QThread used to call function pointer

Pierre-Emmanuel

Hi guys,

I am developing a user interface which will need to run some code in a dedicated thread. I don't want to use the graphical thread because the source code execution may last few minutes to 20-30 minutes and the user interface will freeze.

So far, I am using a sub class of QThread, redefining the method run and job done. However, I would like to have a generic "Working thread" which is taking a function pointer in the constructor and the run function is simply calling the function pointer.

Here is a simple demo app:

working_thread.h:

working_thread.c:

A dummy object to run some code asynchronously:

But the error I got is the following:

I am a java developer and I can use lambda expression to avoid creating a function attached to an object. I investigated this direction, but each time I got compilation errors.

What am I doing wrong here ?

I thought the typedef bool (*Runnable)(); declared in the working_thread.h file was generic enough to accept all kind of functions as soon as it has the correct signature.

I saw as well the moveToThread function, but I'm not sure if it is what I am looking for.

Have a nice day,
Pierre-Emmanuel

jsulm

@Pierre-Emmanuel said in QThread used to call function pointer:

I thought the typedef bool (*Runnable)(); declared in the working_thread.h file was generic enough to accept all kind of functions as soon as it has the correct signature.

No, it's not. You're trying to use a pointer to a method of a class. Your Runnable is a simple function pointer.
Take a look at https://opensource.com/article/21/2/ccc-method-pointers

J.Hilk

Subclassing Thread is in 99% of all cases the wrong approach. But you're coming from Java so I can't fault you for that :p

What you're looking for is QtConcurrent, it accepts a function pointer and runs that in a different thread from an internal thread pool

https://doc.qt.io/qt-6/qtconcurrent.html#run

make sure your function is truly standalone, no access to shared objects etc

Pierre-Emmanuel

Hi @J-Hilk and @jsulm

Thank you for this information. I am trying now to use QtConcurrent::run function but I am facing an issue to be notified after the execution, to get a success status.

Here is what I have done:

async_caller.h:

async_caller.c:

main function:

The output is:

I can see that the task is executed in a different thread, but somehow the slot onAsyncCallComplete is not called.

I watch this youtube tutorial: https://www.youtube.com/watch?v=W9nWyTzxLjs

and added the call to the deleteLater slot to be sure the watcher still exists when the callback should be executed.

PS @J-Hilk: In Java I would not extends the Thread class but use a function to give to the thread constructor ahah. On a more serious topic, would it be a good idea to create a WorkingThread with a lambda expression as input argument for the constructor ?

Have a nice day guys !
Pierre-Emmanuel

jsulm

@Pierre-Emmanuel Please post code as text, not pictures.
You do not have Qt event loop running, that's probably why your slot is not executed: msleep blocks the thread and just after that you terminate the application.

Pierre-Emmanuel

Hi @jsulm,

I updated the main function to initialize the EventLoop it is working as expected.

Thank you very much for your help,
Pierre-Emmanuel de Robien