Solved cast QGraphicsEffect* to QGraphicsColorizeEffect* makes program crash.

[Jakob Clausen]

Hi.
I am trying to get my head around QGraphicsEffect.
I have a bunch of QWidgets and I want to put QGraphicsColorizeEffect on some of them some of the time. So I need to be able to set and remove the effect dynamically.
I can set an effect on a QWidget with QWidget::setGraphicsEffect(), which (I think) works on all types of effects. But there is no function to remove the effect.
But then I thought maybe I can just add an effect to all the widgets, and then use setStrength().
QWidget::setGraphicsEffect() works on all effects and QWidget::graphicsEffect() returns a pointer to a QGraphicsEffect, but setStrength belongs to QGraphicsColorizeEffect.
So maybe I should cast it? But then the program crashes.
My code looks like this:

void activateEffect(QWidget* widget)
{
    QGraphicsColorizeEffect* effect;
    effect = (QGraphicsColorizeEffect*) current->graphicsEffect();
    effect->setStrength(0);

    if (!widget->graphicsEffect())
        widget->setGraphicsEffect(new QGraphicsColorizeEffect(widget));

    effect = (QGraphicsColorizeEffect*) widget->graphicsEffect();
    effect->setStrength(1);

    current = widget;
}

So I guess my questions are

• Is this the right way to do this?
• Why is it crashing. It looks like it crashes at the line with the casting

effect = (QGraphicsColorizeEffect*) current->graphicsEffect();

[Christian Ehrlicher]

Are you sure current is valid? I would guess no. Also I would suggest you to not use C-style casts but the C++ equivalents like const_cast<>, static_cast<>, dynamic_cast<> or reinterpret_cast<> (better avoid using reinterpret cast when possible)

[JonB]

@Jakob-Clausen said in cast QGraphicsEffect* to QGraphicsColorizeEffect* makes program crash.:

effect = (QGraphicsColorizeEffect*) current->graphicsEffect();

As @Christian-Ehrlicher says. Replace with:

Q_ASSERT(current);
effect = dynamic_cast<QGraphicsColorizeEffect*>(current->graphicsEffect());
Q_ASSERT(effect);

What happens?

[Jakob Clausen]

Christian Ehrlicher: Thank you for your input. I think you are right. I will try it on monday. Do you think this is the right way to do something like this?

JonB: Also thank you for your input. I will also try this on monday.

Have a nice weekend.

[mrjj]

Hi
Just as info

You can turn off an effect by calling setGraphicsEffect with nullptr

Its logic is like this

void QWidget::setGraphicsEffect(QGraphicsEffect *effect)
{
    Q_D(QWidget);
    if (d->graphicsEffect == effect)
        return;
    if (d->graphicsEffect) {
        d->invalidateBackingStore(rect());
        delete d->graphicsEffect;
        d->graphicsEffect = 0;
    }
    if (effect) {
        // Set new effect.
        QGraphicsEffectSourcePrivate *sourced = new QWidgetEffectSourcePrivate(this);
        QGraphicsEffectSource *source = new QGraphicsEffectSource(*sourced);
        d->graphicsEffect = effect;
        effect->d_func()->setGraphicsEffectSource(source);
        update();
    }
    d->updateIsOpaque();
}

so nothing bad happens to give it a null pointer since it will delete the one it has.

But if you only use QGraphicsColorizeEffect, i like your idea with setStrength() as
then we dont go around deleting and creating QGraphicsColorizeEffect over and over.

[Jakob Clausen]

Thank you all for your help.
current was not valid. It was a brainfart on my part.
But thank you very much for your input.