[Qt-interest] Qt Signal slot mechanism with QThread

[John McClurkin]

raJkumar subbarayan wrote:
> Hi All,
> I read about QThread behavior along with signal slot mechanism 
> from http://doc.trolltech.com/4.6/threads-qobject.html#signals-and-slots-across-threads and 
> few more places.
> I have couple of questions regarding the article.
> 1)Is it always good to specify whether the connection is Direct 
> Connection 
> <http://doc.trolltech.com/4.6/qt.html#ConnectionType-enum> or Queued 
> Connection 
> <http://doc.trolltech.com/4.6/qt.html#ConnectionType-enum> in QObject::connect 
> function?
>  In case of direct connection the slot is 
> called immediately after emit is encountered. 
> 
>         But in case signal emitted thread and object whose slot is
>         called is in different thread leads to immediately putting the
>         slot's thread to wait and execution of slot in it's thread
>         happens, make sure the slot is reentrant in this case.
>

Whether it is good or not is a value judgement, not a programing issue. 
It is necessary only if you want to guarantee a certain behavior. Qt is 
pretty good at figuring out what type of connection to use.

> 2)I find difficulties understanding how Qt selects one of above 
> connection in case the parameter is left blank, can any one explain ?

The default connection type is Qt::AutoConnection.
 From the documentation on Qt::AutoConnection
"if the signal is emitted from the thread in which the receiving object 
lives,
the slot is invoked directly, as with Qt::DirectConnection;
otherwise the signal is queued, as with Qt::QueuedConnection."

Is this explanation not clear to you?