Qt connect doesn't recognize with lambda expression

Question

I'm designed a QTableWidget with QPushButton, I would like to connect these buttons with a slot to hide some rows.

I'm using a lambda expression to pass a number of a row. But the compiler doesn't recognized this expression :

connect(this->ui->tableWidget->cellWidget(i,0),&QPushButton::clicked,[this,i]{hideRows(i);});

I have this error:

error: no matching function for call to 'SoftwareUdpater::MainWidget::connect(QWidget*, void (QAbstractButton::*)(bool), SoftwareUdpater::MainWidget::displayTable()::<lambda(int)>)'

• The function hideRows(int) is declared as a function. And, as a slot, it doesn't work,
• CONFIG += c++11 is added in pro file,
• My class MainWidget inherits from QWidget,
• Q_OBJECT is added in the header.

So I don't udnerstand why connect() is not recognized by Qt 5.9.1 MinGw 32bit.

Edit: [this,i]() instead of [this](const int i) for the lambda expression

Answer 1

Your connection is wrong. You can't connect a function that doesn't take parameters (clicked()) with a function that takes parameters (your lambda). To verify that this is the case, just do this:

connect(this->ui->tableWidget->cellWidget(i,0),&QPushButton::clicked,[this](){});

And see that it will compile. You have to make your design in such a way that signals and slots are compatible.

Also avoid using lambdas in signals and slots. Read the caveats here.

Answer 2

I was reading your comments on the accepted answer and noticed the root of the problem: This error is being thrown because the effective type of the object — as supplied to QObject::connect; i.e QWidget in your case — does not define the referenced signal QPushButton::clicked.

What likely happened is that the QPushButton pointer was cast into a QWidget and then that pointer was given to connect instead of the original which defines the signal.

Cast the pointer back to a QPushButton * and the error should go away.