Passing another variable to a SLOT in QObject::connect
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Suppose there are two widgets, a QPushButton and a QLabel
How can I write the connecting code, so that when the button is pressed down, a specific string is displayed in the Label?
@QObject::connect(....,SIGNAL(pressed(),......,SLOT(setText("My String"))); @
shows an error...
If I put:
@QObject::connect(....,SIGNAL(pressed(),......,SLOT(setText(QString)));@
where would the value for the string come from?
...I am just starting to learn using Qt... -
Both statements are invalid.
In short:
- connect() wants only the SIGNATURE of the signal/slot (without the return type), not the argument names, nor anything else;
- a slot cannot have more arguments than the ones carried by the signal you're connecting to it (where should they come from?);
- passing values inside connect() is an error, although it's pretty clear what you'd like to do;
- simply use a "proxy" slot that calls the target slot with the arguments, or read the docs about QSignalMapper.
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bq. simply use a “proxy” slot that calls the target slot with the arguments....
How do I do that?
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[quote author="peppe" date="1297240228"]In your (very simple) case, if it's applicable, you can just provide a default argument for the slot. Otherwise, simply create another slot that calls the setText one with the string you want.[/quote]Indeed. With a slot called setText(), providing a default argument can be misleading:
@myLabel->setText();@
What does that do?
Oh well. It's a style issue.
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bq. Otherwise, simply create another slot that calls the setText one with the string you want.
Any way without creating whole new classes?
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"QSignalMapper":http://doc.qt.nokia.com/4.7/qsignalmapper.html might also be an option.
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to solve simple problem like yours I use this solution:
@
connect(button1, SIGNAL(clicked()), SLOT(buttonClicked());
connect(button2, SIGNAL(clicked()), SLOT(buttonClicked());
button1->setProperty("name", "button1");
button2->setProperty("name", "button2");
...
...
MyClass::buttonClicked()
{
QPushButton *button = sender();
if(button->property("name").toString() == "button1")
{
.....
.....
}
else if(button->property("name").toString() == "button2")
{
....
....
}
}
@ -
Luca, that is unsave code.
You don't know what kind of object sender() refers to, or if it is non-0 at all (direct call to buttonClicked()). So, you should at least check that before assuming this. Using a QSignalMapper is the safe way to go, and it still allows other ways to trigger the slot and still work correctly.
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[quote author="Andre" date="1297288071"]Luca, that is unsave code.
You don't know what kind of object sender() refers to, or if it is non-0 at all (direct call to buttonClicked()). So, you should at least check that before assuming this. Using a QSignalMapper is the safe way to go, and it still allows other ways to trigger the slot and still work correctly.
[/quote]
Yes, I usually check if pointer is a push button with a dynamic_cast:
@
QPushButton button = dynamic_cast<QPushButton>(sender());
if(button==NULL)
{
return;
}
@Mine is only a fast example...
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Hi there is a tool which do exactly what you need.
See "QSignalMapper":http://doc.trolltech.com/latest/qsignalmapper.html
There is good example how to use it.