Sending objects as signal arguments between threads in Qt

Question

I am new to Qt and trying to learn the Qt threading mechanism. I am in a situation where I would like a background thread to perform some long running task and report the results to another (or main) thread after processing every 100 items. Right now I am doing this by emitting a signal from the background thread containing a list of the processed objects that is received in a slot in the main thread. Does Qt make a copy of the signal argument when it is received in the slot ? If so, how does how does calling qRegisterMetaType help with that ? This is what I am tying to accomplish in my code :

//background thread
void run(){
   //get a query object from database
   int fireCount = 0;
   QList< QList<QVariant> > data;
   while(query->next()){
        fireCount++;
        QList<QVariant> row;
        //do some calculations on the fields read from the query
        processRow(query,&row);
        data.append(row);
        if(fireCount>100){
            emit publishDataToMainThread(data);
            fireCount = 0;
            data.clear();
        }
   }

}

//slot in main thread
void receiveData(QList< QList<Qvariant> > data){
\\display the data
}

Also , is this a recommended practice for transferring objects between threads ?

ar31 · Accepted Answer · 2012-09-17 07:21:22Z

1

This is a perfectly fine way of doing it. QList uses implicit sharing (i.e. copy on write) so copying it means copying one pointer and increasing the reference count. It only gets copied once you try to modify it. Just remember to use Qt::QueuedConnection when connection the signal to the slot so that the slots gets run in the receivers thread.

qRegisterMetaType or Q_DECLARE_METATYPE are needed so that you can pass parameters by value in signals. It tells the Qt Metatype system (which is sort of like reflection) that this type exists.

answered Sep 17, 2012 at 7:21

ar31

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3 Comments

ar31 Over a year ago

Also maybe using data = QList< QList<QVariant> >() instead of data.clear() might save an unnecessary copy. But maybe that is unnecessary, I didn't look at the source code of clear()

MSalters Over a year ago

There's only one reliable way to save an unnecessary copy, and that's C++11 move semantics. But given that Qt still doesn't support C++98 templates, that might be a bit optimistic. Still, in general .clear() is more efficient in the short term.

ar31 Over a year ago

Disregard my previous comment, clear() internatlly just calls *this = QList<T>().

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