Qt - "No such slot", although there is - c++

Question

I'm trying to connect a button to a function, so when I push the button, the function gets called with a specific parameter. I have
class FieldGridWidget : public QWidget
{
Q_OBJECT
public:
FieldGridWidget(QWidget *parent=0);
~FieldGridWidget();
public slots:
void resizeGrid(int n);
private:
QGridLayout* _gridLayout;
QVector<QPushButton*> _buttonGrid;
};
then the button
_button3 = new QPushButton("3x3", this);
and I'm trying to connect it so if clicked, the resizeGrid function gets called with the parameter 3. For this, I'm trying
connect(_button3, SIGNAL(clicked()), _fieldGrid, SLOT(resizeGrid(3))); //this is line 21
but I get the runtime error
QObject::connect: No such slot FieldGridWidget::resizeGrid(3) in ../filename.cpp:21
What am I doing wrong? Or how can I make it so if I press the button, resizeGrid(3) gets called?
Thank you!

Answer 1

You cannot pass a value to a slot in Qt directly, as in SLOT(resizeGrid(3)). The argument to SLOT should be just the signature of your method (resizeGrid()).
There are two ways how to add an argument to a slot. This question about how to pass arguments to a slot shows some solutions, repeated here. (Go upvote them!)
The modern way : Qt5 and C++11
Instead of connecting a slot, connect to functor, as described by Kuba Ober in this answer:
connect(action1, &QAction::triggered, this, [this]{ onStepIncreased(1); });
connect(action5, &QAction::triggered, this, [this]{ onStepIncreased(5); });
connect(action10, &QAction::triggered, this, [this]{ onStepIncreased(10); });
connect(action25, &QAction::triggered, this, [this]{ onStepIncreased(25); });
connect(action50, &QAction::triggered, this, [this]{ onStepIncreased(50); });
Old fashioned : QSignalMapper
You can use a QSignalMapper to perform what you want, as described by TonyK in this answer:
QSignalMapper* signalMapper = new QSignalMapper (this) ;
connect (action1, SIGNAL(triggered()), signalMapper, SLOT(map())) ;
connect (action5, SIGNAL(triggered()), signalMapper, SLOT(map())) ;
connect (action10, SIGNAL(triggered()), signalMapper, SLOT(map())) ;
connect (action25, SIGNAL(triggered()), signalMapper, SLOT(map())) ;
connect (action50, SIGNAL(triggered()), signalMapper, SLOT(map())) ;
signalMapper -> setMapping (action1, 1) ;
signalMapper -> setMapping (action5, 5) ;
signalMapper -> setMapping (action10, 10) ;
signalMapper -> setMapping (action25, 25) ;
signalMapper -> setMapping (action50, 50) ;
connect (signalMapper, SIGNAL(mapped(int)), this, SLOT(resizeGrid(int))) ;

Answer 2

Signatures of signals and slots must match - if not fully, then at least partially.
For example, you can connect a signal(int, float) to a slot(int), but you can't connect a signal(void) to a slot(int).
It is "sort of possible" to specify a slot parameter in the connection syntax, providing you are using Qt 5:
connect(_button3, &QPushButton::clicked, [=](){ _fieldGrid->resizeGrid(3); });
Of course, you could use the signal mapper in this particular case, but it is a rather clumsy solution - extra verbose and limited to only a single int, string, widget or QObject parameter. With lambdas those limitations do not apply.

Answer 3

clicked() and resizeGrid() signatures should be the same, so you can create function, for example, resizeGridForButton(), where resizeGrid(3) will be executed, or change resizeGrid() signature and transfer int n = 3 into it.
And it's not possible (example in the answer of vsimkus) to send params in connect(...). Also you can send them in emit part through signal params.
Here is a good article about slots and signals.