2

Greeting

I have a following class.

class MyClass : public QObject
{
    Q_OBJECT

public:
    Q_INVOKABLE QVariant status();

public:
    MyClass(ClassX * classX);

public slots:
    void slotA();
    void slotB();

signals:
    void signalA();
    void signalB();

private:
    void init();
    void doSomething();

private:
    ClassX * classX;
    ClassA classA;
    ClassB classB;
};

In MyClass's constructor, I set classX to this.classX and in init(), I connect some of classX signals to MyClass slots and wise-versa and in someFunction() i use classA and classB.

In my controller class in main thread, I create MyClass object and run it inside different thread.

MyClass * myClass = new MyClass(classX);
connect(&myClassThread, SIGNAL(started()), myClass, SLOT(init()));
myClass->moveToThread(&myClassThread);
myClassThread.start();

I see the following warning in qDebugger.

QObject::setParent: Cannot set parent, new parent is in a different thread

Can anyone tell me why i get that warning ?

Thanks in advanced

PS 1: The classX created in main thread.

PS 2 : Remember, Everything work fine and i don't have any problem, I just want to know the reason of this warning and how to fix it.

PS 3 : I also use the following command in main thread to expose the object in javascript.

webFrame->addToJavaScriptWindowObject("myClassObject", myClass);

Edit 1 : QThread myClassThread is class member.

Edit 2 : I believe the lack of information, confused you guys and i'm sorry about that.

The constructor of MyClass is like this :

MyClass::MyClass(ClassX * classX)
{
     this.classX = classX;
}
M.H.
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    Possible duplicate of [QObject: Cannot create children for a parent that is in a different thread](http://stackoverflow.com/questions/3268073/qobject-cannot-create-children-for-a-parent-that-is-in-a-different-thread) – UmNyobe Mar 08 '16 at 13:32
  • easily the nth question about this. `MyClass * myClass = new MyClass(classX);` should be `MyClass * myClass = new MyClass(0);`. Full stop. – UmNyobe Mar 08 '16 at 13:33
  • @UmNyobe Even with no parameters i still get the warnings – M.H. Mar 08 '16 at 13:43
  • @UmNyobe This is not duplicate question, As you can see, I called the methods of myClass via signal-slot! Even if i don't call any methods, or don't even use the object after creation, I still get the warning – M.H. Mar 08 '16 at 13:46
  • I didnt mention signal and slot. If the code snippet above doesnt work with new Myclass(0), then it is different from your real code (as the one above is just for the problem statement). – UmNyobe Mar 08 '16 at 14:27
  • @UmNyobe with MyClass(0) i still get the warning but as i mentioned, The code run and i have no problem, Just want to find the warning reason – M.H. Mar 08 '16 at 14:34
  • show constructor of MyClass. do you pass a QObject as first parameter of the constructor without it being used as a parent? In this case you are confusing us. – UmNyobe Mar 08 '16 at 14:48
  • @UmNyobe Please check Edit 2 – M.H. Mar 08 '16 at 17:09

2 Answers2

7

Let's be clear. Your code is not working as you intended it to work. That is what the framework is telling you.

QObject::setParent: Cannot set parent, new parent is in a different thread

This means all slots and signals of a certain object (suspected myClass) will not be executed in the same thread as the one expected. The issue here revolves around the parent of either myClass or classX objects

Possibility I : myClass->moveToThread(&myClassThread); is failing

Cause: myClass has a parent already set. which is forbidden.

It means that init() will be triggered by the thread of the thread object myClassThread. Thread-wise and event-wise, this is almost the same as if you did 

MyClass * myClass = new MyClass(classX);
QMetaObject::invokeMethod(myClass, "init", Qt::QueuedConnection);

Possibility II : init() is violating thread affinity

Cause: `classX``or a mysterious related object has a parent already set or is not movable to another thread. Think widget.

moveToThread succeed, you have MyClass in one thread, and classX in another thread. You have provided classX when constructing myClass. myClass is now manipulating an object in another thread, and without further code we cannot assume thread safety or correct child parent affinity. Review MyClass::MyClass` and MyClass::init carefully.

Which One is Occurring?

Try putting a break in the debugger, in the controller code and look at the thread id. Then put a break in the debugger in the init method.

  • If it is the same thread, case I
  • Otherwise it is case II
Enter Strandman
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UmNyobe
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1

This is wrong in many ways:

QThread myClassThread;
MyClass * myClass = new MyClass(classX);
connect(&myClassThread, SIGNAL(started()), myClass, SLOT(init()));
myClass->moveToThread(&myClassThread);
myClassThread.start();
  1. You are creating thread object on stack not on heap! This means myClassThread object will be destroyed when this block of code ends.
  2. myClass has a parent. Object which are moved to thread can't have any parent. Only whole three of objects can be moved to threads
Marek R
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  • About 1 : I just added Edit 1, myClassThread is class member actually. About 2 : myClass's parent is QObject. I don't think that causing it – M.H. Mar 08 '16 at 13:53
  • you’re moving myClass to another thread, but classX stays in the main thread. Thus the warning. Don’t use classX as parent for myClass if you want to move myClass to another thread. – Frank Osterfeld Mar 08 '16 at 16:00