Question

I’m trying to make three mutually exclusive radio buttons with three QToolButtons (basically; they’re subclassed). I set Checkable to true in the designer and now have two problems:

1. They need to be mutually exclusive, i.e. clicking one button should uncheck all the others.
2. There is always exactly one button checked.

The first one was easy to solve with the following slot code that was connected to toggled(bool):

void Foo::slot_radio1(bool on) {
    if (on) {
        tb_radio2->setChecked(false);
        tb_radio3->setChecked(false);
    }
}

and likewise for the other two buttons as well. Not nice, but works. The second is a little trickier and that’s where my question comes into play.

In most GUI toolkits you can cancel an event in an event handler if necessary which could be used here to cancel the toggle if the currently-active button would be toggled off. However, that applies to systems that use event handlers and pass around event objects that can be modified. With signals/slots, as far as I understood, you’re handling events after the fact and have no influence on them. Apart from the problem that you don’t have access to the event data anyway. The naïve but stupid approach I tried was

if (!on) {
    tb_radio1->setChecked(true);
}

basically re-checking the button whenever it’s unchecked – which results in a stack overflow as it recursively triggers every other slot again, and again, …

Is there generally a nice and clean approach to achieving this? I’d rather not disconnect slots, change state and connect them again. I’m probably missing something obvious or trivial for anyone who worked with Qt for years, but I guess that’s not me and thus it eludes me.

---

It got a little messy now, but works, based on the blockSignals suggestion by Tim and Andreas:

void WinPhoenix::slot_radio1(bool on) {
    if (on) {
        bool before2 = radio2->blockSignals(true);
        bool before3 = radio3->blockSignals(true);
        radio2->setChecked(false);
        radio3->setChecked(false);
        radio2->blockSignals(before2);
        radio3->blockSignals(before3);
    } else {
        bool before = radio1->blockSignals(true);
        radio1->setChecked(true);
        radio1->blockSignals(before);
    }
}

Answer 1

I’d rather not disconnect slots, change state and connect them again.

You could use QObject::blockSignals() to temporarily disable signals sent by an object:

bool before = tb_radio1->blockSignals(true);
tb_radio1->setChecked(true);
tb_radio1->blockSignals(before);

Other than that, you could consider using a QActionGroup() to implement the mutual exclusiveness.