Why slots in QThread subclasses are unsafe?

[AlphaWolf]

Im am refering to this statement from the QThread documentation page:

Like other objects, QThread objects live in the thread where the object was created -- not in the thread that is created when QThread::run() is called. It is generally unsafe to provide slots in your QThread subclass, unless you protect the member variables with a mutex.

For example I want to tell the thread to do something while it's running. So I create a slot or a standard method and the slot sets a variable which the Qthread::run method reads and then does something. Do I have to use a mutex even if just one thread writes the variable and one only reads it?

[jpn]

It's much safer to add your slots to another object which lives inside the thread:

MyObjectWithFancySlots obj;
QThread thread;
obj.moveToThread(&thread);
thread.start();

MyObjectWithFancySlots obj;
QThread thread;
obj.moveToThread(&thread);
thread.start();

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[AlphaWolf]

Ahh that's cool

Edit: When I call a method of this object will it be executed inside the calling thread or inside the object's thread?

[AlphaWolf]

When I call a method of this object will it be executed inside the calling thread or inside the object's thread?

[jpn]

If you call it directly like a normal C++ function, it will be executed in the callers thread of course. You can use signals and slots, custom events or QMetaObject::invokeMethod() get it executed in the worker thread context.

[AlphaWolf]

ok that's helpful

[AlphaWolf]

Sorry for the double posts but I have another question:

I want the second thread to render into a widget using DirectX all the time to have a smooth scene.

So I have a loop which runs from the beginning till the end of my application. The problem is that my rendering thread needs a message loop in order to execute QMetaObject::invokeMethod() or use signal and slots. You see my problem? When I call QThread::exec() I can't use my loop because Qthread:exec() is blocky by design.

[xavierda]

So I have a loop which runs from the beginning till the end of my application. The problem is that my rendering thread needs a message loop in order to execute QMetaObject::invokeMethod() or use signal and slots. You see my problem? When I call QThread::exec() I can't use my loop because Qthread:exec() is blocky by design.

This is exactly the problem I am facing now - I need to use QUdpSocket, which requires an event loop in order to trigger the readyRead signal, AND I want to have essentially an endless loop such that I can process other things as I need to. (FYI, thread is here.) I think the only solution is to move functionality to another class, possibly another thread altogether. Preferably, I'd like to keep the thread count as low as possible as I want to avoid introding problems that may be harder to pinpoint due to the multithreaded approach. How did you resolve your problem in the end?

Cheers,

Xav.

[xavierda]

How did you resolve your problem in the end?

Actually, just worked this out; my solution is here. See what you think!

Cheers,

Xav.