My tutorial on how to properly use QThreads

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.