Detailed explanation of Qt multithreading

Detailed explanation of Qt multithreading

Qt is a multi-threaded, cross-platform C++ framework designed to help developers handle concurrent tasks in a safe and simple way. In this blog, we will introduce the concept of multi-threading in Qt and its usage.

What is Qt multithreading

In computer science, a thread can be thought of as a lightweight, executable part of a separate process. Compared to traditional single-threaded programs, multi-threaded programs can perform multiple tasks simultaneously and take advantage of the multi-core performance of modern CPUs.

In Qt, multithreading is implemented through the QThread class. The QThread class provides a general method of using threads, which can easily handle various multi-threaded application scenarios.

How to use multithreading in Qt

Inter-thread communication can be made easier using the QObject class and its derived classes, such as QWidget and QCoreApplication. Among them, the most common method is to use the signal and slot mechanism to send signals and receive slots between different threads.

The basic steps for using multithreading are as follows:

Step 1: Create a custom thread class

You need to inherit the QThread class or create a subclass of the thread yourself using the QObject class.

class MyThread : public QThread
{
    
    
    Q_OBJECT
    
public:
    void run() override;

Define the thread's tasks within the run() function you write yourself.

Step 2: Instantiate the new thread

Declare a MyThread thread instance in the main thread and run it explicitly:

MyThread thread;
thread.start();

Step 3: Connect Signals and Slots

The key to multi-threaded programs is communication between threads. You can ensure that the worker thread is deleted after the work is completed by using a signal (QThread::finished()) to connect the worker thread signal to the slot of the object receiving the signal (deleteLater()). Here is some sample code:

connect(&thread, &QThread::finished, worker, &QObject::deleteLater);

Step 4: Handle exceptions when threads stop

To ensure the stability of the application when threads are stopped, we need to handle exceptions when threads are stopped. For example, in the MyThread class you can add the following code:

void MyThread::stop()
{
    
    
    mutex.lock();
        stopped = true;
    mutex.unlock();
}

if (stopped) break;

Note: You need to add mutex protects appropriately according to your code to protect shared data.

Best practices for Qt multithreading

• Avoid doing heavy calculations on the GUI thread.
• Ensure that communication between processes and threads is handled by Qt, rather than directly accessing or changing variables.
• Use queues to manage thread pools whenever possible to avoid overuse of resources.
• Shared data needs to be protected in different threads.

Summarize

Qt multithreading provides a powerful mechanism to handle concurrent tasks. Using the signals and slots mechanism, you can easily implement various inter-thread communications and manage the thread pool and its tasks more easily through the QObject class.

In actual applications, ensuring that you design your code according to Qt multi-threading best practices will allow you to write programs that are efficient, stable, and easy to maintain.