DJI Embedded C++ Qt Interview Insights

Today, I bring you a set of interview insights for embedded Qt development at DJI.

These questions assess both fundamentals and practical applications. Whether or not you are interviewing at DJI, these knowledge points are worth mastering if you are engaged in Qt development.

1. Why did Qt design the object tree mechanism? What problems does it primarily solve?

The object tree mechanism in Qt is primarily designed to address the memory management challenges faced in GUI development with C++. Imagine that manually using new/delete in C++ is like juggling balls in a circus; a slight misstep can lead to “balls all over the ground” (memory leaks).

The core value of the object tree mechanism lies inautomatically managing the object lifecycle, fundamentally reducing memory leaks and dangling pointer issues.

It primarily addresses three major pain points:

• Automatic memory management to avoid memory leaks: When an object is designated as the “parent object” of another object, the child object is added to the parent’s internal list. When the parent object is destroyed, it automatically traverses and destroys all child objects without needing to manually call delete.

• Maintaining logical relationships between objects: In GUI programs, there often exist “containment” or “dependency” relationships between objects (e.g., a window contains buttons). The object tree mechanism binds these relationships through parent-child relationships, allowing the lifecycle of objects to automatically follow this logical association.

• Avoiding double deletion, enhancing safety: In the object tree, when a child object is manually destroyed, it first removes itself from the parent’s list, ensuring that the parent does not attempt to delete the child object again, thus avoiding double deletion errors.

2. What is the purpose of the parent() function in the QObject class?

<span>parent()</span> is one of the core interfaces of the Qt object tree mechanism, akin to a family relationship; it records who the “parent” of the current object is.

Specific functions include:

• Maintaining the hierarchical relationship of the object tree: The parent() function clarifies the position of an object within the entire object tree, for example, the parent of a button is the window, and the parent of the window may be the main window.

• Supporting automatic lifecycle management: The parent object pointed to by parent() is the “manager” of the current object’s lifecycle. When the parent object is destroyed, all child objects are automatically destroyed.

• Facilitating hierarchical interactions between objects: Child widgets can use parent() to access their parent container, allowing them to access the parent’s properties or call methods, enabling cross-level event passing or state synchronization.

3. Does the slot function execute immediately after the signal is emitted?

This question is somewhat “situational”! Whether the slot function executes immediately after the signal is emitted depends on theconnection type and thethread relationship, which can be categorized into three scenarios:

• Direct connection: The slot function executes immediately in the thread that emits the signal, just like a direct function call.

• Queued connection: The signal is encapsulated as an event and placed in the event queue of the thread where the receiver resides, executing only when the event loop processes it.

• Auto connection (default): In the same thread, it is equivalent to a direct connection; across threads, it is equivalent to a queued connection.

In summary: Same thread + direct connection = immediate execution; cross-thread + queued connection = delayed execution.

4. Why can’t Qt manipulate the UI in a child thread?

This is akin to not being able to change the products displayed in the front end while working in the background! Qt does not allow direct manipulation of the UI in a child thread, primarily becauseUI components are not thread-safe.

Specific reasons include:

• Thread safety issues: UI components maintain a large amount of state related to rendering and event handling, and modifications and accesses to this state are not synchronized across threads.

• Single-threaded UI model: Almost all mainstream GUI frameworks adopt this design to simplify UI programming and avoid adding complex thread synchronization logic for every UI operation.

• Event loop dependency: The state updates of UI components depend on the normal operation of the event loop in the UI thread. Directly manipulating the UI in a child thread bypasses the event loop, which may lead to synchronization failures or even freeze the UI thread.

The correct approach: Use Qt’s cross-thread communication mechanisms (signal-slot queued connections, invokeMethod, etc.) to safely trigger UI updates.

5. Why is it recommended to use worker objects instead of directly inheriting QThread in Qt multithreading programming?

This is one of the easiest pitfalls in Qt multithreading programming! Qt has long stated: QThread is the “foreman” responsible for managing the lifecycle of threads; the Worker Object is the “worker” responsible for doing the actual work — do not let the foreman manage threads and do the work simultaneously, as this can lead to confusion in responsibilities!

The core reason: separation of responsibilities

• QThread is the “manager of threads”: Its core function is to manage the lifecycle of operating system threads, not to execute tasks.

• Problems with directly inheriting QThread: This leads to QThread being responsible for both thread management and task logic, violating the “single responsibility principle” and causing thread affinity confusion.

