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.