QThread clarification

[janorcutt]

Hi peeps.
I'm creating a worker thread to keep my app responsive while accessing a database etc,
and had a couple of questions.
1. can i call slots in my worker thread from my main thread before i start the thread?
and if so
2. how do i stop this from happening?

also why can't i create a pointer to my thread i.e.

QThreadClass *thread;

QThreadClass *thread;

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I get a sigsev when i try to connect signals and slots, i have to pass a reference to the connect function i.e.

QThreadClass thread;
connect(&thread, SIGNAL(signal(arg)), this, SLOT(slot(arg)));

QThreadClass thread;
connect(&thread, SIGNAL(signal(arg)), this, SLOT(slot(arg)));

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hope i'm not being too daft.....

[MarekR22]

I recommend to use this kind of pattern:

1. Create simple class inheriting from QObject which handles data base and emits signals and have some slots (lets call it DataBaseHandler).
2. then create thread like that
   

   QThread *thread = new QThread(this);
   DataBaseHandler *database = new DataBaseHandler(thread);
   database->moveToThread(thread);
    
   // example connections:
   connect(this, SIGNAL(buttonWasPressed()),
               database, SLOT(featchSomeData()));
    
   connect(database, SIGNAL(dataReady(QString)),
               someLabel, SLOT(setText(QString)));
    
   thread->start();

   QThread *thread = new QThread(this);
   DataBaseHandler *database = new DataBaseHandler(thread);
   database->moveToThread(thread);
   
   // example connections:
   connect(this, SIGNAL(buttonWasPressed()),
               database, SLOT(featchSomeData()));
   
   connect(database, SIGNAL(dataReady(QString)),
               someLabel, SLOT(setText(QString)));
   
   thread->start();

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Qt will do all tricks required for such simple case.

[wysota]

1. can i call slots in my worker thread from my main thread before i start the thread?

You mean slots of the worker thread or slots of the worker thread object? As for the former - no, because the thread doesn't exist yet so no objects live in that thread. As for the latter - yes but the thread object is associated with the thread that created the object and not with the thread controlled by the object.

also why can't i create a pointer to my thread i.e.

QThreadClass *thread;

QThreadClass *thread;

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What do you mean you "can't"?

i have to pass a reference to the connect function i.e.

QThreadClass thread;
connect(&thread, SIGNAL(signal(arg)), this, SLOT(slot(arg)));

QThreadClass thread;
connect(&thread, SIGNAL(signal(arg)), this, SLOT(slot(arg)));

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It's not a reference, it's an address (aka temporary pointer).

[janorcutt]

ok thanks for the info.
as for the slots, i'm not going to worry about it too much as i only need to emit signals which i can already do, as long as i register the types e.g.

qRegisterMetaType<object>();

qRegisterMetaType<object>();

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as for the pointer, i should have clarified, i can create the pointer however my program sigsev'd at run time at the following code:

connect(thread, SIGNAL(signal(int)), this, SLOT(slot(int)));

connect(thread, SIGNAL(signal(int)), this, SLOT(slot(int)));

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whereas the following worked

// header
QThreadClass thread;
 
//implimentation
connect(&thread, SIGNAL(signal(arg)), this, SLOT(slot(arg)));

// header
QThreadClass thread;

//implimentation
connect(&thread, SIGNAL(signal(arg)), this, SLOT(slot(arg)));

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i'm going to have to read my c++ book again

[wysota]

as for the pointer, i should have clarified, i can create the pointer however my program sigsev'd at run time at the following code:

connect(thread, SIGNAL(signal(int)), this, SLOT(slot(int)));

connect(thread, SIGNAL(signal(int)), this, SLOT(slot(int)));

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Because you created a pointer and not an object. It's like you had a parking space without a car in it. Try opening a door of an empty parking lot

[high_flyer]

Because you created a pointer and not an object.

To clear possible confusion, just creating an object will not do either - you need the pointer to point (have an address) of a created object - this is called "initializing a pointer".

If initializing pointers is something new to you, or how pointers work, I really suggest you don't jump straight in to threads, which is a complex issue on its own.
First be comfortable with the language, then try more complex stuff.