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HomeBackend DevelopmentC++How to Execute a Functor or Lambda in a Specific Qt Thread?

How to Execute a Functor or Lambda in a Specific Qt Thread?

How to execute a functor or a lambda in a given thread in Qt

In Qt, it is possible to execute a lambda or a functor in a specific thread using metacall posting. Here's how it's done:

  1. Create a functoid to wrap the function to be executed:
    Pass the function to be executed as an argument to the std::function class as shown:

     std::function<void> myFunction = []() {
     // Code to be executed in the specified thread
    };</void>
  2. Post the metacall to the specified thread:
    Use the postMetaCall function from the FunctorCallConsumer namespace to post the metacall to the desired thread. The function takes a thread pointer and a function pointer as arguments.

    FunctorCallConsumer::postMetaCall(&myThread, myFunction);
    • Note:
      In Qt 5.10 and later, you can directly use QMetaObject::invokeMethod to execute a function in a specified thread:
    QMetaObject::invokeMethod(&myThread, myFunction);
  3. Implement a metacall event consumer:
    The final step is to implement a QEvent subclass that will receive and execute the metacall. The FunctorCallEvent class can be defined as follows:

    class FunctorCallEvent : public QEvent {
    public:
     FunctorCallEvent(std::function<void> fun) : QEvent(QEvent::None), m_fun(fun) {}
     void execute() { m_fun(); }
    
    private:
     std::function<void> m_fun;
    };
    </void></void>
  4. Connect the event consumer to the thread:
    Once the FunctorCallEvent class has been defined, connect it to the event loop of the specified thread using QCoreApplication::installEventFilter.

    QCoreApplication::instance()->installEventFilter(new FunctorCallEventConsumer(&myThread));

Example:
Here's a complete example that demonstrates how to execute a lambda in a specified thread:

#include <qtcore>

class MyThread : public QThread {
public:
    void run() override {
        // Execute the lambda in this thread
        m_lambda();
    }

    void setLambda(std::function<void> lambda) {
        m_lambda = lambda;
    }

private:
    std::function<void> m_lambda;
};

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

    // Create a new thread
    MyThread thread;

    // Create a lambda to execute in the thread
    std::function<void> lambda = []() {
        qDebug() <p>Output:</p>
<pre class="brush:php;toolbar:false">Lambda executed in thread QThread(0x7f8632802640, name = "MyThread")

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