Why dump in QAbstractAnimation::start

[Toocold]

@jsulm
In this Function

void QAbstractAnimation::start(DeletionPolicy policy)
    
    {
    
        Q_D(QAbstractAnimation);
    
        if (d->state == Running)
    
            return;
    
        d->deleteWhenStopped = policy;
    
        d->setState(Running);
    
    }

My program crashed because of "d" is Null.
But it's strange, the QPropertyAnimation instance is newed in heap.
And I set theirs' parent Null now and start(QAbstractAnimation::KeepWhenStopped) instead of.

[jsulm]

@Toocold said in Why dump in QAbstractAnimation::start:

My program crashed because of "d" is Null.

Then the object is invalid/not initialised.

[Toocold]

@jsulm

        connect(pAnimationBannerOpacity, &QPropertyAnimation::finished,
            [&]
        {
            if (pAnimationTextGeometry)
            {
                pAnimationTextGeometry->start(QAbstractAnimation::KeepWhenStopped);
            }
        });

But I checked it before invoking the function.
And It can work by added to QSequentialAnimationGroup Class.

[jsulm]

@Toocold said in Why dump in QAbstractAnimation::start:

if (pAnimationTextGeometry)

This only checks whether the pointer is not nullptr, but it still can point to not allocated memory.

[JonB]

@Toocold
Because you cannot afford to deference a pointer if it's pointing to garbage, if you use a debugger as @jsulm suggested you can look in its "watch" window at what pAnimationTextGeometry actually points to. It's not foolproof, but if what the debugger shows doesn't look good, it's probably a bad pointer.

[Toocold]

@jsulm @JonB Thanks for your suggestion.
I don't know how to check the pointer is point to an allocated memory.
So I defined the pointer in the lambda function, and it worked.
But why my pAnimationTextGeometry was recycled, I didn't give him a parent.

[Toocold]

@jsulm @JonB
You can try this program, it will crash.

void func()
{
    QLabel* p1 = new QLabel;
    QLabel* p2 = new QLabel;
    QPropertyAnimation*     pA1;
    QPropertyAnimation*     pA2;

    p1->setText("123");
    p2->setText("456");
    p1->setGeometry(QRect(0, 0, 0, 0));
    p2->setGeometry(QRect(0, 0, 0, 0));

    p1->show();
    p2->show();

    pA1 = new QPropertyAnimation(p1, "geometry");
    pA2 = new QPropertyAnimation(p2, "geometry");

    pA1->setDuration(1000);
    pA1->setStartValue(QRect(0, 0, 0, 0));
    pA1->setEndValue(QRect(660, 660, 60, 60));

    pA2->setDuration(1000);
    pA2->setStartValue(QRect(0, 0, 0, 0));
    pA2->setEndValue(QRect(330, 330, 30, 30));

    QObject::connect(pA1, &QPropertyAnimation::finished, [&] {
        pA2->start();
    });

    pA1->start();
}

int main(int argc, char** argv)
{
    QApplication a(argc, argv);

    func();

    return a.exec();
}

[Christian Ehrlicher]

@Toocold said in Why dump in QAbstractAnimation::start:

QObject::connect(pA1, &QPropertyAnimation::finished, [&] {

C++ basics - you catch the variables by reference...

[Toocold]

@Christian-Ehrlicher
Yeah...I didn't notice it.
Thank you.

[Christian Ehrlicher]

@Toocold Then please mark this topic as solved, thx.