface is almost ready, but it behaves “strangely”—elements overlap, don’t respond to events, or suddenly disappear. Where exactly does the GUI logic break, and why do familiar techniques stop working?
This chapter carefully reveals the hidden foundation of Qt interfaces. Here you’ll discover why a widget isn’t just a button or window but a full-fledged object with behavior, and you’ll learn the secret that professional developers use to build scalable and predictable GUIs. The “before/after” contrast becomes especially noticeable: interfaces stop being a collection of chaotic elements and become a managed system.
The chapter covers two different Qt hierarchies, shows how the same widget can be a container, breaks down 3 key methods of geometry management, and demonstrates how the backing store mechanism can improve interface responsiveness without complex optimization. Widget stacks, frames, and scroll views also make an appearance—just enough to make you want to keep reading.
Skip this chapter and you’ll forever remain in “why does it work this way?” mode.
This chapter includes ready-to-use code examples.
Chapter Self-Check
What’s the fundamental difference between class hierarchy and object hierarchy in Qt?Answer
Correct answer: Class hierarchy is inheritance (QPushButton inherits from QAbstractButton), while object hierarchy is parent-child relationships between objects (a button is placed inside a window and belongs to it).
Why will a top-level widget remain invisible even after creation, and what needs to be done?Answer
Correct answer: After creating a top-level widget, you must call the show() method, otherwise both the window itself and all child widgets will be invisible.
Why is the autoFillBackground property needed, and what happens if it’s not set to true?Answer
Correct answer: By default, this property is false, causing the widget not to fill with a background and remain invisible. Setting it to true makes the widget automatically fill the background and become visible.
How does the Backing Store technique improve application performance?Answer
Correct answer: The technique caches raster images of widgets in memory, allowing them to be displayed quickly without calling paint events (PaintEvent), significantly speeding up interface updates.
What happens to child widgets if a parent widget becomes disabled or invisible?Answer
Correct answer: Child widgets automatically inherit the parent’s state—they become disabled or invisible along with it, simplifying management of widget groups.
Why use Qt::WindowStaysOnTopHint when creating a window?Answer
Correct answer: This flag advises the system to keep the window always in the foreground, preventing other windows from overlapping it—useful for important notifications or tool panels.
When and why should you call setOverrideCursor()?Answer
Correct answer: During long operations, this sets a wait cursor for the entire application, informing the user that the application is busy and temporarily not responding to commands.
Why is there no separation between controls and containers in Qt?Answer
Correct answer: Any widget can serve as a container for other widgets thanks to object hierarchy, providing flexibility and consistency in building interfaces.
When is calling setGeometry(5, 5, 260, 330) equivalent to two separate method calls?Answer
Correct answer: It’s equivalent to sequential calls to move(5, 5) and resize(260, 330), allowing simultaneous setting of position and size of the widget.
What does the setWidget() method of the QScrollArea class do with the passed widget?Answer
Correct answer: This method automatically makes the passed widget a child of the viewport widget in the object hierarchy, adding it to the scroll window.
Why does QStackedWidget only show one child at a time?Answer
Correct answer: This is its main purpose—to organize multiple widgets so only one is visible at any given time, useful for creating multi-page interfaces or wizards.
How do you create a custom mouse cursor image, and what’s required?Answer
Correct answer: You can use a QPixmap object with a PNG file containing the image and bitmap mask, passing it to the QCursor constructor, then setting it via setCursor().
What happens if you create a widget without passing a parent parameter to the constructor?Answer
Correct answer: The widget becomes a top-level widget with its own window, since the pwgtParent parameter defaults to nullptr.
Why create a QPalette object first when setting a widget’s background?Answer
Correct answer: The palette allows setting the widget’s background (color or image) via setColor() or setBrush() methods, after which the palette is applied to the widget via setPalette().
When does QScrollArea automatically add scrollbars?Answer
Correct answer: Scrollbars appear automatically when at least one child widget extends beyond the viewport boundaries, allowing hidden parts to be scrolled into the visible area.
Practical Exercises
Easy Level
Colored Widgets with Different Backgrounds
Create a Qt application with a top-level widget containing three child widgets. Set a different background color for each (red, green, blue), position them so they partially overlap, creating an interesting composition. The main window should have the title “Color Palette”.
Hints: Create QPalette objects for each child widget. Use setColor() with Qt::blue, Qt::red, Qt::green. Don’t forget setAutoFillBackground(true) for each widget. Use move() and resize() for positioning.
Medium Level
Image Gallery with Scrolling
Create a gallery application that displays several images (3-5) vertically inside a QScrollArea. Each image should be in a separate QLabel. Implement functionality where scrollbars are always visible. Add a frame (QFrame) around the entire gallery with a Sunken effect.
Hints: Use QVBoxLayout for vertical placement of QLabels with images. Set ScrollBarAlwaysOn for both scrollbars. Wrap content in QFrame and use setFrameStyle(QFrame::Box | QFrame::Sunken). For loading images, use QPixmap.
Hard Level
Multi-Page Settings Wizard
Create a wizard application with three settings pages using QStackedWidget. First page—welcome with text, second—theme selection (light/dark) via buttons, third—completion. Add “Back” and “Next” buttons for page navigation. When selecting dark theme, change the entire application’s background. Implement cursor change to PointingHandCursor when hovering over navigation buttons.
Hints: Use QStackedWidget::setCurrentIndex() for page switching. Create slots to handle button clicks. For theme switching, use QPalette with setColor() for background. Override enterEvent() for buttons to change cursor via setCursor(). Store current page index in a class variable.
💬 Join the Discussion!
Got a handle on the widget concept and hierarchy? Have questions about the differences between class hierarchy and object hierarchy?
Maybe you’ve found an elegant way to work with palettes or created an unusual custom cursor?
Share your experience, ask questions, or help other readers master the fundamental concepts of Qt!
