Calling an intermediate function when connecting Signals and Slots

Smeeth

I am trying to connect the valueChanged() signal from QSlider to the setText() slot of a QLabel, but I want to call an intermediate function to format the string and check for some edge cases. For instance, if the QSlider is at its minimum value of 0, the text in the QLabel should be "---". not "0".

Is there a way I can call this function with the int value from QSlider::valueChanged() signal, and then pass that value to the QLabel::setText() slot?

I have tried this, but the syntax is not correct. (I get a "No such slot" error).

connect(mySld, SIGNAL(valueChanged(int)), myLbl, SLOT(setText(getLabelText(int))));

QString MyClass::getLabelText(int value){
    if (value > 0)
        return QString::number(value);
    else
        return "---";
}

Chris Kawa

You can use a lambda for this:

connect(mySld, &QSlider::valueChanged, myLbl,  [=](int v) { myLbl->setText(getLabelText(v)); });

Smeeth

That works great! I was trying to stick to the updated SIGNAL/SLOT syntax but this does exactly what I needed it to do. Thank you.

Chris Kawa

SIGNAL/SLOT macros are the remnant of the pre c++11 era. I would suggest to stick to the pointer based syntax - faster, more flexible and less error prone.

Kien Bui

@Chris-Kawa said in Calling an intermediate function when connecting Signals and Slots:

[=](int v) { myLbl->setText(getLabelText(v)); }

I set config on .pro file

CONFIG   += c++11

But I still can't use SIGNAL/SLOT syntax with lambda

Christian Ehrlicher

What error do you get? What Qt version do you use?

Chris Kawa

@Kien-Bui SIGNAL/SLOT macro syntax is based on parsing strings. It doesn't work with lambdas.

mrjj

@Kien-Bui
hi
for an intro to the new syntax , please see here
https://wiki.qt.io/New_Signal_Slot_Syntax