Advantages of the Worker Object pattern:

• Clear responsibilities: QThread manages threads, while the Worker performs tasks.

• Correct thread affinity: By using moveToThread, the Worker is moved to the child thread, ensuring its slot functions execute in the child thread.

• Flexible use of the event loop: The event loop in the child thread can properly handle signals, slots, timers, and other events.

In simple terms, it allowsthe manager to only manage, and the worker to only work — this is a more reasonable division of labor.

6. What are the core classes of the Qt drawing system? What are their differences?

The Qt drawing system is like a professional painting team, each with its own role:

• QPainter (the brush): The one doing the actual work; all drawing operations (drawing lines, rectangles, text, etc.) are performed by it.

• QPaintDevice (the canvas): The target medium for drawing, such as QWidget (window), QPixmap (pixmap), QImage (image), QPrinter (printer).

• QPaintEngine (the translator): The bridge between QPainter and QPaintDevice, converting drawing commands into instructions that the device can understand.

Additionally, there areQPen (pen style), QBrush (brush), QFont (font) and other auxiliary classes that together form a complete drawing system.

7. What are the differences between Qt’s custom library and STL?

• Different design goals: STL is the C++ standard, universal! It can be used with any framework as long as it is C++, pursuing “cross-compiler, standardized”; the Qt library is a “dedicated tool” for the Qt ecosystem, tightly bound to signals and slots, serving only Qt projects.

• Different functional scopes: STL focuses on “basic data structures + algorithms”, such as vector for storing data and sort for sorting, with pure functionality; the Qt library is more versatile, including not only containers like QList and QVector but also QString (natively supports Unicode, making it easy to handle Chinese), QFile (file operations), QNetwork (network) — and it can directly work with UI components, such as displaying QString on buttons, which STL cannot do.

• Different memory management: STL containers require manual deletion of pointers (unless using smart pointers), for example, if pointers are stored in a vector, the memory pointed to by the pointers remains when the container is deleted; Qt’s QObject-derived classes have a “parent object mechanism” that automatically deletes objects in containers if they have a parent, making it more beginner-friendly.

• Different usage scenarios: Use STL for pure C++ projects, which are universal and framework-agnostic; for Qt projects (especially GUI), prioritize using the Qt library, which integrates smoothly with signals and slots, reducing the need for adaptation code — for example, using QList in Qt is more convenient than std::list, as it can be used directly as a signal parameter.

Selection advice: Use STL for pure C++ projects, and prioritize the Qt library for Qt projects for smoother integration.

8. What are the advantages of UDP compared to TCP?

UDP is like sending a postcard, simple and direct; TCP is like sending a registered letter, reliable but cumbersome.

Advantages of UDP:

• No connection establishment, low overhead: TCP requires three-way handshake to establish a connection and four-way handshake to disconnect, which wastes time; UDP allows you to send data directly without these steps — it is favored for online gaming and voice calls, as even a few milliseconds of delay can affect the experience.

• High transmission efficiency, low latency: TCP, for reliability, requires acknowledgment, retransmission, and congestion control, and has a larger header (starting at 20 bytes); UDP has a header of only 8 bytes, sending data without waiting for acknowledgment — suitable for live broadcasts and real-time video, where occasional packet loss is acceptable, but high latency is detrimental.

• Low resource usage, capable of handling many clients: TCP must maintain the state of each connection (sliding window, sequence number), which can cause the server to lag when handling many clients; UDP does not maintain connections, allowing it to handle hundreds or thousands of clients without issue — it is used for DNS queries, where each request corresponds to a response, making it efficient!

• Supports broadcasting/multicasting, unlike TCP: TCP can only facilitate point-to-point communication, while UDP can broadcast to all devices on a local area network or multicast to a specific group of devices — for example, finding devices on a local network or streaming media broadcasts relies on UDP.

Applicable scenarios: Scenarios requiring high real-time performance, such as voice calls, online gaming, and live streaming, where a small amount of packet loss is tolerable but low latency is essential.

These are the core technical questions from the DJI embedded Qt interview. From object trees to multithreading, from the drawing system to network communication, it covers multiple key knowledge points in Qt development. In fact, Qt interviews are not difficult; the key is to thoroughly understand the fundamentals and relate them to practical scenarios..

If you are interested in Qt development or preparing for Qt-related interviews, you might want to use these questions to assess your mastery level. In actual learning and work, the depth of understanding of these fundamental concepts often determines the quality of the code and the stability of the program.

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