My tutorial on how to properly use QThreads
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Thank you very much Maya, this tutorial has been very useful for my project!!
Regards.
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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. -
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()
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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?
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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.
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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 -
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. -
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.@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.
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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 ?
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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 ?
@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.
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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 ?
@wally123
As I read it, she meant other objectsWorker::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.
