Every developer has encountered a situation where Qt’s standard widgets are no longer sufficient: the interface is almost ready, but a “minor detail”—non-standard behavior, special input, or custom visualization—turns into a source of compromises and workarounds. Sound familiar?
This chapter carefully demolishes the myth that custom controls are time-consuming, complex, and risky. It reveals how developers design widgets so they naturally fit into the layout system, scale correctly, and remain reusable. You’ll discover that choosing the right base class saves hours of work.
It shows practices of inheriting from QLineEdit and QFrame, working with paintEvent(), sizeHint(), and QSizePolicy, as well as the signal and slot architecture for reusable components. One real example—and it becomes clear why this approach works many times more stable.
Skip this chapter—and you’ll continue battling standard solution limitations instead of taking control of the interface into your own hands.
This chapter includes ready-to-use code examples.
Chapter Self-Check
Why is choosing the right base class critically important when creating a custom widget?Answer
Correct answer: A well-chosen base class already contains most of the necessary properties and methods, significantly saving development time and eliminating the need to implement functionality from scratch.
What does the sizeHint() method return and how is it used by the layout system?Answer
Correct answer: Returns a QSize object with recommended widget dimensions. Layout classes use this value to calculate optimal widget sizes during placement, interpreting it according to the QSizePolicy.
What is the difference between the QSizePolicy::Expanding and QSizePolicy::Preferred constants?Answer
Correct answer: Both allow the widget to stretch and shrink, but Expanding tells the layout that the widget preferably wants to stretch and should be given maximum space, whereas Preferred has no such priority.
Why does the CustomWidget example use QSizePolicy::Minimum for width and Fixed for height?Answer
Correct answer: Minimum for width allows the widget to stretch horizontally but not shrink below sizeHint(), while Fixed for height strictly fixes the height at the sizeHint() value, which is suitable for a progress indicator.
Why emit the progressChanged() signal in CustomWidget if no one connects to it in the example?Answer
Correct answer: This is a reusable component design principle—the widget should provide signals for all important events so it can be used in other applications where state change tracking will be needed.
Why is it recommended to use update() instead of repaint() for widget repainting in Qt6?Answer
Correct answer: The update() method is more efficient as it places the repaint request in the event queue and allows optimization of multiple requests, while repaint() performs immediate repainting.
Why is the drawFrame() call made at the very end in the CustomWidget example’s paintEvent() method?Answer
Correct answer: So the frame is drawn on top of all widget content and is not covered by the gradient or text, ensuring the correct drawing layer order.
What function does std::clamp(n, 0, 100) perform in the slotSetProgress() slot?Answer
Correct answer: Limits the value n to the range from 0 to 100, ensuring the progress indicator value doesn’t exceed acceptable limits, simplifying bounds checking.
Why does QScrollArea use the QSizePolicy::Expanding policy in both directions?Answer
Correct answer: Thanks to scrollbars, QScrollArea can work with any size, but the larger the size, the more convenient to use, so Expanding tells the layout to provide maximum available space.
How do you ensure dark theme support in a custom widget instead of using hardcoded colors?Answer
Correct answer: Use QPalette and get colors from the widget’s current palette via palette().color(), for example QPalette::Text, QPalette::Base, QPalette::Highlight, which automatically adapts to the theme.
In what cases should widget class attributes be declared as protected instead of private?Answer
Correct answer: When further inheritance of the created widget class is planned and derived classes will need access to the base class’s internal data.
Which event methods need to be overridden for correct widget operation with keyboard focus?Answer
Correct answer: The focusInEvent() method for handling focus gain and focusOutEvent() for handling focus loss, allowing the widget to react to focus state changes.
If you need a widget that can stretch and shrink, but the layout should strive to give it maximum space—which policy to choose?Answer
Correct answer: QSizePolicy::MinimumExpanding (if there’s a minimum size) or QSizePolicy::Expanding (if there’s no minimum size), as these policies tell the layout to provide as much space as possible.
Practical Exercises
Easy Level
Colored Label Widget
Create a custom widget inheriting from QLabel that displays text on a colored background with the ability to change the background color via a public slot setBackgroundColor(QColor). The widget should work correctly with layouts and have reasonable dimensions.
Hints: Inherit from QLabel to use ready text functionality. Override paintEvent() to draw colored background. Use palette().color(QPalette::Text) for text color. Override sizeHint() to return an appropriate size (e.g., 150×50).
Medium Level
Interactive Counter Indicator
Develop a counter widget that displays a numeric value as a filling circle (progress from 0 to 100). The widget should have slots for incrementing/decrementing the value, emit a signal on change, work correctly with themes, and support mouse clicks to change the value.
Hints: Inherit from QWidget. Override paintEvent() using QPainter::drawPie() for the circle. Override mousePressEvent() for interactivity. Use QPalette for colors. Define a valueChanged(int) signal. Set QSizePolicy::Fixed for both directions to keep the widget square. Use std::clamp() to limit values.
Hard Level
Multifunctional Graph Widget
Create a widget for displaying a line graph with zooming and scrolling. The widget should accept an array of data points, automatically scale to window size, support dark and light themes, have customizable line colors via slots, emit a signal when clicking on a data point, and work correctly in any layouts with Expanding policy.
Hints: Inherit from QWidget. Store data in QVector<QPointF>. Override paintEvent() with coordinate scaling. Use QPainterPath for drawing lines. Override mousePressEvent() and wheelEvent() for interactivity. Implement resizeEvent() to recalculate scale. Use palette() for all colors. Define signals pointClicked(int) and dataChanged(). Set setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding). Add antialiasing via setRenderHint().
💬 Join the Discussion!
Figured out creating custom widgets? Have questions about overriding event methods or working with QSizePolicy?
Share your widgets, talk about difficulties implementing paintEvent(), or discuss best practices for dark theme support!