My tutorial on how to properly use QThreads

[MayaPosch]

http://developer.qt.nokia.com/wiki/QThreads_general_usage

There you go... feel free to link to it from other articles in an appropriate manner :)

Feedback is also very welcome.

[BelenMuñoz]

Thank you very much Maya, this tutorial has been very useful for my project!!

Regards.

[MayaPosch]

You're quite welcome, BelenMuñoz :)

[Sam]

Thanks for such a nice article, made the picture more clear about the implementation.

Regards

[jigar.patel]

But what about child qobjects (heap objects)?
By this thread creation method, child objects are not moved in to thread.
how can i move them into thread?
please help me.

[JKSH]

Hi,

When you move a parent QObject, all its children are automatically moved too.

Remember though that member variables do not automatically become children; the parent-child relationship must be set by either:

• passing the parent's pointer into the child's constructor, or
• calling QObject::setParent()

[jigar.patel]

class SecondaryOperation: public QObject
{
Q_OBJECT
private:
bool printtype;
QTimer *tt;
public:
SecondaryOperation(bool);
public slots:
void runsecondaryfunction();
void ttevent();
signals:
void taskfinish();
};

i am creating one object of above class in main thread. i am also creating one thread, to which i am passing "runsecondaryfunction()" public slot. in "runsecondaryfunction()" there is one while loop, that will continuous run in thread. i have initialized timer tt in class constructor. timeout event of tt timer is linked with "ttevent()". but there is no timeout event occur. also if i am declaring other seperate timer object in "runsecondaryfunction()", program stop unexpectedly. also i want to use one object of qnetworkaccessmanager in SecondaryOperation class. then how can i use it?

[JKSH]

Hi jigar.patel,

There are many parts in your question. Please "start a new discussion":http://qt-project.org/forums/newtopic/10/ and show more of your code (especially the implementation of runsecondaryfunction() and ttevent()) -- your descriptions don't have enough details.

Also, please put '@' before and after your code to give it proper highlighting. That makes it easier to read.

[deleted28]

Hello Maya,

beginner question on this sentence "... you should NEVER allocate heap objects (using new) in the constructor of the QObject class"
I do not undestand which is the constructor of the QObject class.

This constructor ? : Worker::Worker() {} ?

Can i do this way, or is this the constructor meant above ?

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QThread* thread = new QThread;
    Worker* worker = new Worker();
    worker->moveToThread(thread);

If this is ok ( and it works in this simple version) where not to never allocate ... ?
thank you

[mrjj]

as far as I can read, she means in the worker constructor as we later do

Worker* worker = new Worker();
worker->moveToThread(thread);

so if we in Worker constructor create new other objects,
they are not moved and stays in main thread.

[JKSH]

@mrjj said:

so if we in Worker constructor create new other objects,
they are not moved and stays in main thread.

Only if the other objects aren't parented properly. See the QObject documentation:

Note: A QObject's member variables do not automatically become its children. The parent-child relationship must be set by either passing a pointer to the child's constructor, or by calling setParent(). Without this step, the object's member variables will remain in the old thread when moveToThread() is called.

[deleted28]

Hello mrjj,

if I see this line in the Worker constructor, i tend to assume the code would create a neverending number of Worker objects. For me this looks recursive, (?)
That's why I asked what Maya meant by NEVER do ... but "what ?"

Worker::Worker(QObject *parent) : QObject(parent)
{
Worker* worker = new Worker();
...
}

In which point my thinking is bad ?

[koahnig]

@wally123

Creating an instance of the same object within the constructor is always deadly. That has nothing to do with QThreads.

@JKSH said:

Only if the other objects aren't parented properly. See the QObject documentation:

Note: A QObject's member variables do not automatically become its children. The parent-child relationship must be set by either passing a pointer to the child's constructor, or by calling setParent(). Without this step, the object's member variables will remain in the old thread when moveToThread() is called.

That is a good point.

[mrjj]

@wally123
As I read it, she meant other objects

Worker::Worker(QObject *parent) : QObject(parent)
{
SomeWidget*  MyWid= new SomeWidget();
}

and as JKSH then teach us is that you -can- do that if you then also do

MyWid->setParent(this)

I dont think she warned about

Worker::Worker(QObject *parent) : QObject(parent)
{
Worker* worker = new Worker();
}

And yes, this will be recursive to the point of blowing up.