How to pass a QString to a Qt slot from a QMenu via QSignalMapper or otherwise

Question

I have a QMenu with many submenus. These are dynamically created i.e. the names menus come from a db and created in a loop. Now i wanted to fire the same slot triggered() or similar when a menu is clicked, but i needed the QString menu name to be passed to slot so i could perform menu specific actions. I have tried this i.e. passing a QAction * to the triggered event and used setData but i am getting the run time error.

object::connect: No such signal QAction::triggered(QAction *)

for(int j=0; j<channelTypes[i].getNumChannels() ; j++){
        QAction *subMenuAct = subMenu->addAction(tr(c_name)); // c_name the menu name
        subMenuAct->setData(ch_name);
        connect(subMenuAct, SIGNAL(triggered(QAction *)), this, SLOT(playChannel(QAction *))); // playChannel is the slot
}

void <ClassName>::playChannel(QAction *channelAction)
{
     QString str = channelAction->data().toString();
    qDebug() << "Selected - " << str;
}

Alternately, i have also tried QSignalMapper where signalMapper is a data member initialized in the constructor

signalMapper = new QSignalMapper(this);

and

for(int j=0; j<channelTypes[i].getNumChannels() ; j++){
       QAction *subMenuAct = subMenu->addAction(tr(c_name));

       connect(subMenuAct, SIGNAL(triggered()), signalMapper, SLOT(map()));
       signalMapper->setMapping(subMenu, ch_name);
       connect(signalMapper, SIGNAL(mapped(QString)), this, SLOT(playChannel(QString)));
}

In the second case, i don't get any error, however the slot function playChannel is not being called. Would really appreciate if some one could help resolving it.

Update 1: The only difference that i see from other examples i have seen is that usually people are connecting signals from several widgets to a single slot (say different buttons). In my case i am connecting several sub menus (with different names) to a single slot. Should this make any difference?

Update 2: It worked after the correction suggested in the solution below for QSignalMapper. Also the fact that i was using SubMenu as argument to setMapping , where as MenuAction item should have been used instead. But now i am getting event fired multiple times i.e. as many times as there are entries in the main menu for the selected sub menu category. If channel type is English (main menu) with four entries), HBO, star movies etc. (sub menu), and i choose HBO, then event is fired four times with string HBO. It works fine if i create a separate signal mapper for each submenu. But i was hoping a single mapper should be used and i am doing something incorrectly here. Some more details in the comments to the answer.

Answer 1

After adding the QAction to the menu, you only have to connect QMenu to the slot. You don't connect each action individually to the slot:

for(int j=0; j<channelTypes[i].getNumChannels() ; j++){
    ch_name = <name from the database for the channel j>;
    QAction *subMenuAct = subMenu->addAction(tr(ch_name));
    subMenuAct->setData(ch_name);
}

connect(subMenu, SIGNAL(triggered(QAction *)), 
        this, SLOT(playChannel(QAction *)), Qt::UniqueConnection);

As I don't know how you if you delete subMenu each time the dynamic menu is filled, the Qt::UniqueConnection ensure that the slot won't be reconnected multiple times.

---

For the signal mapper version, you should only connect the actions to the mapper in the loop. The connection from the mapper to the slot should only be done once.

for(int j=0; j<channelTypes[i].getNumChannels() ; j++){
   ch_name = <name from the database for the channel j>;
   QAction *subMenuAct = subMenu->addAction(tr(ch_name));
   connect(subMenuAct, SIGNAL(triggered()), signalMapper, SLOT(map()));
   signalMapper->setMapping(subMenuAct, ch_name);   
}
connect(signalMapper, SIGNAL(mapped(QString)), this, SLOT(playChannel(QString)));

And for that case, the slot playChannel should accept a QString instead of a QAction*.