Error if I pass arguments to connect in

[Qt Enthusiast]

connect(act, SIGNAL(triggered()), this, SLOT(writeimage(QString("currrent"))));

gives me error Object::connect: No such slot GQActions::writeimage(QString("all"))
Object::connect: No such slot GQActions::writeimage(QString("all"))

but If I do not pass any argument in writeimage then I do not get error
connect(act, SIGNAL(triggered()), this, SLOT(writeimage());

it is working fine

[jsulm]

@Qt-Enthusiast you can not pass parameters to slots in connect!
Parameters are passed from signal when it is emitted.

[Pradeep Kumar]

@Qt-Enthusiast

As said by @jsulm ,

connect(act, SIGNAL(triggered()), this, SLOT(writeimage()));
both the signatures must be same.

If you have any values to be passed, can provide only datatypes.

Ex:
connect(act, SIGNAL(triggered(bool)), this, SLOT(writeimage(bool)));

Thanks,

[jsulm]

@Pradeep-Kumar @Qt-Enthusiast Actually the signal can have more parameters than the slot - the slot then just ignores all the other parameters.
The thing is:

connect(act, SIGNAL(triggered()), this, SLOT(writeimage(QString("currrent"))));

this is not a valid connect call as QString("currrent") creates a QString instance. If writeimage has a QString parameter then it should be:

connect(act, SIGNAL(triggered()), this, SLOT(writeimage(QString)));

but even then it is wrong as the signal does not have QString parameter.

[J.Hilk]

@jsulm @Pradeep-Kumar @Qt-Enthusiast

can pass a default parameters to the SIGNAL, to somewhat circumvent the issue

signals:
   void signalDefault(QString s = "foo");

private slots:
void slotDefault(QString s){
 // do stuff
}

//in cpp
connect(this, SIGNAL(signalDefault(QString)), this, SLOT(slotDefault(QString)));
emit signalDefault();

[jsulm]

@J.Hilk I never tried, did you?
But even if it is working I wouldn't do this because you're hiding in your code what you actually pass to the connected slots.

[Pradeep Kumar]

@J.Hilk ,

can we use as u mentioned,

signals:
void signalDefault(QString s = "foo");

and emit without argument?.

emit signalDefault();

Thanks,

[jsulm]

@Pradeep-Kumar just try

[J.Hilk]

@jsulm said in Error if I pass arguments to connect in:

@J.Hilk I never tried, did you?
But even if it is working I wouldn't do this because you're hiding in your code what you actually pass to the connected slots.

I did, I have one or 2 such case in my current project, it even work with both new and old SIGNAL/SLOT syntax. I believe that changed recently, as I remember it working only with the old one.

[J.Hilk]

@Pradeep-Kumar yes you can.

[Pradeep Kumar]

@jsulm , @J-Hilk ,

yes i tried, i am getting the value in slot,

so is it the new way, or was present earlier also. just curious.

Thanks,

[jsulm]

@Pradeep-Kumar I don't think it's new - it is probably working since C++ allows default values :-)

[Pradeep Kumar]

@jsulm

well i tried for the first time, got the output.

Thanks,

[VRonin]

You can use the new connect syntax to make this work:
connect(act, &QAction::triggered, this, [this]()->void{writeimage(QStringLiteral("currrent"));});