can we define a std::thread on QThread

stackprogramer

Hi, my experience in using QThread, I see that QThread vs std::thread is slower, I have a question about how to use them. For managing signal/slots and GUI connection I need QThread, but it is slow for heavy computing use....is It possible to define a std::thread on QThread with a while flag...I want to manage of standard thread with QThread.... (starting and stopping)
Can anyone guide or help, thanks in advance

#include <iostream>
#include <thread>

class A {
public:
    void print(bool& flag) {
        while (flag) {
            std::cout << "Printing..." << std::endl;
            // You can add more code logic here
            // ...
        }
    }
};

int main() {
    bool flag = true;

    A obj;
    std::thread threadObj(&A::print, &obj, std::ref(flag));
    
    // You can do other operations in the main thread here
    
    // Setting the flag to false to stop the printing thread
    flag = false;
    
    // Waiting for the thread to finish before exiting
    threadObj.join();
    
    return 0;
}

JonB

@stackprogramer
Once a thread is running it should run at whatever speed regardless of how it was created. So what exactly do you claim is "slow"? QThreads (with default run()/exec()) are more intended for receiving Qt signals requesting they do some work rather than "starting and stopping". Qt also has QtConcurrent.

stackprogramer

@JonB qthread is managed with qt event loop, but standard thread is managed with OS, https://stackoverflow.com/questions/40880949/what-is-an-event-loop-in-qt I test two case on qthread there's overload and is slower....I need use std::thread on qthread.

Christian Ehrlicher

QThread is not 'managed' by an event loop. You can use QThread without a spinning event loop but then you can not receive signals from another thread.

JonB

@stackprogramer said in can we define a std::thread on QThread:

@JonB qthread is managed with qt event loop

No, not per se, only like I said if you use default run()/exec(). And anyway, depending on your usage, this is likely to be negligible, e.g. if your thread does a bit of computation in response to a signal. The thread itself just runs, whether you start it from QThread or std::thread, by the OS. Additionally I mentioned QtConcurrent which does not use an event loop.

I cannot imagine creating some std::thread inside a QThread is helpful.

SimonSchroeder

The only place where I could imagine a QThread to be slower than a std::thread is during creation/destruction. Once it is running, there is no difference. The default implementation of QThread starts a QEventLoop. There is a chance that you could start your own event loop inside a std::thread. Though Qt might use calls to check for the corresponding QThread of a QObject. You could certainly emit signals from a std::thread as these are "regular" functions calls. You might have to make sure that your connections are Qt::QueuedConnections.

Both QThread and std::thread need to use the OSes underlying thread functions to control threads. So, if you want to wrap a std::thread into a QThread, a QThread is already wrapping a std::thread, but without the std::thread in the middle.