Connecting two signals with different arguments - c++

Question

I want to get a QTreeView widget to emit a clicked(const QModelIndex&) signal whenever a pushbutton is clicked. This is so that I can get a list of all the items that are selected within the QTreeView at the time of clicking the pushbutton. Now I thought I could connect two signals with distinct arguments (Qt Connect signals with different arguments), however when I try to call
connect(ui.pbAddVideo, SIGNAL(clicked()), ui.treeView_video, SIGNAL(clicked(const QModelIndex&)));
I get the error message:
QObject::connect: Incompatible sender/receiver arguments QPushButton::clicked() --> QTreeView::clicked(QModelIndex)
Have I misunderstood the whole signal forwarding concept?
As always, many thanks.

Answer 1

Firtsly, what index you must send by clicking a button from your tree?
Secondly, since c++11 standart you can do something like that:
connect(ui.pbAddVideo, &QPushButton::clicked, [=] { emit ui.treeView_video->clicked(ui.treeView_video->currentIndex()); });

Answer 2

I would solve your problem with the following approach:
First you have to handle the button click:
connect(ui.pbAddVideo, SIGNAL(clicked()), this, SLOT(onLoadVideo()));
In the slot you need to get the list of selected items from the tree view and do something with them:
void MyClass::onLoadVideo()
{
QItemSelectionModel *selectionModel = ui.treeView_video->selectionModel();
QModelIndexList selectedVideos = selectionModel->selectedIndexes();
foreach (QModelIndex *video, selectedVideos) {
// Play the video.
}
}

Answer 3

You are connecting one SIGNAL() to another SIGNAL() which is perfectly ok but their parameters should match. In your case the second signal has a parameter(i.e QModelIndex) that the first signal does not have.

Answer 4

Have I misunderstood the whole signal forwarding concept?
Yes.
When a signal is emitted, Qt takes the signal's arguments and passes them to the connected slot/signal.
Valid signal-slot connection
For example, suppose you connect the QSlider::valueChanged(int) signal to the QSpinBox::setValue(int) slot. When the valueChanged() signal is emitted, this is effectively how the slot gets called:
// "Emitting a signal" == "Calling the signal function"
QSlider::valueChanged(int sliderSignalValue)
{
QSpinBox *receiver = findReceiver();
receiver->setValue(sliderSignalValue);
}
Valid signal-signal connections
Similarly, if you connect the QSlider::valueChanged(int) signal to the QSpinBox::valueChanged(int) signal, the code behaves like this:
QSlider::valueChanged(int sliderSignalValue)
{
QSpinBox *receiver = findReceiver();
emit receiver->valueChanged(sliderSignalValue);
}
Now, if you want to connect in the opposite direction (connect(ui.treeView_video, SIGNAL(clicked(const QModelIndex&)), ui.pbAddVideo, SIGNAL(clicked()));, it's perfectly fine:
QTreeView::clicked(const QModelIndex& viewSignalValue)
{
QPushButton *receiver = findReceiver();
emit receiver->clicked(); // No problem. The viewSignalValue is simply ignored.
}
Invalid signal-slot connection
However, for the connection that you wanted to make, the code will need to behave like this:
QPushButton::clicked()
{
QTreeView *receiver = findReceiver();
emit receiver->clicked(/*???*/); // ERROR: You need to pass a QModelIndex value!
}
You've got a parameter mismatch. QTreeView::clicked() requires a QModelIndex value, but QPushButton::clicked() cannot provide this. Therefore, you cannot connect these two together.
Does this make sense?

Answer 5

Many thanks to #vahancho whose answer I have adopted. There is no point in using the "clicked()" signal from the qTreeView since I do not need to wait for this to access the data inside. Hence:
connect(ui.pbAddVideo, SIGNAL(clicked()), this, SLOT(addVideo()));
void VigilWidget::addVideo() {
QItemSelectionModel *selectionModel = ui.treeView_video->selectionModel();
QModelIndexList selectedVideos = selectionModel->selectedIndexes();
foreach (QModelIndex video, selectedVideos) {
qDebug().nospace() << video.data(0);
}
}
As to my question about how signal to signal connections work, thanks to everyone for taking the time to explain this :)