Qt slot with default arguments not working

[JuhaSim]

Having slot

private slots:
void slot(bool param = true);

And connecting

QTimer::singleShot(6000, this, &Class::slot);

Creates a compilation error:

qglobal.h:121:49: error: static assertion failed: The slot requires more arguments than the signal provides.
qobject.h:239:9: note: in expansion of macro ‘Q_STATIC_ASSERT_X’

Qt 5.12.1

https://doc.qt.io/qt-5/signalsandslots.html#signals-and-slots-with-default-arguments
The documentation says that: The signatures of signals and slots may contain arguments, and the arguments can have default values.

Is this a bug?

[J.Hilk]

@JuhaSim said in Qt slot with default arguments not working:

Is this a bug?

no, default arguments for slots is a feature for Qt4 Syntax only, the Qt5 one does not support it, sadly enough.

You have two options:

QTimer::singleShot(6000, this, SLOT(slot());

This may be wrong, been a while since I used Qt4 with default argument!

or a lambda

QTimer::singleShot(6000, this, [=]()->void{slot();});

[Pablo J. Rogina]

@JuhaSim said in Qt slot with default arguments not working:

void slot(bool param = true);

Setting Qt 4.x vs 5.x syntax for connect() aside, for me the question is when the called slot will receive a false value? what will make the argument have a different value from the default one?

Assuming conditionX I can think of an interim slot called by the timer off which in turns calls the current slot. Pseudo-code:

private slots:
void slotTemp();

private:
void slot(bool param = true);

....

QTimer::singleShot(6000, this, &Class::slotTemp);

void slotTemp() {
    slot(conditionX);
}

[SGaist]

Hi and welcome to devnet,

AFAIK, you can't have fewer signal parameters using the syntax (see here).

However, you can do what you want using a lambda or use the old syntax.